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  • Articles  (7,968)
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  • Articles  (7,968)
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  • 2010-2014  (7,968)
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  • 1
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    Incorporating numerical modeling into estimates of the detection capability of the IMS infrasound network (2012)
    Wiley
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    Publication Date: 2012-03-09
    Description: To monitor compliance with the Comprehensive Nuclear-Test ban Treaty (CTBT), a dedicated International Monitoring System (IMS) is being deployed. Recent global scale observations recorded by this network confirm that its detection capability is highly variable in space and time. Previous studies estimated the radiated source energy from remote observations using empirical yield-scaling relations which account for the along-path stratospheric winds. Although the empirical wind correction reduces the variance in the explosive energy versus pressure relationship, strong variability remains in the yield estimate. Today, numerical modeling techniques provide a basis to better understand the role of different factors describing the source and the atmosphere that influence propagation predictions. In this study, the effects of the source frequency and the stratospheric wind speed are simulated. In order to characterize fine-scale atmospheric structures which are excluded from the current atmospheric specifications, model predictions are further enhanced by the addition of perturbation terms. A theoretical attenuation relation is thus developed from massive numerical simulations using the Parabolic Equation method. Compared with previous studies, our approach provides a more realistic physical description of long-range infrasound propagation. We obtain a new relation combining a near-field and a far-field term, which account for the effects of both geometrical spreading and absorption. In the context of the future verification of the CTBT, the derived attenuation relation quantifies the spatial and temporal variability of the IMS infrasound network performance in higher resolution, and will be helpful for the design and prioritizing maintenance of any arbitrary infrasound monitoring network.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
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  • 2
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    Estimation of errors in the inverse modeling of accidental release of atmospheric pollutant: Application to the reconstruction of the cesium-137 and iodine-131 source terms from the Fukushima Daiichi power plant (2012)
    Wiley
    Details
    Publication Date: 2012-03-09
    Description: A major difficulty when inverting the source term of an atmospheric tracer dispersion problem is the estimation of the prior errors: those of the atmospheric transport model, those ascribed to the representativity of the measurements, those that are instrumental, and those attached to the prior knowledge on the variables one seeks to retrieve. In the case of an accidental release of pollutant, the reconstructed source is sensitive to these assumptions. This sensitivity makes the quality of the retrieval dependent on the methods used to model and estimate the prior errors of the inverse modeling scheme. We propose to use an estimation method for the errors' amplitude based on the maximum likelihood principle. Under semi-Gaussian assumptions, it takes into account, without approximation, the positivity assumption on the source. We apply the method to the estimation of the Fukushima Daiichi source term using activity concentrations in the air. The results are compared to an L-curve estimation technique and to Desroziers's scheme. The total reconstructed activities significantly depend on the chosen method. Because of the poor observability of the Fukushima Daiichi emissions, these methods provide lower bounds for cesium-137 and iodine-131 reconstructed activities. These lower bound estimates, 1.2 × 1016 Bq for cesium-137, with an estimated standard deviation range of 15%–20%, and 1.9 − 3.8 × 1017 Bq for iodine-131, with an estimated standard deviation range of 5%–10%, are of the same order of magnitude as those provided by the Japanese Nuclear and Industrial Safety Agency and about 5 to 10 times less than the Chernobyl atmospheric releases.
    Print ISSN: 0148-0227
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  • 3
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    Simultaneous assimilation of AIRS Xco2 and meteorological observations in a carbon climate model with an ensemble Kalman filter (2012)
    Wiley
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    Publication Date: 2012-03-09
    Description: This study is our first step toward the generation of 6 hourly 3-D CO2 fields that can be used to validate CO2 forecast models by combining CO2 observations from multiple sources using ensemble Kalman filtering. We discuss a procedure to assimilate Atmospheric Infrared Sounder (AIRS) column-averaged dry-air mole fraction of CO2 (Xco2) in conjunction with meteorological observations with the coupled Local Ensemble Transform Kalman Filter (LETKF)-Community Atmospheric Model version 3.5. We examine the impact of assimilating AIRS Xco2 observations on CO2 fields by comparing the results from the AIRS-run, which assimilates both AIRS Xco2 and meteorological observations, to those from the meteor-run, which only assimilates meteorological observations. We find that assimilating AIRS Xco2 results in a surface CO2 seasonal cycle and the N-S surface gradient closer to the observations. When taking account of the CO2 uncertainty estimation from the LETKF, the CO2 analysis brackets the observed seasonal cycle. Verification against independent aircraft observations shows that assimilating AIRS Xco2 improves the accuracy of the CO2 vertical profiles by about 0.5–2 ppm depending on location and altitude. The results show that the CO2 analysis ensemble spread at AIRS Xco2 space is between 0.5 and 2 ppm, and the CO2 analysis ensemble spread around the peak level of the averaging kernels is between 1 and 2 ppm. This uncertainty estimation is consistent with the magnitude of the CO2 analysis error verified against AIRS Xco2 observations and the independent aircraft CO2 vertical profiles.
    Print ISSN: 0148-0227
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  • 4
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    Periodic bedrock ridges on Mars (2012)
    Wiley
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    Publication Date: 2012-03-09
    Description: Evidence for sediment transport and erosion by wind is widespread over the surface of Mars today and was likely a major geomorphic process for much of its geological past. Although Martian surface features resembling aeolian dunes and ripples have been recognized since the Mariner and Viking missions, such features have been interpreted previously as active, indurated, or exhumed sedimentary forms. Here we report evidence based on High Resolution Imaging Science Experiment images that show some megaripple forms are eroded into cohesive substrate rather than being composed of loose granular material or fossilized dunes. Exposure of stratigraphic continuity within layered, cohesive material extending crest to trough through features with mean wavelengths of 18 to 51 m demonstrates the primarily erosional formation of what we term periodic bedrock ridges (PBRs). Hence some surfaces on Mars previously considered to be covered by wind-deposited material are actually wind-carved exposures that offer windows into Martian history. PBRs lack the distinctive streamlining associated with wind-parallel yardangs and comparison of PBR orientation to yardangs, megayardangs, and active sedimentary dunes in the same vicinity confirm that these PBRs formed transverse to prevailing winds. Observed wavelengths of PBRs are comparable to those predicted by a simple model for erosional wavelengths of periodic transverse bed forms owing to the spacing of flow separations within the flow. Recognition of these transverse aeolian erosional forms brings up the question of how widespread Martian PBRs are and how many have been misinterpreted as active or indurated (fossilized) sedimentary dunes.
    Print ISSN: 0148-0227
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  • 5
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    Hydroxyl radical and NOx production rates, black carbon concentrations and light-absorbing impurities in snow from field measurements of light penetration and nadir reflectivity of onshore and offshore coastal Alaskan snow (2012)
    Wiley
    Details
    Publication Date: 2012-03-09
    Description: Photolytic production rates of NO, NO2 and OH radicals in snow and the total absorption spectrum due to impurities in snowpack have been calculated for the Ocean-Atmosphere-Sea-Ice-Snowpack (OASIS) campaign during Spring 2009 at Barrow, Alaska. The photolytic production rate and snowpack absorption cross-sections were calculated from measurements of snowpack stratigraphy, light penetration depths (e-folding depths), nadir reflectivity (350–700 nm) and UV broadband atmospheric radiation. Maximum NOx fluxes calculated during the campaign owing to combined nitrate and nitrite photolysis were calculated as 72 nmol m−2 h−1 for the inland snowpack and 44 nmol m−2 h−1 for the snow on sea-ice and snowpack around the Barrow Arctic Research Center (BARC). Depth-integrated photochemical production rates of OH radicals were calculated giving maximum OH depth-integrated production rates of ∼160 nmol m−2 h−1 for the inland snowpack and ∼110–120 nmol m−2 h−1 for the snow around BARC and snow on sea-ice. Light penetration (e-folding) depths at a wavelength of 400 nm measured for snowpack in the vicinity of Barrow and snow on sea-ice are ∼9 cm and 14 cm for snow 15 km inland. Fitting scaled HULIS (HUmic-LIke Substances) and black carbon absorption cross-sections to the determined snow impurity absorption cross-sections show a “humic-like” component to snowpack absorption, with typical concentrations of 1.2–1.5 μgC g−1. Estimates of black carbon concentrations for the four snowpacks are ∼40 to 70 ng g−1 for the terrestrial Arctic snowpacks and ∼90 ng g−1 for snow on sea-ice.
    Print ISSN: 0148-0227
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  • 6
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    Juan de Fuca subduction zone from a mixture of tomography and waveform modeling (2012)
    Wiley
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    Publication Date: 2012-03-09
    Description: Seismic tomography images of the upper mantle structures beneath the Pacific Northwestern United States display a maze of high-velocity anomalies, many of which produce distorted waveforms evident in the USArray observations indicative of the Juan de Fuca (JdF) slab. The inferred location of the slab agrees quite well with existing contour lines defining the slab's upper interface. Synthetic waveforms generated from a recent tomography image fit teleseismic travel times quite well and also some of the waveform distortions. Regional earthquake data, however, require substantial changes to the tomographic velocities. By modeling regional waveforms of the 2008 Nevada earthquake, we find that the uppermost mantle of the 1D reference model AK135, the reference velocity model used for most tomographic studies, is too fast for the western United States. Here, we replace AK135 with mT7, a modification of an older Basin-and-Range model T7. We present two hybrid velocity structures satisfying the waveform data based on modified tomographic images and conventional slab wisdom. We derive P and SH velocity structures down to 660 km along two cross sections through the JdF slab. Our results indicate that the JdF slab is subducted to a depth of 250 km beneath the Seattle region, and terminates at a shallower depth beneath Portland region of Oregon to the south. The slab is about 60 km thick and has a P velocity increase of 5% with respect to mT7. In order to fit waveform complexities of teleseismic Gulf of Mexico and South American events, a slab-like high-velocity anomaly with velocity increases of 3% for P and 7% for SH is inferred just above the 660 discontinuity beneath Nevada.
    Print ISSN: 0148-0227
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  • 7
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    Multimodality of a particle size distribution of cohesive suspended particulate matters in a coastal zone (2012)
    Wiley
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    Publication Date: 2012-03-09
    Description: Particle size distributions (PSDs) of suspended particulate matters in a coastal zone are lognormal and multimodal in general. The multimodal PSD, which is caused by the mixing of multiple particle and aggregate size groups under flocculation and erosion/resuspension, is a record of the particle and aggregate dynamics in a coastal zone. Curve-fitting software was used to decompose the multimodal PSD into subordinate lognormal PSDs of primary particles, flocculi, microflocs, and macroflocs. The curve-fitting analysis for a time series of multimodal PSDs in the Belgian coastal zone showed the dependency of the multimodality on (1) shear-dependent flocculation in a flood and ebb tide, (2) breakage-resistant flocculation in the spring season, and (3) silt-sized particle erosion and advection in a storm surge. Also, for modeling and simulation purposes, the curve-fitting analysis and the settling flux estimation for the multimodal PSDs showed the possibility of using discrete groups of primary particles, flocculi, microflocs, and macroflocs as an approximation of a continuous multimodal PSD.
    Print ISSN: 0148-0227
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  • 8
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    A comparison of lower thermospheric winds derived from range spread and specular meteor trail echoes (2012)
    Wiley
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    Publication Date: 2012-03-09
    Description: Interferometry measurements of range spread meteor trail echoes (RSTEs; also known as nonspecular echoes) have provided new insights into both the irregularity structures in meteor trails and lower-thermospheric winds (LTWs). In this study, we used trail echoes observed with the newly installed Sanya (18.4°N, 109.6°E) 47.5 MHz VHF coherent radar and the Sanya all-sky meteor radar to estimate instantaneous zonal and hourly averaged meridional winds from RSTEs and hourly averaged zonal and meridional winds from large numbers of specular meteor echoes. The mean height variations in both the zonal and meridional winds estimated from the RSTEs were generally consistent with those estimated from specular meteor echoes below 96 km. This gives validity to the technique proposed recently by Oppenheim et al. (2009) and suggests that RSTE measurements made with a small radar can be used to investigate LTWs, whereas this had previously been limited to larger radars such as the Jicamarca radar. However, some observations show significant differences in wind magnitude at individual heights at times. The results of RSTE measurements show the presence of an intense westward wind with a speed near 100 ms−1. In contrast, the specular meteor zonal winds were generally less than 50 ms−1. On the other hand, the meridional drift of RSTEs derived from the meridional Doppler velocity at higher altitudes shows a very poor correlation with the specular meteor meridional wind. Potential causes for the discrepancy in wind estimates obtained from RSTE and specular meteor trail echoes are discussed.
    Print ISSN: 0148-0227
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  • 9
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    Observed and modeled solar cycle variation in geocoronal hydrogen using NRLMSISE-00 thermosphere conditions and the Bishop analytic exosphere model (2012)
    Wiley
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    Publication Date: 2012-03-09
    Description: High precision observations during Solar Cycle 23 using the Wisconsin H-alpha Mapper (WHAM) Fabry-Perot quantify a factor of 1.5 ± 0.15 higher Balmer α column emission intensity during near-solar-maximum than during solar minimum conditions. An unresolved question is how does the observed solar cycle variation in the hydrogen column emission compare with that calculated from the hydrogen distribution in atmospheric models? We have compared WHAM solar minimum and near-solar-maximum column intensity observations with calculations using the thermospheric hydrogen density profile and background thermospheric conditions from the Mass Spectrometer Incoherent Scatter (NRLMSISE-00) empirical model extended to exospheric altitudes using the analytic exosphere model of Bishop (1991). Using this distribution, we apply the lyao_rt global resonance radiative transfer code of Bishop (1999) to calculate expected intensities that would be observed from the ground for the viewing conditions of the observations. The observed intensities are brighter than those calculated for the corresponding conditions, indicating that when MSIS is used as the thermospheric hydrogen distribution the derived intensities are too low. Additionally, both the observed and calculated WHAM hydrogen column emission intensities are higher for near-solar-maximum than for solar minimum conditions. There is better agreement between observations and intensities calculated using the evaporative analytic exosphere model at solar maximum, suggesting an underestimation of modeled satellite atoms at high altitudes. This result is consistent with sensitivity studies using the option for a quasi-exobase for satellite atoms to account for the creation of satellite orbits from charge exchange collisions.
    Print ISSN: 0148-0227
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  • 10
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    Evaluation of OVATION Prime as a forecast model for visible aurorae (2012)
    Wiley
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    Publication Date: 2012-03-09
    Description: This study evaluates the ability of the OVATION Prime auroral precipitation model to provide operational forecasts of the visible aurora. An operational implementation would primarily provide the general public with some guidance for viewing the aurora. We evaluate the likelihood that if aurorae are predicted to be visible at a location, they will be seen there within the hour. Nighttime model forecasts were validated with Polar Ultraviolet Imager data for Kp ≥ 3 and for the years 1997 and 1998. The overall forecasts for a visible aurora to occur or to not occur were correct 77% of the time. The most important prediction for public auroral viewing is that the visible aurora will occur, and these forecasts were correct 86% of the time.
    Print ISSN: 1539-4964
    Electronic ISSN: 1542-7390
    Topics: Geosciences , Physics
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  • 11
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    Influence of atmospheric circulation patterns on the oxygen isotope ratio of tree rings in the Alpine region (2012)
    Wiley
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    Publication Date: 2012-03-09
    Description: The oxygen isotope ratio of precipitation and tree rings is a complex function of climate variables and atmospheric dynamics, which often makes the interpretation of δ18O for palaeoclimate research challenging. Here we analyzed monthly precipitation δ18O series for 1973–2004 and annually resolved tree ring δ18O chronologies for 1945–2004 for three sites in Switzerland: one north of the Alps, one at high-elevation within the Alps, and one south of the Alps. The goal of the study was to improve the understanding of the tree ring archive by a systematic analysis of nonlocal parameters related to atmospheric circulation, in particular, geopotential height field anomalies and the frequency of synoptic weather situations, in addition to the usual local climate parameters like temperature, sunshine duration, and relative humidity. We observed that on average high-pressure situations during summer were associated with relatively high δ18O and low-pressure situations were associated with relatively low δ18O, for both the isotope ratio in precipitation and tree rings. However, correlations to the frequency of weather types were not higher than simple correlations to local temperature. Accordingly, we constructed a combined index from temperature and air pressure that proved to be a good predictor of δ18O in precipitation and used this as the source water term in a tree ring isotope fractionation model. This enabled us to use the model beyond the period where isotope values for precipitation are available, opening new perspectives in the interpretation of long tree ring δ18O chronologies.
    Print ISSN: 0148-0227
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  • 12
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    The genesis of an inter-field marine sandwave and the associated anti-asymmetry migration of neighboring crests (2012)
    Wiley
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    Publication Date: 2012-02-22
    Description: A series of high resolution multibeam echo sounding surveys near San Francisco, CA suggests the occasional migration of marine sandwave crests in the direction opposite to that indicated by the shape asymmetry of the individual sandwaves, in contrast to all previous observations of the relationship between bedform shape asymmetry and migration. The anti-asymmetry migration occurs over approximately the same time period that a new sand wave crest is formed within the large and relatively stable field. The new sandwave crest appears approximately midway between two larger crests, in the vicinity of structural defects in a nearly two-dimensional portion of the field. The sandwaves in the vicinity of the new crest are found to have migrated away from the new crest regardless of their shape asymmetry. Later a large section of the new crest disappears, and the neighboring sandwaves migrated back toward the vacated crest location. This migration occurs for sandwaves up to at least ten wavelengths away from the new crest.
    Print ISSN: 0094-8276
    Electronic ISSN: 1944-8007
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  • 13
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    Anisotropic Rayleigh-wave tomography of Ireland's crust: Implications for crustal accretion and evolution within the Caledonian Orogen (2012)
    Wiley
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    Publication Date: 2012-02-22
    Description: The Irish landmass, now at the western extremity of the Eurasian Plate, was formed in the Caledonian Orogeny during the Palaeozoic assembly of Pangea. The associated closure of the Iapetus Ocean is recorded in the NE–SW structural trends that dominate the tectonic set-up of Ireland today. The deep-crustal dynamics of the orogeny and the effect on the crust of the subsequent extension and magmatism in the North Atlantic are debated. Fabrics within deep crustal rocks preserve a record of deformation during and after the continental collisions. Here, we measured Rayleigh-wave phase velocities using seismograms recorded by permanent and temporary intermediate-band stations in Ireland and inverted the data for phase-velocity maps, including azimuthal anisotropy. The observed isotropic phase-velocity heterogeneity reflects moderate crustal thickness and seismic velocity variations across Ireland. Anisotropy of Rayleigh waves at 10–20 s periods shows a NE–SW fast-propagation direction and is largest (up to 2%) at a 15 s period, at which Rayleigh waves sample primarily the middle and lower crust. The NE–SW trend of the deep-crustal anisotropic fabric is parallel to tectonic trends, in particular the Iapetus Suture Zone, which indicates that suture-parallel flow in the middle and lower crust accommodated the continental collision. The apparent preservation of the Caledonian-age fabric also shows that the deep crust of the Eurasian margin in Ireland was neither stretched by the NW–SE extension associated with the opening of the North Atlantic, nor modified significantly by the Cenozoic magmatism in the region.
    Print ISSN: 0094-8276
    Electronic ISSN: 1944-8007
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  • 14
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    Air quality over the Canadian oil sands: A first assessment using satellite observations (2012)
    Wiley
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    Publication Date: 2012-02-22
    Description: Results from the first assessment of air quality over the Canadian oil sands–one of the largest industrial undertakings in human history–using satellite remote sensing observations of two pollutants, nitrogen dioxide (NO2) and sulfur dioxide (SO2), are presented. High-resolution maps were created that revealed distinct enhancements in both species over an area (roughly 30 km × 50 km) of intensive surface mining at scales of a few kilometers. The magnitude of these enhancements, quantified in terms of total mass, are comparable to the largest seen in Canada from individual sources. The rate of increase in NO2 between 2005 and 2010 was assessed at 10.4 ± 3.5%/year and resulted from increases both in local values as well as the spatial extent of the enhancement. This is broadly consistent with both surface-measurement trends and increases in annual bitumen production. An increase in SO2 was also found, but given larger uncertainties, it is not statistically significant.
    Print ISSN: 0094-8276
    Electronic ISSN: 1944-8007
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  • 15
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    Thermotidal and land-heating forcing of the diurnal cycle of oceanic surface winds in the eastern tropical Pacific (2012)
    Wiley
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    Publication Date: 2012-02-22
    Description: The diurnal cycle in the oceanic surface winds in the tropical eastern Pacific is shown, through numerical experiments with a regional atmospheric model, to be associated with the migrating diurnal atmospheric thermal tide, forced by absorption of solar near-IR radiation by tropospheric water vapor, and a topographically-modified extended sea-breeze, forced by diurnal land heating. Idealized experiments prove capable of discriminating the effects of both processes, showing that beyond 2000 km from the coast, the thermal tide is dominant, while closer to the coast both processes are of the same order. The shortwave forcing due to water vapor is also found to produce a diurnal cycle in precipitation, but the process appears to be independent from the thermal tide and it is proposed that this effect is mediated by the radiatively-forced changes in the column stability.
    Print ISSN: 0094-8276
    Electronic ISSN: 1944-8007
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  • 16
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    Reconciling two approaches to attribution of the 2010 Russian heat wave (2012)
    Wiley
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    Publication Date: 2012-02-22
    Description: In the summer 2010 Western Russia was hit by an extraordinary heat wave, with the region experiencing by far the warmest July since records began. Whether and to what extent this event is attributable to anthropogenic climate change is controversial. Dole et al. (2011) report the 2010 Russian heat wave was “mainly natural in origin” whereas Rahmstorf and Coumou (2011) write that with a probability of 80% “the 2010 July heat record would not have occurred” without the large-scale climate warming since 1980, most of which has been attributed to the anthropogenic increase in greenhouse gas concentrations. The latter explicitly state that their results “contradict those of Dole et al. (2011).” Here we use the results from a large ensemble simulation experiment with an atmospheric general circulation model to show that there is no substantive contradiction between these two papers, in that the same event can be both mostly internally-generated in terms of magnitude and mostly externally-driven in terms of occurrence-probability. The difference in conclusion between these two papers illustrates the importance of specifying precisely what question is being asked in addressing the issue of attribution of individual weather events to external drivers of climate.
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    Electronic ISSN: 1944-8007
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  • 17
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    Gravity inversion of 2D bedrock topography for heterogeneous sedimentary basins based on line integral and maximum difference reduction methods (2012)
    Wiley
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    Publication Date: 2012-02-22
    Description: ABSTRACT Based on the line integral (LI) and maximum difference reduction (MDR) methods, an automated iterative forward modelling scheme (LI-MDR algorithm) is developed for the inversion of 2D bedrock topography from a gravity anomaly profile for heterogeneous sedimentary basins. The unknown basin topography can be smooth as for intracratonic basins or discontinuous as for rift and strike-slip basins. In case studies using synthetic data, the new algorithm can invert the sedimentary basins bedrock depth within a mean accuracy better than 5% when the gravity anomaly data have an accuracy of better than 0.5 mGal. The main characteristics of the inversion algorithm include: (1) the density contrast of sedimentary basins can be constant or vary horizontally and/or vertically in a very broad but a priori known manner; (2) three inputs are required: the measured gravity anomaly, accuracy level and the density contrast function, (3) the simplification that each gravity station has only one bedrock depth leads to an approach to perform rapid inversions using the forward modelling calculated by LI. The inversion process stops when the residual anomalies (the observed minus the calculated) falls within an ‘error envelope’ whose amplitude is the input accuracy level. The inversion algorithm offers in many cases the possibility of performing an agile 2D gravity inversion on basins with heterogeneous sediments. Both smooth and discontinuous bedrock topography with steep spatial gradients can be well recovered. Limitations include: (1) for each station position, there is only one corresponding point vertically down at the basement; and (2) the largest error in inverting bedrock topography occurs at the deepest points.
    Print ISSN: 0016-8025
    Electronic ISSN: 1365-2478
    Topics: Geosciences , Physics
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  • 18
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    Statistical indices of the northward rainfall progression over eastern Africa (2012)
    Wiley
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    Publication Date: 2012-02-22
    Description: Two statistical indices are developed to represent the northward progression of rainfall between late January and late July over eastern Africa. The first, a Seasonal Location Index, extracts the seasonal phase associated with a daily rainfall map using a principal component analysis. The second, a Rainfall Cluster Index, describes the seasonal cycle as a progression through a series of rainfall patterns that have been defined using a cluster analysis. The indices are used to identify large seasonal shifts in the latitude of rainfall, to examine when these shifts occur, and to investigate circulation features associated with perturbations ahead of and behind the mean seasonal cycle. These indices are potentially useful for exploring the predictability of rainfall transitions at both short-range and seasonal timescales. Copyright © 2012 Royal Meteorological Society
    Print ISSN: 0899-8418
    Electronic ISSN: 1097-0088
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  • 19
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    High resolution climate data for Austria in the period 2008–2040 from a statistical climate change model (2012)
    Wiley
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    Publication Date: 2012-02-22
    Description: Climate change data for Austria have been produced for the period from 2008 to 2040, with a temporal/spatial resolution of 1 d and 1 km 2 . The climate change data are based on historical daily weather station data from 1975 to 2007, and linear regression modelling with repeated bootstrapping. The spatial resolution is based on 60 climate clusters which represent homogenous climates with respect to mean annual precipitation sums and mean annual temperatures from the period 1961 to 1990. For each climate cluster, a regression model fit has been performed and extrapolated for the period 2008–2040. The integral parts of our regression model are: (1) the extrapolation of the observed linear temperature trend from 1975 to 2007, by using an average national trend of approximately 0.05 °C per year derived from a homogenized dataset, and (2) the repeated bootstrapping of historical temperature residuals, and of the observations for some other weather parameters, such as solar radiation, precipitation, relative humidity and wind speed. Thus, we ensure consistent physical, spatial and temporal correlations. Precipitation scenarios have been developed to account for any possible wider range of precipitation patterns. These scenarios include increased/decreased annual precipitation sums, as well as unchanged annual precipitation sums, but with different seasonal distributions. These climate change data are available at: http://www.landnutzung.at/Klima_Daten.html Copyright © 2012 Royal Meteorological Society
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    Synoptic and climatological controls of particulate matter pollution in a Southern Hemisphere coastal city (2012)
    Wiley
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    Publication Date: 2012-02-22
    Description: A complex interaction of local meteorology and source characteristics regularly leads to nocturnal smog events during winter in Christchurch, New Zealand. The main focus of this article is on improving understanding of the relationship between atmospheric processes operating at a range of scales that leads to poor air quality in such urban environments. This research therefore aims to provide a quantitative analysis of atmospheric influences on particulate matter pollution in Christchurch across a wide range of spatial and temporal scales, from local to hemispheric and daily to interannual. The probability of exceeding the National Environmental Standard for PM 10 for a range of local atmospheric conditions is calculated using the classification and regression trees technique, and links between these probabilities, local meteorology and synoptic weather situations are established. The effect of the transition between synoptic types on local air quality is also examined, and the progression of anticyclones across the country is identified to be the dominant synoptic control mechanism. It is shown that variation in latitudinal location of the path of anticyclones over New Zealand influences the predicted exceedance probability. On interdecadal and hemispheric scales, it is found that the particular combination of local and synoptic atmospheric conditions that favours air quality degradation shows a reoccurring pattern of frequency maxima (and minima) with a periodicity of approximately 14-16 years. In relation to the identified interdecadal variability of synoptic circulation, a close relationship to Southern Hemisphere pressure anomalies at high latitudes is revealed. The results of this research show that, in addition to daily weather variation, air quality in Christchurch is influenced by longer-term climatic processes that operate on interannual hemispheric scales with the implication that, in general, air pollution potential can also be expected to vary on a periodic interdecadal time scale. Copyright © 2012 Royal Meteorological Society
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    Tree-ring-based temperature reconstruction for the northern Greater Higgnan Mountains, China, since A.D. 1717 (2012)
    Wiley
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    Publication Date: 2012-02-22
    Description: In this paper, ring-width chronologies of pine trees ( Pinus sylvestris var. mongolica ) from one sampling site in the northern Greater Higgnan Mountains, China, were constructed. The results of growth-climate responses show that mean temperature is the limiting factor affecting radial growth of pine trees in the study area. Consequently, mean temperature from May to October from 1717 to 2008 has been reconstructed using the standard chronology. For the calibrated period (1957-2008), the explained variance of the reconstruction is 57%. The characteristics of the reconstruction expose that mean temperature has increased since the 1970s, and the decade 2000s and 1990s are also ranked as the warm decades on record. However, this period from 1970s to now is not exceptional within the past 300 years. By applying an 11-year moving average to the reconstruction, three warm periods and three cold periods are evident. The warm and cold periods of the reconstructed mean temperature correspond well with other reconstructions. Power spectral and wavelet analysis demonstrated the existence of significant ∼70- and ∼100-year cycles of variability. Furthermore, the reconstruction and North Atlantic Oscillation Index showed a significant positive correlation ( r = 0.34, n = 136, p 〈 0.0001). Copyright © 2012 Royal Meteorological Society
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    Evaluation of climate change effects on future corn (Zea mays L.) yield in western Turkey (2012)
    Wiley
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    Publication Date: 2012-02-22
    Description: A comparative performance analysis was studied on well-known drought indices [Standardized Precipitation Index (SPI), Palmer Drought Severity Index (PDSI) and its moisture anomaly index (Orig-Z), self-calibrated Palmer Drought Severity Index (SC-PDSI) and its moisture anomaly index (SC-Z)] to determine the most appropriate index for assessing corn ( Zea mays L.) yield in four crop regions (Aydın, Denizli, Afyon, Uşak) in western Turkey and to evaluate the vulnerability of corn production to climate change with future projections provided by the Hadley Centre for Climate Prediction and Research ENSEMBLES project (HadCM3Q0). A series of curvilinear regression-based crop-yield models were developed for each of the crop region based on the drought indices. The crop-yield model that performed best at high-drought risk years was the SC-PDSI in Aydın region and the PDSI in Denizli, Afyon and Uşak regions. The SC-PDSI index in Aydın region described 75.1% of the measured variability. The PDSI index in Denizli Ayfon and Uşak regions explained 69.8, 71.3 and 66.4% of the measured yield variability, respectively. The vulnerability of the corn yield to HadCM3Q0 projections was evaluated for Aydın and Afyon regions due to the resolution of the regional climate model. For the high-drought risk years, the expected decrease in corn yields was 2.1 ton ha −1 in Aydın region and 0.014 ton ha −1 for Afyon region. For the low drought risk years, the crop yield models predicted the expected decrease in corn yield as 0.104 ton ha −1 in Afyon region. However, there was a positive yield response by 0.022 ton ha −1 in Aydın region. Copyright © 2012 Royal Meteorological Society
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    Optical properties and chemical composition of aerosol particles at an urban location: An estimation of the aerosol mass scattering and absorption efficiencies (2012)
    Wiley
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    Publication Date: 2012-02-22
    Description: We investigated aerosol optical properties, mass concentration and chemical composition over a 1 year period (from March 2006 to February 2007) at an urban site in Southern Spain (Granada, 37.18°N, 3.58°W, 680 m above sea level). Light-scattering and absorption measurements were performed using an integrating nephelometer and a MultiAngle Absorption Photometer (MAAP), respectively, with no aerosol size cut-off and without any conditioning of the sampled air. PM10 and PM1 (ambient air levels of atmospheric particulate matter finer than 10 and 1 microns) were collected with two high volume samplers, and the chemical composition was investigated for all samples. Relative humidity (RH) within the nephelometer was below 50% and the weighting of the filters was also at RH of 50%. PM10 and PM1 mass concentrations showed a mean value of 44 ± 19 μg/m3 and 15 ± 7 μg/m3, respectively. The mineral matter was the major constituent of the PM10–1 fraction (contributing more than 58%) whereas organic matter and elemental carbon (OM+EC) contributed the most to the PM1 fraction (around 43%). The absorption coefficient at 550 nm showed a mean value of 24 ± 9 Mm−1 and the scattering coefficient at 550 nm presented a mean value of 61 ± 25 Mm−1, typical of urban areas. Both the scattering and the absorption coefficients exhibited the highest values during winter and the lowest during summer, due to the increase in the anthropogenic contribution and the lower development of the convective mixing layer during winter. A very low mean value of the single scattering albedo of 0.71 ± 0.07 at 550 nm was calculated, suggesting that urban aerosols in this site contain a large fraction of absorbing material. Mass scattering and absorption efficiencies of PM10 particles exhibited larger values during winter and lower during summer, showing a similar trend to PM1 and opposite to PM10–1. This seasonality is therefore influenced by the variations on PM composition. In addition, the mass scattering efficiency of the major aerosol constituents in PM10 were also calculated applying the multilinear regression (MLR) analysis. Among all of them, the most efficient in terms of scattering was sulfate ion (7 ± 1 m2g−1) while the least efficient was the mineral matter (0.2 ± 0.3 m2g−1). On the other hand, we found that the absorption process was mainly dominated by carbonaceous particles.
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    Particle transport over rough hillslope surfaces by dry ravel: Experiments and simulations with implications for nonlocal sediment flux (2012)
    Wiley
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    Publication Date: 2012-02-24
    Description: Past studies of hillslope evolution have typically assumed that soil creep processes are governed by a linear relationship between local hillslope angle and transport distance. The assumption of “linear diffusion” has fallen out of favor because, when coupled with an expression of mass continuity, it yields unrealistic hillslope profiles. As a consequence, a better understanding of the mechanics of sediment transport is needed. Here we report results from a series of flume experiments performed to investigate sediment transport by dry ravel, a common soil creep process in arid and semiarid environments. We find that, at gentle slopes, transport distances follow distributions characteristic of local transport. As gradients steepen, a fraction of the particles begins to exhibit nonlocal transport, and that fraction increases rapidly with slope. A stochastic discrete element model that couples an effective friction term with a shock term reproduces the results from the flume experiments, suggesting that it can be used to explore the nature of particle transport on rough surfaces. The model predicts that exponential distributions of transport distances on gentle slopes evolve into quasi-uniform distributions on steep slopes, and the transition occurs as slopes approach the angle of repose. Our results support previous findings that the angle of repose represents a threshold between friction and inertial regimes. In addition, we propose that the angle of repose represents a fuzzy boundary between local and nonlocal transport.
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    Simulating the dynamics and intensification of cyclonic Loop Current Frontal Eddies in the Gulf of Mexico (2012)
    Wiley
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    Publication Date: 2012-02-24
    Description: The dynamics associated with the Loop Current (LC) variability in the Gulf of Mexico (GoM) are studied using a 5-year, free-running numerical simulation with the Hybrid Coordinate Ocean Model (HYCOM). The dynamics of major GoM circulation features are represented: the extension of the LC and the associated anticyclonic, warm core Loop Current Eddies (LCEs) and cyclonic Loop Current Frontal Eddies (LCFEs). The study focuses on the dynamics of the LCFEs and their role during the LCEs shedding, which dramatically affects the GoM circulation. We analyze several characteristics of the LC frontal dynamics. Modeled LCFEs have a coherent vertical structure, which extends to the deep layers of the GoM. They may split in two separate upper and lower layer eddies. Deep and surface remnants from different frontal eddies are able to align to form new, coherent structures. LCFEs intensify along the extended LC northern edge when flowing over the deep northern GoM shelf slope that forms the Mississippi Fan, through a “promontory effect” in which the incoming cyclone aggregates positive potential vorticity anomalies in lower layers, leading to the intensification of the whole vortex structure. LCFEs may also expand further along the LC path by horizontal vortex merging, when they are blocked between the LC and the northeast corner of the continental shelf in the GoM. The intensification and merging due to topographic effects explain the enlarged frontal eddies observed on the eastern side of the Loop Current. These larger eddies further migrate along the LC front and may play a role in the shedding sequence.
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    Melt inclusion constraints on the magma source of Eyjafjallajökull 2010 flank eruption (2012)
    Wiley
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    Publication Date: 2012-02-24
    Description: The 2010 eruptive activity at the Eyjafjallajökull volcanic system began 20 March with a basaltic flank eruption on a 300 m long fissure on the Fimmvörðuháls Pass, in between Eyjafjallajökull and Mýrdalsjökull volcanoes. The magma expelled from the fissure is olivine- and plagioclase-bearing mildly alkali basalt that exhibits uniform and rather primitive whole-rock composition. This event provides a rare opportunity to assess deep magmatic processes in Iceland. Melt inclusions (MIs) hosted in olivine phenocrysts were analyzed for their major, trace and volatile element concentrations to enable identification of magmatic source(s) for Eyjafjallajökull volcano and to better constrain processes occurring at depth. The MIs, in particular those in Mg-rich olivines, record primary magma composition before homogenisation and differentiation during magma ascent. The olivine phenocrysts hosting the MIs have a large compositional range, extending from Fo73 to Fo87, reflecting changes in the magma characteristics from the source to the surface. The MI compositions exhibit significant variations with MgO ranging from 5.2 to 7.2 wt%. This compositional range was caused by a binary mixing of two basaltic end-members followed by fractional crystallization process. The sources of these end-members are identical to those of Katla and Surtsey basalts, with a dominant role of the Katla source. Trace element characteristics of the Fimmvörðuháls MIs suggest important proportions of recycled oceanic crust in their mantle sources.
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    Precursory deformation and fracture before brittle rock failure and potential application to volcanic unrest (2012)
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    Publication Date: 2012-02-24
    Description: Small-magnitude earthquakes and ground deformation are the precursors most frequently recorded before volcanic eruptions. Analogous signals (using acoustic emissions) have also been reported before the bulk brittle failure of crustal rock in the laboratory. Models based on laboratory and field data have focused on precursory behavior during deformation under a constant stress. A new model is proposed for extending analyses to deformation under an increasing stress. It describes how precursory time series can be determined from a parent relation between fracturing and stress, together with time-dependent changes in applied stress and rock resistance. The model applies to rock in which these stresses do not interact with each other and occupy volumes much smaller than the total volume being deformed. It identifies how the amounts of fracturing observed during deformation are controlled not only by stress concentrations at macroscopic heterogeneities, such as crack tips but also by rock composition, temperature, confining pressure, and the distribution of energy among atoms. The results appear to be scale independent, and so may be used to investigate whether the approach to bulk failure is limited by changes in applied stress or in rock weakening. When applied to pre-eruptive data from Hawaii, the analysis suggests that precursory signals are controlled by an increase in applied stress, rather than by creep deformation under a constant stress.
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    Impact of modifying the longwave water vapor continuum absorption model on community Earth system model simulations (2012)
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    Publication Date: 2012-02-24
    Description: The far-infrared (wavelengths longer than 17 μm) has been shown to be extremely important for radiative processes in the earth's atmosphere. The strength of the water vapor continuum absorption in this spectral region has largely been predicted using observations at other wavelengths that have been extrapolated using semiempirical approaches such as the Clough-Kneizys-Davies (CKD) family of models. Recent field experiments using new far-infrared instrumentation have supported a factor of 2 decrease in the modeled strength of the foreign continuum at 50 μm and a factor of 1.5 increase in the self-continuum at 24 μm in the Clough-Kneizys-Davies continuum model (CKD v2.4); these changes are incorporated in the Mlawer-Tobin-CKD continuum model (MT_CKD v2.4). The water vapor continuum in the Community Earth System Model (CESM v1.0) was modified to use the newer model, and the impacts of this change were investigated by comparing output from the original and modified CESM for 20 year integrations with prescribed sea surface temperatures. The change results in an increase in the net upward longwave flux of order 0.5 W m−2 between 300 and 400 mb, and a decrease in this flux of about the same magnitude for altitudes below 600 mb. The radiative impact results in a small but statistically significant change in the mean temperature and humidity fields, and also a slight decrease (order 0.5%) of high-cloud amount. The change in the cloud amount modified the longwave cloud radiative forcing, which partially offset the radiative heating caused by the change in the water vapor continuum absorption model.
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    Asymmetric seasonal temperature trends (2012)
    Wiley
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    Publication Date: 2012-02-25
    Description: Current consensus on global climate change predicts warming trends driven by anthropogenic forcing, with maximum temperature changes projected in the Northern Hemisphere (NH) high latitudes during winter. Yet, global temperature trends show little warming over the most recent decade or so. For longer time periods appropriate to the assessment of trends, however, global temperatures have experienced significant warming trends for all seasons except winter, when cooling trends exist instead across large stretches of eastern North America and northern Eurasia. Hence, the most recent lapse in global warming is a seasonal phenomenon, prevalent only in boreal winter. Additionally, we show that the largest regional contributor to global temperature trends over the past two decades is land surface temperatures in the NH extratropics. Therefore, proposed mechanisms explaining the fluctuations in global annual temperatures should address this apparent seasonal asymmetry.
    Print ISSN: 0094-8276
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    On the threshold of flow in a tight natural rock (2012)
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    Publication Date: 2012-02-25
    Description: The behaviour of hydraulically ‘tight’ barrier rocks is a key determinant of the long-term integrity of potential underground storage sites for the waste products from low-carbon emission energy production technologies (including nuclear waste and CO2 captured from fossil fuels). Here we isolate the relationship between crack-induced permeability and porosity using an initially crack-free natural crystalline material. We vary secondary porosity from an initial value of zero, and demonstrate that the bulk permeability K varies with total connected porosity Φ above the percolation threshold Φc as K = K0(Φ − Φc)n, where n = 3.8 ± 0.4, i.e., similar to results obtained for higher porosity rocks, indicating universality of this scaling law. Close to the percolation threshold a modest change in total porosity from 1% to 5% or so results in a massive change in permeability of 7 orders of magnitude or more. The results are consistent with a continuum percolation model that reflects the microstructure of the pore/induced microcrack network in the natural material.
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    Eyjafjallajökull ash concentrations derived from both lidar and modeling (2012)
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    Publication Date: 2012-02-25
    Description: Following the eruption of the Icelandic volcano Eyjafjallajökull on the 14 April 2010, ground-based N2-Raman lidar (GBL) measurements were used to trace the temporal evolution of the ash plume from 16 to 20 April 2010 above the southwestern suburb of Paris. The nighttime overpass of the Cloud-Aerosol LIdar with Orthogonal Polarization onboard Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation satellite (CALIPSO/CALIOP) on 17 April 2010 was an opportunity to complement GBL observations. The plume shape retrieved from GBL has been used to assess the size range of the particles size. The lidar-derived aerosol mass concentrations (PM) have been compared with model-derived PM concentrations held in the Eulerian model Polair3D transport model, driven by a source term inferred from the SEVIRI sensor onboard Meteosat satellite. The consistency between model and ground-based wind lidar and CALIOP observations has been checked. The spatial and temporal structures of the ash plume as estimated by each instrument and by the Polair3D simulations are in agreement. The ash plume was associated with a mean aerosol optical thickness of 0.1 ± 0.06 and 0.055 ± 0.053 for GBL (355 nm) and CALIOP (532 nm), respectively. Such values correspond to ash mass concentrations of ∼400 ± 160 and ∼720 ± 670 μg m−3, respectively, within the ash plume, which was lower than 0.5 km in width. The relative uncertainty is ∼75% and mainly due to the assessment of the specific cross-section assuming an aerosol density of 2.6 g cm−3. The simulated ash plume is smoother leading to integrated mass of the same order of magnitude (between 50 and 250 mg m−2).
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    Thin filament simulations for Earth's plasma sheet: Tests of validity of the quasi-static convection approximation (2012)
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    Publication Date: 2012-02-25
    Description: The main goal of this paper is to estimate the errors involved in applying a quasi-static convection model such as the Rice Convection Model (RCM) or its equilibrium version (RCM-E), which neglect inertial currents, to treat the injection of fresh particles into the inner magnetosphere in a substorm expansion phase. The approach is based on the idea that the dipolarization process involves earthward motion of a bubble that consists of flux tubes that have lower values of the entropy parameter than the surrounding medium. Our tests center on comparing MHD simulations with RCM- and RCM-E-like quasi-static approximations, for cases where the bubble is considered to be a thin ideal-MHD filament. Those quasi-static solutions miss the interchange oscillations that are often a feature of the MHD results. RCM and, to a lesser extent, RCM-E calculations tend to overestimate the westward electric field at the ionospheric footprint of the bubble and underestimate its duration. However, both get the time integral of the E × B drift velocity right as well as the net energization of the particles in the filament. The quasi-static approximation is most accurate if its computed value of the braking time of the bubble's earthward motion is long compared to the period of the relevant interchange oscillation. Comparison of MHD filament simulations of interchange instability with corresponding RCM calculations suggests a similar validity criterion. For plasma sheet conditions, the quasi-static approximation is typically best if the background medium has low β, worst if it consists of highly stretched field lines.
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    Strong magnetic field fluctuations within filamentary auroral density cavities interpreted as VLF saucer sources (2012)
    Wiley
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    Publication Date: 2012-02-25
    Description: The Geoelectrodynamics and Electro-Optical Detection of Electron and Suprathermal Ion Currents (GEODESIC) sounding rocket encountered more than 100 filamentary density cavities associated with enhanced plasma waves at ELF (
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    VS and density structure beneath the Colorado Plateau constrained by gravity anomalies and joint inversions of receiver function and phase velocity data (2012)
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    Publication Date: 2012-02-25
    Description: The Colorado Plateau is a physiographic province in the western US with an average elevation of ∼1.9 km where, in contrast to neighboring provinces, there is little evidence of large scale tectonic deformation or magmatism. Recent availability of Earthscope/USArray seismic data allow us to better examine the crust and upper mantle structure beneath the region and test proposed explanations for the plateau's uplift and relative stability. Using phase velocities for fundamental mode Rayleigh waves and P receiver functions, we perform over 800 joint inversions for 1-D shear wave velocity VS profiles sampling the plateau and surrounding regions down to 150 km depth. We image a sharp change in crustal thickness at the western edge of the Colorado Plateau with a more gradual increase eastward moving into the Rocky Mountains. A relatively thick (≳100 km) lithosphere beneath the plateau extends into the Rocky Mountains to the north. We use empirical scaling relations to estimate densities from our VS results, and predict the associated gravity anomalies, which are inconsistent with the observed distribution of the Bouguer gravity anomalies. We somewhat reconcile the prediction and observations by assuming that lateral density variations below 50 km can be ignored and the lithospheric root is therefore neutrally buoyant. While there is some evidence for small scale convection and lithospheric removal at its edges, the shape of the lithospheric mantle anomaly is consistent with a large scale uplift of the plateau by heating since removal of the Farallon slab. We conclude that the lithospheric root is key to the long term stability of the Colorado Plateau, leading to a colder, stronger crust.
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    Thin filament simulations for Earth's plasma sheet: Interchange oscillations (2012)
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    Publication Date: 2012-02-25
    Description: This paper presents a quantitative theory of “interchange oscillations,” which occur as an earthward-moving low-entropy plasma bubble slows and eventually comes to rest. Our theoretical picture is based on an idealized situation where an ideal-MHD magnetic filament moves without friction through a stationary background that represents the plasma sheet. If the relevant region of the background plasma sheet is interchange stable, then the filament usually executes a damped oscillation about an equilibrium position, where its entropy parameter matches the local background. The oscillations are typically dramatic only if the equatorial plasma beta is greater than about one. We derive an approximate analytic formula for the oscillation period, which is not simply related to slow- or intermediate-wave travel times. For an oscillation that Panov and collaborators carefully studied using THEMIS data, our simple theory, though based on an unrealistic 2D background magnetic field, predicted an oscillation period that agrees with the observations within about 40%. The simulations suggest that the ionospheric oscillation should lag behind the magnetospheric one by between 40 and 90 degrees. Ionospheric conductance affects the damping rate, which maximizes for an auroral zone conductance ∼2 S. Adding a friction force acting between the filament and the background increases the decay rate of the oscillation.
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    Large-scale circulation features in the tropical western Pacific and their representation in climate models (2012)
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    Publication Date: 2012-02-25
    Description: Within the Western Pacific Monsoon (WPM) region the zonal component of the low-level winds tends to weaken and reverse from east to west during the peak monsoon season, which also marks a peak in rainfall. This study examines how well climate models can simulate these phenomena prior to evaluating their projections for later this century. While a seasonal wind reversal or weakening appears to be reasonably well simulated by most models over much of the WPM, the relationships between large-scale average winds and rainfall are not always well simulated. This allows us to discriminate among the models in order to see if this affects the projections. However, it so happens that this has relatively little effect, and the predominant signal is for an increase in rainfall with a weakening or negligible change to the low-level monsoon winds. These results indicate that the WPM climate will respond more to global scale drivers such as an increase in atmospheric water vapor content and a weakening of the global circulation, rather than to more regional changes such as an increase in the land/ocean temperature contrast.
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    Climatology and characteristics of stratospheric sudden warmings in the Whole Atmosphere Community Climate Model (2012)
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    Publication Date: 2012-02-25
    Description: Major stratospheric sudden warmings (SSW) occurring during Northern Hemisphere winter were identified in four runs of the Whole Atmosphere Community Climate Model (WACCM). Their characteristics are compared to those found by other authors using reanalysis data. The comparison shows that the frequency of occurrence of major SSW in the model is very similar to that found in reanalysis data, as is the occurrence of vortex splitting and displacement events. The main difference with respect to observations is that the modeled SSW are relatively longer lasting. WACCM simulates quite accurately some dynamical features associated with major SSW, despite the presence of outlier cases; however, the recently reported relationship between regional blocking and the type of SSW is only partially reproduced by WACCM. In general, the observed climatological and dynamical signatures of displacement SSW tend to be better reproduced by the model than those associated with splitting SSW. We also find that SSW in the model are often associated with an elevated polar cap stratopause, in agreement with recent observations. However, the simulations also show that there is not in general a close correspondence between major SSW and elevated polar cap stratopause events.
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    A Paleoproterozoic giant radiating dyke swarm in the Dharwar Craton, southern India (2012)
    Wiley
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    Publication Date: 2012-02-28
    Description: Identical high precision U-Pb baddeleyite ages, together with paleomagnetic and geochemical data, on mafic dykes occurring over an area of 140,000 km2, define a Paleoproterozoic giant dyke swarm at ca. 2.367 Ga in the Dharwar craton, south India, referred to here as the Dharwar giant dyke swarm. All six U-Pb ages on these dykes are identical within error and suggest emplacement of this swarm within a geologically short time span of ∼5 Myr. A systematic southward progression in the trend of dykes from N48°E to N90°E, defines a fan angle of about 40° with convergence to a focal point about 300 km west of the present-day Dharwar craton boundary, resulting in a spectacular radiating dyke swarm extending across the entire eastern Dharwar craton. The large areal extent, radiating dyke pattern and short duration imply a mantle plume origin for the Dharwar giant dyke swarm. Despite their large areal distribution, all dykes in this swarm are geochemically coherent and have similar primitive mantle-normalized trace element patterns and rare earth element characteristics. Although the NE part of the swarm is magnetically overprinted, a remanence survives that has the same direction as primary magnetizations from dykes in the southern part of the swarm.
    Electronic ISSN: 1525-2027
    Topics: Chemistry and Pharmacology , Geosciences , Physics
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    Elasto-plastic and hydromechanical models of failure around an infinitely long magma chamber (2012)
    Wiley
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    Publication Date: 2012-03-13
    Description: Surface displacements solutions of elastic deformation around an inflating magma chamber generally assume that the associated internal overpressure is limited by the bedrock tensile strength. When considering stress equilibrium in the bedrock adjacent to a spherical or infinitely long cylinder, the gravity body force actually resists tensile failure, thus leading to a much larger pressure threshold. And when considering a Coulomb failure criterion, analytical and numerical models predict that shear failure develops instead of tensile failure. Here, three numerical codes are used to compare elasto-plastic solutions of surface displacements and patterns of failure in plane-strain. Shear failure propagates independently from the surface downward, then from the chamber walls upwards, and finally the two plasticized domains connect. Another test with internal underpressure (simulating source deflation) fits standard solutions from tunneling engineering. The effect of pore fluid pressures is also explored. In case of lithostatic fluid pore pressure in the bedrock, the gravity effect cancels out, and tensile failure is enabled for an overpressure close to the tensile strength. Coupled hydromechanical models in undrained conditions indicate that the initial bedrock porosity modifies the evolution of fluid pressure, volumetric strain and effective normal stress, and consequently also the pressure threshold for the onset of failure. We show that a bedrock of low porosity is more prone to fail than a bedrock of high porosity. In summary, our elasto-plastic and hydromechanical models illustrate the contexts for either tensile or shear failure around magmatic bodies, at the same time complementing and delimiting predictions deduced from elasticity.
    Electronic ISSN: 1525-2027
    Topics: Chemistry and Pharmacology , Geosciences , Physics
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    Severe 2011 ozone depletion assessed with 11 years of ozone, NO2, and OClO measurements at 80°N (2012)
    Wiley
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    Publication Date: 2012-03-13
    Description: Unusually cold conditions in Arctic winter 2010/11 led to large stratospheric ozone loss. We investigate this with UV-visible measurements made at Eureka, Canada (80.05°N, 86.42°W) from 1999–2011. For 8–22 March 2011, OClO was enhanced, indicating chlorine activation above Eureka. Ozone columns were lower than in any other year in the record, reaching minima of 237 DU and 247 DU in two datasets. The average NO2 column inside the vortex, measured at visible and UV wavelengths, was 46 ± 30% and 45 ± 27% lower in 2011 than the average NO2 column from previous years. Ozone column loss was estimated from two ozone datasets, using a modeled passive ozone tracer. For 12–20 March 2011, the average ozone loss was 27% and 29% (99 DU and 108 DU). The largest percent ozone loss in the 11-year record of 47% (250 DU and 251 DU) was observed on 5 April 2011.
    Print ISSN: 0094-8276
    Electronic ISSN: 1944-8007
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    Turbulent flow structures and aeolian sediment transport over a barchan sand dune (2012)
    Wiley
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    Publication Date: 2012-03-13
    Description: The turbulent structure of airflow over a barchan sand dune is determined using quadrant analysis of wind velocity data derived from sonic anemometers. Results indicate an increased frequency of ejection and sweep events in the toe region of the dune, characteristic of the turbulent bursting process. In contrast, at the crest there was a significant increase in the occurrence of outward interactions. Combined with high frequency saltation data our analyses show that turbulent structures characterised by a positive streamwise fluctuating velocity (+u′; sweeps at the toe and outward interactions at the crest) have a dominant influence on sand transport on the dune, together accounting for up to 83% and 95% of transporting events at the toe and crest respectively.
    Print ISSN: 0094-8276
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  • 42
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    The role of terrestrial vegetation in atmospheric Hg deposition: Pools and fluxes of spike and ambient Hg from the METAALICUS experiment (2012)
    Wiley
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    Publication Date: 2012-03-13
    Description: As part of the Mercury Experiment to Assess Atmospheric Loading in Canada and the U.S. (METAALICUS), different stable Hg(II) isotope spikes were applied to the upland and wetland areas of a boreal catchment between 2001 and 2006 to examine retention of newly deposited Hg(II). In the present study, a Geographical Information Systems (GIS)-based approach was used to quantify canopy and ground vegetation pools of experimentally applied upland and wetland spike Hg within the METAALICUS watershed over the terrestrial loading phase of the experiment. A chemical kinetic model was also used to describe the changes in spike Hg concentrations of canopy and ground vegetation over time. An examination of the fate of spike Hg initially present on canopy vegetation using a mass balance approach indicated that the largest percentage flux from the canopy over one year post-spray was emission to the atmosphere (upland: 45%; wetland: 71%), followed by litterfall (upland: 14%; wetland: 10%) and throughfall fluxes (upland: 12%; wetland: 9%) and longer term retention of spike in the forest canopy (11% for both upland and wetland). Average half-lives (t1/2) of spike on deciduous (110 ± 30 days) and coniferous (180 ± 40 days) canopy and ground vegetation (890 ± 620 days) indicated that retention of new atmospheric Hg(II) on terrestrial (especially ground) vegetation delays downward transport of new atmospheric Hg(II) into the soil profile and runoff into lakes.
    Print ISSN: 0886-6236
    Electronic ISSN: 1944-9224
    Topics: Biology , Chemistry and Pharmacology , Geography , Geosciences , Physics
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    A cloudier Arctic expected with diminishing sea ice (2012)
    Wiley
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    Publication Date: 2012-03-13
    Description: Arctic sea ice cover has decreased dramatically over the last three decades. Global climate models under-predicted this decline, most likely a result of the misrepresentation of one or more processes that influence sea ice. The cloud feedback is the primary source of uncertainty in model simulations, especially in the polar regions. A better understanding of the interaction between sea ice and clouds, and specifically the impact of decreased sea ice on cloud cover, will provide valuable insight into the Arctic climate system and may ultimately help in improving climate model parameterizations. In this study, an equilibrium feedback assessment is employed to quantify the relationship between changes in sea ice and clouds, using satellite-derived sea ice concentration and cloud cover over the period 2000–2010. Results show that a 1% decrease in sea ice concentration leads to a 0.36–0.47% increase in cloud cover, suggesting that a further decline in sea ice cover will result in an even cloudier Arctic.
    Print ISSN: 0094-8276
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    Explosion swarms at Stromboli volcano: A proxy for nonequilibrium conditions in the shallow plumbing system (2012)
    Wiley
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    Publication Date: 2012-03-14
    Description: In this paper we look at changes of the statistics of the stationary explosive process at a basaltic volcano as a proxy for departures from thermodynamic equilibrium conditions in the shallow plumbing system. Specifically, we investigate the explosion process of Stromboli volcano that occurred during 2002–2003, 2006–2007 and 2010–2011. The first two periods were characterized by eruptions with significant lava effusion and strong paroxysmal events, while the last one shows persistent explosive activity accompanied by minor episodes of lava flow. We use three-component seismic data recorded by broadband stations operating on the volcano and, for 2007 and 2010–2011 cases, strainmeter data from a Sacks-Evertson borehole dilatometer. For each time interval we study the explosive process by looking at the inter-occurrence times and at the amplitude distribution. Moreover, we analyze its waveforms, spectral content and polarization properties. In all three cases we find sharp increases of the explosion rate associated with swarms. Swarms are characterized by quasi-monochromatic seismic events with frequency peak close to about 3 Hz, higher amplitude than the usual explosions and variability coefficient of the inter-occurrence times close to 0.5. In correspondence to the swarms, we also observe negative variations in the strain signals, which indicate a depressurization in the shallow plumbing system. This depressurization emerges clearly from the data collected during 2010–2011, whereas it is less sharp for the 2007 episode, and has been estimated in about 105 Pa. From the polarization analysis we infer that this depressurization affects the upper 0.3–0.8 km of the plumbing system.
    Electronic ISSN: 1525-2027
    Topics: Chemistry and Pharmacology , Geosciences , Physics
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    Effects of native forest restoration on soil hydraulic properties, Auwahi, Maui, Hawaiian Islands (2012)
    Wiley
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    Publication Date: 2012-03-14
    Description: Over historic time Hawai‘i's dryland forests have been largely replaced by grasslands for grazing livestock. On-going efforts have been undertaken to restore dryland forests to bring back native species and reduce erosion. The reestablishment of native ecosystems on land severely degraded by long-term alternative use requires reversal of the impacts of erosion, organic-matter loss, and soil structural damage on soil hydraulic properties. This issue is perhaps especially critical in dryland forests where the soil must facilitate native plants' optimal use of limited water. These reforestation efforts depend on restoring soil ecological function, including soil hydraulic properties. We hypothesized that reforestation can measurably change soil hydraulic properties over restoration timescales. At a site on the island of Maui (Hawai‘i, USA), we measured infiltration capacity, hydrophobicity, and abundance of preferential flow channels in a deforested grassland and in an adjacent area where active reforestation has been going on for fourteen years. Compared to the nearby deforested rangeland, mean field-saturated hydraulic conductivity in the newly restored forest measured by 55 infiltrometer tests was greater by a factor of 2.0. Hydrophobicity on an 8-point scale increased from average category 6.0 to 6.9. A 4-point empirical categorization of preferentiality in subsurface wetting patterns increased from an average 1.3 in grasslands to 2.6 in the restored forest. All of these changes act to distribute infiltrated water faster and deeper, as appropriate for native plant needs. This study indicates that vegetation restoration can lead to ecohydrologically important changes in soil hydraulic properties over decadal time scales.
    Print ISSN: 0094-8276
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    Radiocarbon and stable-isotope geochemistry of organic and inorganic carbon in Lake Superior (2012)
    Wiley
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    Publication Date: 2012-03-14
    Description: We present a lake-wide investigation of Lake Superior carbon and organic matter biogeochemistry using radiocarbon, stable isotope, and carbon concentrations. Dissolved inorganic carbon (DIC) abundance in the lake was 121–122 Tg C, with offshore concentration and δ13C values being laterally homogenous and tightly coupled to the physical and thermal regime and biochemical processes. Offshore Δ14C of DIC (50–65‰) exhibited lateral homogeneity and was more 14C enriched than co-occurring atmospheric CO2 (∼38‰); nearshore Δ14C of DIC (36–38‰) was similar to atmospheric CO2. Dissolved organic carbon (DOC) abundance was 14.2–16.4 Tg C. DOC's concentration and δ13C were homogenous in June (mixed lake), but varied laterally during August (stratification) possibly due to spatial differences in lake productivity. Throughout sampling, DOC had modern radiocarbon values (14–58‰) indicating a semilabile nature with a turnover time of ≤60 years. Lake particulate organic carbon (POC, 0.9–1.3 Tg C) was consistently 13C depleted relative to DOC. The δ15N of epilimnetic particulate organic nitrogen shifted to more negative values during stratification possibly indicating greater use of nitrate (rather than ammonium) by phytoplankton in August. POC's radiocarbon was spatially heterogeneous (Δ14C range: 58‰ to −303‰), and generally 14C depleted relative to DOC and DIC. POC 14C depletion could not be accounted for by black carbon in the lake but, because of its spatial and temporal distribution, is attributed to sediment resuspension. The presence of old POC within the epilimnion of the open lake indicates possible benthic-pelagic coupling in the lake's organic carbon cycle; the ultimate fate of this old POC bears further investigation.
    Print ISSN: 0886-6236
    Electronic ISSN: 1944-9224
    Topics: Biology , Chemistry and Pharmacology , Geography , Geosciences , Physics
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    An improved model of the lightning electromagnetic field interaction with the D-region ionosphere (2012)
    Wiley
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    Publication Date: 2012-03-14
    Description: We present an improved time-domain model of the lightning electromagnetic pulse (EMP) interaction with the lower ionosphere. This improved model inherently accounts for the Earth's curvature, includes an arbitrary number of ion species, and uses a convolutional Perfectly Matched Layer (PML) boundary. We apply an improved model of electron heating due to the lightning EMP and electrostatic fields, and we include ionization, attachment, and detachment. In addition to modeling lightning, this model can be used for long-distance VLF wave propagation in the Earth-ionosphere waveguide, heating of the lower ionosphere by VLF transmitters, and heating in the F-region ionosphere by lightning. In this paper we present three initial results of this model. First, we compare results of ionospheric heating and electron density disturbances with and without electron detachment taken into account. We find that detachment is important only for the QE effects on time scales longer than 1 ms. Second, we find a simple explanation for the recently-reported “elve doublets”, which we find are an effect of the rise and fall times of the lightning waveform. In particular, we find that all elves are doublets, and the rise and fall times of the current pulse control the brightness and separation in time of the two successive halves of the elve. Third, we find a similar simple explanation for “ring” sprites, whole columns appear in a circle symmetric around the discharge axis. We find that ring sprites can be initiated for particular current waveforms, where the QE and EMP fields in the mesosphere produce a maximum reduced field away from the discharge axis.
    Print ISSN: 0148-0227
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    Seasonal composition of remote and urban fine particulate matter in the United States (2012)
    Wiley
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    Publication Date: 2012-03-13
    Description: Speciated aerosol composition data from the rural Interagency Monitoring for Protected Visual Environments (IMPROVE) network and the Environmental Protection Agency's urban/suburban Chemical Speciation Network (CSN) were combined to evaluate and contrast the PM2.5 composition and its seasonal patterns at urban and rural locations throughout the United States. We examined the 2005–2008 monthly and annual mean mass concentrations of PM2.5 ammonium sulfate (AS), ammonium nitrate (AN), particulate organic matter (POM), light-absorbing carbon (LAC), mineral soil, and sea salt from 168 rural and 176 urban sites. Urban and rural AS concentrations and seasonality were similar, and both were substantially higher in the eastern United States. Urban POM and LAC concentrations were higher than rural concentrations and were associated with very different seasonality depending on location. The highest urban and rural POM and LAC concentrations occurred in the southeastern and northwestern United States. Wintertime peaks in AN were common for both urban and rural sites, but urban concentrations were several times higher, and both were highest in California and the Midwest. Fine soil concentrations were highest in the Southwest, and similar regional patterns and seasonality in urban and rural concentrations suggested impacts from long-range transport. Contributions from sea salt to the PM2.5 budget were non-negligible only at coastal sites. This analysis revealed spatial and seasonal variability in urban and rural aerosol concentrations on a continental scale and provided insights into their sources, processes, and lifetimes.
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    Observations of the vertical structure of turbulent oscillatory boundary layers above fixed roughness using a prototype wideband coherent Doppler profiler: 2. Turbulence and stress (2012)
    Wiley
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    Publication Date: 2012-03-13
    Description: Measurements of turbulence and shear stress in oscillatory boundary layers are reported from experiments carried out with a prototype wideband coherent Doppler profiler above fixed roughness beds of 0.37 mm diameter sand and 3.9 mm diameter gravel. The 10 s oscillation period and 0.75 m to 1.5 m oscillation excursions correspond to roughness Reynolds numbers for the gravel bed in the 290 to 490 range, assuring fully rough turbulent conditions. Bottom stress was estimated via the law-of-the-wall, the vertical integral of the defect acceleration, and the Reynolds stress. The Reynolds stress was obtained from the second moment of the beam-coordinate velocities. Bed friction factors, fw, from the defect stresses are in reasonable agreement with predictions based on Swart's empirical relation as modified by Nielsen (1992) and with values determined using Laser Doppler Anemometry (LDA) by Sleath (1987) via the defect method and by Jensen (1988) via the law-of-the-wall. The fw values determined here from the law-of-the-wall are higher than predicted (ca. 50% higher for the gravel bed), likely due to background vertical shear associated with residual motions in the tank. The Reynolds stresses are lower than the predictions by a factor of 3 to 4, compared to the factor of 5 to 10 obtained by Sleath (1987). Beam coordinate turbulent kinetic energy spectra indicate that the vertical momentum flux is mostly associated with fluctuations between the forcing frequency and the inertial subrange, the latter contributing typically less than 10% of the total observed Reynolds stress.
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    Anomalous intraseasonal events in the thermocline ridge region of Southern Tropical Indian Ocean and their regional impacts (2012)
    Wiley
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    Publication Date: 2012-03-13
    Description: The present study explores the mechanisms responsible for the strong intraseasonal cooling events in the Thermocline Ridge region of the southwestern Indian Ocean. Air sea interface and oceanic processes associated with Madden Julian Oscillation are studied using an Ocean General Circulation Model and satellite observations. Sensitivity experiments are designed to understand the ocean response to intraseasonal forcing with a special emphasis on 2002 cooling events, which recorded the strongest intraseasonal perturbations during the last well-observed decade. This event is characterized by anomalous Walker circulation over the tropical Indian Ocean and persistent intraseasonal heat flux anomaly for a longer duration than is typical for similar events (but without any favorable preconditioning of ocean basic state at the interannual timescale). The model heat budget analysis during 1996 to 2007 revealed an in-phase relationship between atmospheric fluxes associated with Madden Julian Oscillation and the subsurface oceanic processes during the intense cooling events of 2002. The strong convection, reduced shortwave radiation and increased evaporation have contributed to the upper ocean heat loss in addition to the slower propagation of active phase of convection, which supported the integration of longer duration of forcing. The sensitivity experiments revealed that dynamic response of ocean through entrainment at the intraseasonal timescale primarily controls the biological response during the event, with oceanic interannual variability playing a secondary role. This study further speculates the role of oceanic intraseasonal variability in the 2002 droughts over Indian subcontinent.
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    Evaluation of an ice cloud parameterization based on a dynamical-microphysical lifetime concept using CloudSat observations and the ERA-Interim reanalysis (2012)
    Wiley
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    Publication Date: 2012-03-13
    Description: This study validates the cloud ice water content (IWC, non-precipitating ice/non-snow) produced by a unique prognostic cloud ice parameterization when used in the UCLA atmospheric general circulation model against CloudSat observations, and also compares it with the ERA-Interim reanalysis. A distinctive aspect of this parameterization is the novel treatment of the conversion of cloud ice to precipitating snow. The ice-to-snow autoconversion time scale is a function of differential infrared radiative heating and environmental static stability. The simulated IWC is in agreement with CloudSat observations in terms of its magnitude and three-dimensional structure. The annual and seasonal means of the zonal-mean IWC profiles from the simulations both show a local maximum in the upper troposphere in the tropics associated with deep convection, and other local maxima in the mid-troposphere in midlatitudes in both hemispheres associated with storm tracks. In contrast to the CloudSat values, the reanalysis shows much smaller IWC values in the tropics and much larger values in the lower troposphere in midlatitudes. The different vertical structures and magnitudes of IWC between the simulations and the reanalysis are likely due to differences in the parameterization of various processes in addition to the ice-to-snow autoconversion, including ice sedimentation, temperature thresholds for ice deposition and cumulus detrainment of cloud ice. However, a series of sensitivity experiments supports the conclusion that the model with a constant autoconversion time scale cannot reproduce the correct IWC distribution in both the tropics and midlatitudes, which strongly suggests the importance of physically based effects on the autoconversion timescale.
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    Impact of Caribbean cyclones on the detachment of Loop Current anticyclones (2012)
    Wiley
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    Publication Date: 2012-03-13
    Description: The western boundary current in the North Atlantic is characterized by an intense flow (reaching 2.5 ms−1, at the surface) that enters the Caribbean Sea through the Lesser Antilles passages, crosses the entire Caribbean and enters the Gulf of Mexico through the Yucatan Channel, where it is known as the Loop Current. A characteristic feature of the Loop Current is eddy-shedding events (i.e., the detachment of large anticyclonic eddies) at irregular intervals. Moored current measurements between January 2005 and July 2009 in the Loop Current, the Yucatan Channel, and the Caribbean coastal waters of Mexico (i.e., the Western Cayman Sea), along with AVISO altimetry, are used to evidence the northward propagation of cyclonic anomalies along the Caribbean coast of Mexico and the marked eastward displacement of the Loop Current at 23°N latitude, just before several anticyclonic eddy shedding events. After entering the Gulf of Mexico, these cyclonic anomalies might initiate or enhance existing Campeche Bank cyclonic eddies, which are related to many of the Loop Current detachment events. Sixteen of the twenty-one detachments that occurred during the study period (76%) are related to the cyclonic eddies in the Western Caribbean Sea; six of them were not reattached again to the Loop Current. Observations, thus, clearly indicate that cyclonic eddies in the Western Cayman Sea contribute significantly to the Loop Current eddy-shedding process, which is complex and in principle not unique.
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    Tide transformation in the Guadalquivir estuary (SW Spain) and process-based zonation (2012)
    Wiley
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    Publication Date: 2012-03-13
    Description: This study analyzes tide transformation in the Guadalquivir estuary (SW Spain). When fresh water discharges are less than 40 m3/s, the estuary is tidally-dominated (flood-dominated) and well mixed. Under such conditions, the estuary can be divided into three stretches, each characterized by a different tide propagation process. In the first stretch of 25 km, the dominant process is diffusion. In the next stretch, approximately over 35 km length, convergence and friction processes are in balance. At the head of the estuary, in the last stretch, the tidal motion is partially standing because of tidal reflection on the Alcalá del Río dam, located 110 km upstream from the estuary mouth. The reflection coefficient R varies with the frequency; for diurnal constituents its magnitude ∣RD∣ is 0.25; this value increases in the case of semi-diurnal (∣RS∣ ≈ 0.40), and quarter-diurnal constituents (∣RQ∣ ≈ 0.65), and reaches its minimum at the sixth-diurnal components (∣RX∣ ≈ 0.10). The tidal reflection can generate residual currents that have consequences in the bed morphology. Furthermore, when the fresh water discharges are greater than 400 m3/s, the estuary is fluvially-dominated and the water level can be calculated as the linear superposition of tide and river contributions. However, superposition arguments do not hold for currents at any point in the estuary.
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    Lunar radiation environment and space weathering from the Cosmic Ray Telescope for the Effects of Radiation (CRaTER) (2012)
    Wiley
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    Publication Date: 2012-03-13
    Description: The Cosmic Ray Telescope for the Effects of Radiation (CRaTER) measures linear energy transfer by Galactic Cosmic Rays (GCRs) and Solar Energetic Particles (SEPs) on the Lunar Reconnaissance Orbiter (LRO) Mission in a circular, polar lunar orbit. GCR fluxes remain at the highest levels ever observed during the space age. One of the largest SEP events observed by CRaTER during the LRO mission occurred on June 7, 2011. We compare model predictions by the Earth-Moon-Mars Radiation Environment Module (EMMREM) for both dose rates from GCRs and SEPs during this event with results from CRaTER. We find agreement between these models and the CRaTER dose rates, which together demonstrate the accuracy of EMMREM, and its suitability for a real-time space weather system. We utilize CRaTER to test forecasts made by the Relativistic Electron Alert System for Exploration (REleASE), which successfully predicts the June 7th event. At the maximum CRaTER-observed GCR dose rate (∼11.7 cGy/yr where Gy is a unit indicating energy deposition per unit mass, 1 Gy = 1 J/kg), GCRs deposit ∼88 eV/molecule in water over 4 billion years, causing significant change in molecular composition and physical structure (e.g., density, color, crystallinity) of water ice, loss of molecular hydrogen, and production of more complex molecules linking carbon and other elements in the irradiated ice. This shows that space weathering by GCRs may be extremely important for chemical evolution of ice on the Moon. Thus, we show comprehensive observations from the CRaTER instrument on the Lunar Reconnaissance Orbiter that characterizes the radiation environment and space weathering on the Moon.
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    Improved detection of sulphur dioxide in volcanic plumes using satellite-based hyperspectral infrared measurements: Application to the Eyjafjallajökull 2010 eruption (2012)
    Wiley
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    Publication Date: 2012-03-14
    Description: The explosive phase of the eruption of the Eyjafjallajökull volcano in Iceland beginning on 14 April 2010 caused extensive disruption to aviation in Europe with serious social and economic consequences. Despite its impact, the explosive phase was modest in size and the amount of sulphur dioxide (SO2) released was low. The potential of hyperspectral thermal infrared measurements to discriminate emissions from similar events by measuring SO2 is examined using the Infrared Atmospheric Sounding Interferometer (IASI) on board MetOp-A. The transported plume in the initial stages of the explosive phase contained low amounts of SO2 at low altitude which placed it at the detection limit of space-based sensors used to monitor the volcanic threat to aviation using current methods. A recently developed technique for the fast retrieval of SO2 from IASI is applied in the context of the Eyjafjallajökull eruption to show that IASI is easily capable of sensing the SO2 in the plume at this stage where existing methods fail. The fast SO2 retrieval is calibrated against a fully quantitative optimal estimation retrieval of SO2 total column amount and plume altitude to derive the detection limit for the plume on 15 April 2010. An estimate of the general detection limit for the instrument is placed conservatively at 0.3 Dobson Units (DU) which is an order of magnitude lower than previously thought.
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    The distinct behaviors of Pacific and Indian Ocean warm pool properties on seasonal and interannual time scales (2012)
    Wiley
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    Publication Date: 2012-03-14
    Description: The seasonal and interannual variabilities of warm pool properties in the Pacific and Indian Ocean sectors are examined and contrasted. The properties examined are the size, mean and maximum sea surface temperatures (SSTs), and central position. The seasonal variability is more vigorous in the Indian Ocean sector, but the interannual variability is comparable in the Pacific and Indian Ocean sectors. The variability is associated with significant longitudinal and latitudinal displacements on seasonal time scales but only with longitudinal displacements on interannual time scales. As for the controlling factors, while the seasonal variability of the warm pool is controlled by the annual march of the Sun in the Pacific sector and by the Indian summer monsoon in the Indian Ocean sector, the interannual variability in both sectors is related mostly to El Niño–Southern Oscillation (ENSO). ENSO is closely correlated with the size variations and longitudinal displacements of the warm pool. Interestingly, the warm pool intensity in both sectors is not highly correlated with ENSO until 5 to 6 months after ENSO peaks. The possible causes of this delayed ENSO influence are discussed. Only size and intensity (i.e., mean SST) variations in the Indian Ocean warm pool are significantly correlated with quasi-biennial variability in the Indian monsoon, which indicates that the Indian Ocean warm pool may be a potential predictor for Indian monsoon variations.
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    Analysis of a ray-tracing model for gravity waves generated by tropospheric convection (2012)
    Wiley
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    Publication Date: 2012-03-14
    Description: The Vadas-Fritts ray-tracing model for convectively generated gravity waves is analyzed using the stationary phase approximation and is interpreted in terms of a ray Jacobian approximated by the density of rays. The Vadas-Fritts model launches rays from the convective source region, with initial conditions for the ray-tracing deduced from a near-field integral representation. In the far-field the rays are binned in space-time grid cells. The contribution of each ray to the spatial wave amplitude is determined by its spectral amplitude and by the local density of rays within the grid cells. The present analysis accomplishes two things. First, the stationary phase analysis gives the formal initial conditions for the ray-tracing, which mostly agree with the Vadas-Fritts initialization but also suggest some refinements. Secondly, the Jacobian and ray-density analysis shows how the Vadas-Fritts model can be generalized to follow a beam of rays with a single moving grid cell.
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    The role of winter precipitation and temperature on northern Eurasian streamflow trends (2012)
    Wiley
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    Publication Date: 2012-03-14
    Description: Eurasian river discharge into the Arctic Ocean has steadily increased during the 20th century, and many studies have documented the spatial distribution of the trends and hypothesized the causes. There is a large variation in the scope of these studies, including the spatial scale of interest, and they often lack consistency in the time period analyzed. Studies have shown a connection between changes in the seasonal snowpack and discharge, but they have been constrained by the limitations of the snow observational network, which contains few long-term stations. This study overcomes these problems by using both in situ observations and a land surface model to evaluate the role snowpack changes have had on increases in runoff across northern Eurasia from 1936 through 1999. Our analysis shows consistent trends in both observations and model predictions. Increases in cold season precipitation propagate into increases in maximum snow water equivalent, which lead to increases in runoff. A series of model experiments demonstrate that the nonlinear interaction between winter precipitation and temperature has driven changes in the snowpack, which are manifested in the modeled runoff trends. Given that winter precipitation is expected to continue to increase and temperatures to warm during the 21st century in this region, these results point to the importance in understanding how the projected changes will influence the seasonal snowpack, which may have important consequences for streamflow in this region and freshwater export to the Arctic Ocean.
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    A method for unmixing magnetic hysteresis loops (2012)
    Wiley
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    Publication Date: 2012-03-15
    Description: Hysteresis loops provide essential information concerning both induced and remanent magnetizations and are an important tool for characterizing magnetic mineral assemblages. Although the hysteresis behavior of mixed natural magnetic assemblages has been a focal point of much recent work, little progress has been made in unmixing of hysteresis loops into characteristic components. Unmixing strategies can act as cornerstones for interpretation of rock magnetic data and have become popular for characterizing isothermal remanent magnetization acquisition curves. Unmixing of hysteresis loops is, however, a challenging task because the individual component loops in the mixture must meet stringent shape constraints. We present a new technique for decomposing an ensemble of hysteresis loops into a small number of end-members based on linear mixing theory. The end-members are not based on type curves but instead are derived directly from the hysteresis data. Particular attention is paid to the form of the end-members, ensuring they meet the shape constraints expected for hysteresis loops of natural magnetic mineral assemblages. Marine sediments from the Southern Ocean and lake sediments from Butte Valley, northern California, provide case studies on which the proposed unmixing method is tested.
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    Field and satellite observations of the formation and distribution of Arctic atmospheric bromine above a rejuvenated sea ice cover (2012)
    Wiley
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    Publication Date: 2012-03-15
    Description: Recent drastic reduction of the older perennial sea ice in the Arctic Ocean has resulted in a vast expansion of younger and saltier seasonal sea ice. This increase in the salinity of the overall ice cover could impact tropospheric chemical processes. Springtime perennial ice extent in 2008 and 2009 broke the half-century record minimum in 2007 by about one million km2. In both years seasonal ice was dominant across the Beaufort Sea extending to the Amundsen Gulf, where significant field and satellite observations of sea ice, temperature, and atmospheric chemicals have been made. Measurements at the site of the Canadian Coast Guard Ship Amundsen ice breaker in the Amundsen Gulf showed events of increased bromine monoxide (BrO), coupled with decreases of ozone (O3) and gaseous elemental mercury (GEM), during cold periods in March 2008. The timing of the main event of BrO, O3, and GEM changes was found to be consistent with BrO observed by satellites over an extensive area around the site. Furthermore, satellite sensors detected a doubling of atmospheric BrO in a vortex associated with a spiral rising air pattern. In spring 2009, excessive and widespread bromine explosions occurred in the same region while the regional air temperature was low and the extent of perennial ice was significantly reduced compared to the case in 2008. Using satellite observations together with a Rising-Air-Parcel model, we discover a topographic control on BrO distribution such that the Alaskan North Slope and the Canadian Shield region were exposed to elevated BrO, whereas the surrounding mountains isolated the Alaskan interior from bromine intrusion.
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    Unsteady late Pleistocene incision of streams bounding the Colorado Front Range from measurements of meteoric and in situ 10Be (2012)
    Wiley
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    Publication Date: 2012-03-15
    Description: Dating of gravel-capped strath terraces in basins adjacent to western U.S. Laramide Ranges is one approach to document the history of late Cenozoic fluvial exhumation. We use in situ 10Be measurements to date the broad surfaces adjacent to the eastern edge of the Rocky Mountains in Colorado, and compare these calculated ages with results from meteoric 10Be measurements. We analyze three sites near Boulder, Colorado (Gunbarrel Hill, Table Mountain, and Pioneer) that have been mapped as the oldest terrace surfaces with suggested ages ranging from 640 ka to the Plio-Pleistocene transition. Our in situ 10Be results reveal abandonment ages of 95 ± 129 ka at Table Mountain, 175 ± 27 ka at Pioneer, and ages of 251 ± 10 ka and 307 ± 15 ka at Gunbarrel Hill. All are far younger than previously thought. Inventories of meteoric 10Be support this interpretation, yielding ages that are comparable to Table Mountain and ∼20% lower than Pioneer in situ ages. We argue that lateral beveling by rivers dominated during protracted times of even moderate glacial climate, and that vertical incision rates of several mm/yr likely occurred during times of very low sediment supply during the few interglacials that were characterized by particularly warm climate conditions. In contrast to the traditional age chronology in the area, our ages suggest that the deep exhumation of the western edge the High Plains occurred relatively recently and at an unsteady pace.
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    Nanoparticle chemical composition and diurnal dependence at the CalNex Los Angeles ground site (2012)
    Wiley
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    Publication Date: 2012-03-15
    Description: The Nano Aerosol Mass Spectrometer (NAMS) was deployed to the California Nexus Los Angeles ground site in Pasadena, California during May–June 2010 to study nanoparticles in the 20–25 nm size range. NAMS gives a quantitative measure of the elemental composition of individual particles, and molecular apportionment of the elemental data allows the O/C mole ratio of carbonaceous matter in each particle to be determined. Abrupt increases in nanoparticle number concentration were observed in the afternoon on sunny days, and coincided with a shift in the wind direction from the southeast to the southwest. Nanoparticles analyzed during these time periods were found to contain enhanced levels of sulfur and silicon relative to particles analyzed earlier in the day, and the O/C ratios of carbonaceous matter changed from a distribution dominated by primary motor vehicle emissions (O/C ratio 〈 0.25) to one dominated by “fresh” secondary organic aerosol (O/C ratio between 0.25 and 0.65). The wind direction and chemical composition dependencies suggest that the afternoon increase in number concentration originated from motor vehicle emissions in the downtown Los Angeles area that were photochemically processed during transport to the measurement site. It is likely that photochemical processing led to both a change in the composition of preexisting particles and to the formation of new particles.
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    Regions of rapid sea ice change: An inter-hemispheric seasonal comparison (2012)
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    Publication Date: 2012-03-16
    Description: This bi-polar analysis resolves ice edge changes on space/time scales relevant for investigating seasonal ice-ocean feedbacks and focuses on spatio-temporal changes in the timing of annual sea ice retreat and advance over 1979/80 to 2010/11. Where Arctic sea ice decrease is fastest, the sea ice retreat is now nearly 2 months earlier and subsequent advance more than 1 month later (compared to 1979/80), resulting in a 3-month longer summer ice-free season. In the Antarctic Peninsula and Bellingshausen Sea region, sea ice retreat is more than 1 month earlier and advance 2 months later, resulting in a more than 3-month longer summer ice-free season. In contrast, in the western Ross Sea (Antarctica) region, sea ice retreat and advance are more than 1 month later and earlier respectively, resulting in a more than 2 month shorter summer ice-free season. Regardless of trend magnitude or direction, and at latitudes mostly poleward of 70° (N/S), there is strong correspondence between anomalies in the timings of sea ice retreat and subsequent advance, but little correspondence between advance and subsequent retreat. These results support a strong ocean thermal feedback in autumn in response to changes in spring sea ice retreat. Further, model calculations suggest different net ocean heat changes in the Arctic versus Antarctic where autumn sea ice advance is 1 versus 2 months later. Ocean-atmosphere changes, particularly in boreal spring and austral autumn (i.e., during ∼March-May), are discussed and compared, as well as possible inter-hemispheric climate connections.
    Print ISSN: 0094-8276
    Electronic ISSN: 1944-8007
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    Increase of global monsoon area and precipitation under global warming: A robust signal? (2012)
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    Publication Date: 2012-03-16
    Description: Monsoons, the most energetic tropical climate system, exert a great social and economic impact upon billions of people around the world. The global monsoon precipitation had an increasing trend over the past three decades. Whether or not this increasing trend will continue in the 21st century is investigated, based on simulations of three high-resolution atmospheric general circulation models that were forced by different future sea surface temperature (SST) warming patterns. The results show that the global monsoon area, precipitation and intensity all increase consistently among the model projections. This indicates that the strengthened global monsoon is a robust signal across the models and SST patterns explored here. The increase of the global monsoon precipitation is attributed to the increases of moisture convergence and surface evaporation. The former is caused by the increase of atmospheric water vapor and the latter is due to the increase of SST. The effect of the moisture and evaporation increase is offset to a certain extent by the weakening of the monsoon circulation.
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    Development of a multimodel-based seasonal prediction system for extreme droughts and floods: a case study for South Korea (2012)
    Wiley
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    Publication Date: 2012-03-16
    Description: An experimental, district-level system was developed to forecast droughts and floods over South Korea to properly represent local precipitation extremes. The system is based on the Asia-Pacific Economic Cooperation (APEC) Climate Center (APCC) multimodel ensemble (MME) seasonal prediction products. Three-month lead precipitation forecasts for 60 stations in South Korea for the season of March to May are first obtained from the coarse-scale MME prediction using statistical downscaling. Owing to the relatively small variance of the MME and regression-based downscaling outputs, the downscaled MME (DMME) products need to be subsequently inflated. The final station-scale precipitation predictions are then used to produce drought and flood forecasts on the basis of the Standardized Precipitation Index (SPI). The performance of three different inflation schemes was also assessed. Of these three schemes, the method that simply rescales the variance of predicted rainfall to that based on climate records, irrespective of the prediction skill or the DMME variance itself at a particular station, gives the best overall improvement in the SPI predictions. However, systematic biases in the prediction system cannot be removed by variance inflation. This implies that DMME techniques must be further improved to correct the bias in extreme drought/flood predictions. Overall, it is seen that DMME, in conjunction with variance inflation, can predict hydrological extremes with reasonable skill. Our results could inform the development of a reliable early warning system for droughts and floods, which is invaluable to policy makers and stakeholders in agricultural and water management sectors, and so forth and is important for mitigation and adaptation measures. Copyright © 2012 Royal Meteorological Society
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    Aqua-planet simulations of the formation of the South Atlantic convergence zone (2012)
    Wiley
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    Publication Date: 2012-03-16
    Description: The impact of Amazon Basin convection and cold fronts on the formation and maintenance of the South Atlantic convergence zone (SACZ) is studied using aqua-planet simulations with a general circulation model. In the model, a circular patch of warm sea-surface temperature (SST) is used to mimic the effect of the Amazon Basin on South American monsoon convection. The aqua-planet simulations were designed to study the effect of the strength and latitude of Amazon Basin convection on the formation of the SACZ. The simulations indicate that the strength of the SACZ increases as the Amazon convection intensifies and is moved away from the equator. Of the two controls studied here, the latitude of the Amazon convection exerts the strongest effect on the strength of the SACZ. An analysis of the synoptic-scale variability in the simulations shows the importance of frontal systems in the formation of the aqua-planet SACZ. Composite time series of frontal systems that occurred in the simulations show that a robust SACZ occurs when fronts penetrate into the subtropics and become stationary there as they cross eastward of the longitude of the Amazon Basin. Moisture convergence associated with these frontal systems produces rainfall not along the model SACZ region and along a large portion of the northern model Amazon Basin. Simulations in which the warm SST patch was too weak or too close to the equator did not produce frontal systems that extended into the tropics and became stationary, and did not form a SACZ. In the model, the SACZ forms as Amazon Basin convection strengthens and migrates far enough southward to allow frontal systems to penetrate into the tropics and stall over South America. This result is in agreement with observations that the SACZ tends to form after the onset of the monsoon season in the Amazon Basin. Copyright © 2012 Royal Meteorological Society
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    A new application of a multifrequency submillimeter radiometer in determining the microphysical and macrophysical properties of volcanic plumes: A sensitivity study (2012)
    Wiley
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    Publication Date: 2012-03-16
    Description: In this paper, the potential of a multifrequency submillimeter radiometer to characterize ash plumes in the near-field of a volcanic eruption is evaluated. The radiometer's sensitivity to mass concentration and particle effective dimension is shown to depend most critically on aerosol altitude and ejected water vapor concentration. There is also some dependence on temperature, aerosol shape and complex refractive index. For this study, the volcanic aerosols are assumed to be randomly oriented solid hexagonal silicates of aspect ratio unity. The T-matrix method is used to calculate the single-scattering properties of the aerosols at 36 frequencies between 90 GHz and 880 GHz, and the aerosol bulk scattering properties are derived assuming lognormal size distribution functions. A midlatitude standard summer atmosphere and a perturbed midlatitude summer atmosphere are used to quantify the sensitivity, using the delta-Eddington two-stream approximation, of the radiometer to the presence of aerosol. It is shown that at 34 frequencies, between 113 GHz and 880 GHz, the sensitivity to aerosol is a maximum if the following four conditions are satisfied: (i) The altitude of the aerosol layer should be ≫ 3 km, (ii) 0.1 g m−3 〈 mass concentration 〈 30 g m−3 (iii) the aerosol effective dimension, De, 20 μm 〈 De 〈 1000 μm and (iv) water vapor ejected by a volcano into the atmosphere should be 〈 1000 times greater than the background water vapor concentration. The paper demonstrates the potential usefulness of using spectrally resolved submillimeter measurements in the near-field of volcanic eruptions to characterize plume properties.
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    Atlantic hurricane activity following two major volcanic eruptions (2012)
    Wiley
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    Publication Date: 2012-03-16
    Description: In 1982 and 1991, two major volcanic eruptions loaded the stratosphere with long-lived sulfate aerosols, altering the global climate by redistributing longwave and shortwave radiation at the surface and throughout the atmosphere, cooling the surface and subsurface waters of the tropical oceans. Theory and observations demonstrate, through direct and indirect mechanisms, a causal relationship between tropical North Atlantic sea surface temperatures and seasonal Atlantic hurricane frequency, duration, and intensity. Therefore, it is plausible that hurricane activity in the seasons immediately following these eruptions is diminished. However, to date, such a theory remains untested. Here I use observations, reanalysis data, and output from a numerical model to suggest that the number, duration, and intensity of hurricanes in the years following the eruptions of El Chichón (1982) and Mount Pinatubo (1991) decreased via the aerosol direct effect. Determining the effects of each eruption on seasonal cyclone activity is complicated by simultaneous positive ENSO events; thus further study of the relationship between Atlantic tropical cyclones and major volcanic eruptions is warranted.
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    A model based method for obtaining the open/closed field line boundary from the cusp auroral 6300 Å[OI] red line (2012)
    Wiley
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    Publication Date: 2012-03-16
    Description: Ground based optical instruments are invaluable tools for studies of processes associated with the cusps and auroral morphology. In this work we present a method for obtaining the magnetic latitude of the open/closed field line boundary (OCB) from the cusp 6300 Å[OI] auroral red line using a meridian scanning photometer. The method which is based on a pre-defined reference cusp aurora produced by the GLOW model is examined with respect to uncertainties, and we describe how a set of equations describing the error is constructed. The method is applicable to data from optical instruments located at high latitude observation sites such as Svalbard and Antarctica. Equations describing both errors and the mapping altitude for transforming the OCB from instrument centered coordinates to magnetic latitude for instrumentation located in Svalbard (Longyearbyen) are presented. Further, by applying the GLOW model we present results illustrating the great variability in the altitude profile of the atomic oxygen 6300 Å[OI] red line emission in the cusp. A simple calculation showing how a poleward neutral wind will change the latitudinal shape of the cusp aurora is also performed.
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    Electron and wave characteristics observed by the THEMIS satellites near the magnetic equator during a pulsating aurora (2012)
    Wiley
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    Publication Date: 2012-03-16
    Description: Based on conjugate ground and THEMIS satellite observations, we show electron spectra and wave characteristics near the magnetic equatorial plane during a pulsating aurora event on the high latitude side of the auroral oval. The pulsating aurora was observed by a 30-Hz sampled all-sky imager (ASI) at Gillam (56.4°N, 265.4°E), Canada, at ∼0840-0910 UT on 8 January 2008. The auroral intensity pulsation at the possible THEMIS D (THD) footprints had frequency peaks at ∼0.1–0.2 Hz. The footprint of THD was in the poleward part of the proton aurora observed by a meridian-scanning photometer. After auroral pulsation began at ∼0842 UT, both THD and THEMIS E which was near THD in the mid-tail at 11.6–11.8 RE, observed enhanced field-aligned electron fluxes at energies of 1–10 keV. However, the amplitudes of whistler mode waves and electrostatic cyclotron harmonics (ECH) waves observed by THD with the highest sampling rate of 8 kHz were not significant, showing a marked contrast to the recent report of clear correlation between whistler mode waves and auroral pulsations observed at 5–9 RE. We suggest that the observed field-aligned electrons, which are probably caused by Fermi-type acceleration associated with earthward plasma flow in the mid-tail plasma sheet, are modulated by some wave processes to cause pulsating auroras.
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    Hall magnetohydrodynamic effects for three-dimensional magnetic reconnection with finite width along the direction of the current (2012)
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    Publication Date: 2012-03-16
    Description: We performed three-dimensional Hall magnetohydrodynamic (MHD) simulations of magnetic reconnection with finite width along the direction perpendicular to the antiparallel magnetic field (i.e., the direction of the electric current). Previous similar simulations including the Hall term have shown that the localized reconnection region itself can broaden in the anticurrent direction when the initial current is carried only by electrons. However, there is still no clear understanding of the behavior of the reconnection region in the presence of the initial ion current as in the Earth's magnetotail plasma sheet since no simulations have been carried out under such situations. In this study, we performed a systematic parametric survey considering the cases in which the initial current is carried not only by electrons but also by ions and found that the speed and direction of the current-aligned broadening of the reconnection region are almost equal to those of background ion and electron flows that carry the current. This result means that location and size of the localized reconnection region vary with time, depending on plasma conditions in the background current sheet in Hall MHD regime. The rate of the localized reconnection can reach close to the value in the two-dimensional case, even when reconnection starts in an extremely narrow region with its current-aligned width equal to an ion inertial length. The localized reconnection process also produces the asymmetry of the current-aligned structure of the reconnection jet. These results can explain various observational features related to magnetic reconnection in the near-Earth magnetotail.
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    The diurnal cycle of rainfall in South Africa in the austral summer (2012)
    Wiley
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    Publication Date: 2012-03-16
    Description: The diurnal cycle of precipitation over South Africa during summer is analysed for the first time using hourly precipitation data from 103 stations for the period 1998–2007. Using harmonic analysis, we found the presence of a distinct diurnal cycle over most of South Africa regarding both the frequency and amount of precipitation events. The standardized amplitudes, indicative of the strength of the diurnal cycle across a region, are strongest over the interior and along the east coast of South Africa with up to 70% explained variance associated with the diurnal cycle. The time of maximum precipitation is late afternoon to early evening in the interior, and midnight to early morning along the Agulhas Current as well as inland in the northeast of the county. The proximity of the warm Agulhas Current plays a role in the diurnal cycle of rainfall at coastal stations. There is an early morning maximum in precipitation in the South West of the country with small amplitude in the diurnal cycle there. On average, the peak in precipitation amount leads the peak in frequencies of precipitation by 30 min to 1 h. Together with a high resolution climatological summer rain rate presented here and a detailed table for all stations, this study is a benchmark upon which model output or satellite estimate of the diurnal cycle can be compared within the region. Copyright © 2012 Royal Meteorological Society
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    Effect of mid-latitude blocking anticyclones on the weather of the Arabian Peninsula (2012)
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    Publication Date: 2012-03-16
    Description: The statistical relationships among the various 10°–70°E mid-latitude blocking anticyclone parameters and the weather of the Arabian Peninsula (AP) (35°–60°E, 12°–32°N) over a 40-year period (1968-2007), on seasonal, interannual, decadal and long-term scales, are studied. The studied parameters include the number of blocking anticyclone events, the duration, the intensity, and the longitude at the blocking anticyclone onset. It is found that 31% of the Northern Hemisphere mid-latitude blocking anticyclone events occurred over the 10°–70°E longitudes, and out of these, the maximum number of mid-latitude blocking anticyclone event onsets are at 30°E (24%). On the seasonal basis, the annual and decadal relationships of the 10°–70° blocking anticyclones with the El-Niño Southern Oscillation (ENSO), the North Atlantic Oscillation (NAO) and the Arctic Oscillation (AO) indices are presented. The results show that the number of days the blocking anticyclones persists is sensitive to the ENSO phase. The mid-latitude blocking anticyclone occurrence over the 10°–70°E longitudes is indicative of the reduced surface temperature variance, both upstream and downstream, during the blocking anticyclone period, over the AP. A shift in the mean surface temperature distribution occurs, in all seasons, during the blocking anticyclone period. The blocking anticyclones initiate a surface temperature change (both positive and negative) that persists even after the blocking anticyclone's decay. The AP surface weather during the months of October, November, and December is affected more by the occurrence of mid-latitude blocking anticyclones over the 10°–70°E longitudes in the presence the of El-Niño phase. Copyright © 2012 Royal Meteorological Society
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    Assessment of the effects of acid-coated ice nuclei on the Arctic cloud microstructure, atmospheric dehydration, radiation and temperature during winter (2012)
    Wiley
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    Publication Date: 2012-03-16
    Description: Owing to the large-scale transport of pollution-derived aerosols from the mid-latitudes to the Arctic, most of the aerosols are coated with acidic sulfate during winter in the Arctic. Recent laboratory experiments have shown that acid coating on dust particles substantially reduces the ability of these particles to nucleate ice crystals. Simulations performed using the Limited Area version of the Global Multiscale Environmental Model (GEM-LAM) are used to assess the potential effect of acid-coated ice nuclei on the Arctic cloud and radiation processes during January and February 2007. Ice nucleation is treated using a new parameterization based on laboratory experiments of ice nucleation on sulphuric acid-coated and uncoated kaolinite particles. Results show that acid coating on dust particles has an important effect on cloud microstructure, atmospheric dehydration, radiation and temperature over the Central Arctic, which is the coldest part of the Arctic. Mid and upper ice clouds are optically thinner while low-level mixed-phase clouds are more frequent and persistent. These changes in the cloud microstructures affect the radiation at the top of the atmosphere with longwave negative cloud forcing values ranging between 0 and − 6 W m −2 over the region covered by the Arctic air mass. Copyright © 2012 Royal Meteorological Society
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    Long-term occurrence, variability and tracks of deep cyclones over Krakow (Central Europe) during the period 1900–2010 (2012)
    Wiley
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    Publication Date: 2012-03-16
    Description: This article discusses patterns in the long-term and seasonal occurrence of deep cyclones over Krakow. This study analysed the frequency of occurrence of air pressure values equal to or lower than the 1st percentile (equivalent to ≤995.3 hPa) of all air pressure values recorded at 12:00 UTC over a period of 110 years (1900/1901–2009/2010). Special attention was devoted to the tracks of deep cyclones. No distinct changes were found in the frequency of occurrence of deep cyclones during the study period. Overall the frequency peaked in December, but in recent years there has been an increase in frequency towards the end of winter and beginning of spring. A similar general lack of noticeable change in the number of days with deep cyclones can also be found in specific tracks. There were minor increases in the frequency of occurrence of cyclones from the Norwegian Sea (T1), the Atlantic (T3), Bay of Biscay (T6) and the Mediterranean (T7) after 1950. The study also found confirmation of the theory that cyclone tracks had shortened at their northeastern extremities. Copyright © 2012 Royal Meteorological Society
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    Trends in extreme temperature indices in South Africa: 1962–2009 (2012)
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    Publication Date: 2012-03-16
    Description: Trends in daily maximum and minimum extreme temperature indices were investigated for 28 weather stations in South Africa, not only for the common period of 1962–2009, but also for longer periods which the individual record lengths of the stations would allow. The utilized weather stations had limited gaps in their time series, did not undergo major moves, or had their exposure compromised during the study period, as to influence the homogeneity of their time series. The indices calculated were forthcoming from those developed by the WMO/CLIVAR Expert Team on Climate Change Detection and Indices (ETCCDI), but only those applicable to the South African climate were selected. The general result is that warm extremes increased and cold extremes decreased for all of the weather stations. The trends however vary on a regional basis, both in magnitude and statistical significance, broadly indicating that the western half, as well as parts of the northeast and east of South Africa, show relatively stronger increases in warm extremes and decreases in cold extremes than elsewhere in the country. These regions coincide to a large degree with the thermal regimes in South Africa which are susceptible to extreme temperatures. The annual absolute maximum and minimum temperatures do not reflect the general trends displayed by the other indices, showing that individual extreme events cannot always be associated with observed long-term climatic trends. The analyses of longer time series than the common period indicate that it is highly likely that warming accelerated since the mid-1960s in South Africa. Copyright © 2012 Royal Meteorological Society
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    On the temporal clustering of US floods and its relationship to climate teleconnection patterns (2012)
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    Publication Date: 2012-03-16
    Description: This article examines whether the temporal clustering of flood events can be explained in terms of climate variability or time-varying land-surface state variables. The point process modelling framework for flood occurrence is based on Cox processes, which can be represented as Poisson processes with randomly varying rate of occurrence. In the special case that the rate of occurrence is deterministic, the Cox process simplifies to a Poisson process. Poisson processes represent flood occurrences which are not clustered. The Cox regression model is used to examine the dependence of the rate of occurrence on covariate processes. We focus on 41 stream gauge stations in Iowa, with discharge records covering the period 1950–2009. The climate covariates used in this study are the North Atlantic Oscillation (NAO) and the Pacific/North American Teleconnection (PNA). To examine the influence of land-surface forcing on flood occurrence, the antecedent 30 d rainfall accumulation is considered. In 27 out of 41 stations, either PNA or NAO, or both are selected as significant predictors, suggesting that flood occurrence in Iowa is influenced by large-scale climate indices. Antecedent rainfall, used as a proxy for soil moisture, plays an important role in driving the occurrence of flooding in Iowa. These results point to clustering as an important element of the flood occurrence process. Copyright © 2012 Royal Meteorological Society
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    Separation of the time periods with dominant climatologic parameters by multivariate statistical analysis (2012)
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    Publication Date: 2012-03-16
    Description: Climatology is one of the areas that rely on collecting huge quantities of data. The longer a time period is observed, the better; the more parameters are included, the better. But the human mind cannot easily extract useful information from the abundance of data; thus, many valuable facts may be overlooked. Having that in mind, the authors of this paper have focused on the data condensation with the goal of gathering more information about the underlying trends of the main climatologic parameters change to show climatic variability. The data from the Belgrade Meteorological Observatory are analysed using a number of different methods of multivariate statistical analysis. Separation of the years and time periods with similar weather pattern characteristics was successful and indicates that there is a trend of temperature increase, as well as a trend of the temperature range decrease. Copyright © 2012 Royal Meteorological Society
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    Synoptic situations of severe local convective storms during the pre-monsoon season in Bangladesh (2012)
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    Publication Date: 2012-03-16
    Description: This study examines synoptic situations of severe local convective storms (mesoscale severe weather associated with deep convections such as tornado and hail) during the pre-monsoon season (from March to May) in Bangladesh. We compared composite meteorological fields on severe local convective storm days (SLCSD) with those on non-severe local convective storm days (NSLCSD). Moisture inflow from the Bay of Bengal is enhanced with intensification of southwesterly wind at 950 hPa on SLCSD compared with NSLCSD. The temperature is higher at 800 hPa over the inland area of the Indian subcontinent including Bangladesh on SLCSD than NSLCSD. At 550 hPa, a trough over Bangladesh develops on SLCSD compared with NSLCSD. This leads to the development of a thermal trough over the inland area of the Indian subcontinent and enhancement of cold advection from the northwest into Bangladesh on SLCSD at this level. This synoptic situation produces great potential instability of the atmosphere in Bangladesh on SLCSD during the pre-monsoon season. Composite distributions of lifted index, precipitable water and convective available potential energy on SLCSD and NSLCSD over south Asia show distinct differences of these parameters between these two categories with statistical significance especially in and around Bangladesh. These differences indicate that the atmospheric environment has great potential instability especially in and around Bangladesh on SCLSD under the synoptic situations shown in this study. Copyright © 2012 Royal Meteorological Society
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    Modelling variable glacier lapse rates using ERA-Interim reanalysis climatology: an evaluation at Vestari- Hagafellsjökull, Langjökull, Iceland (2012)
    Wiley
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    Publication Date: 2012-03-16
    Description: The near-surface air temperature lapse rate is an important tool for spatially distributing temperatures in snow- and ice-melt models, but is difficult to parameterize, as it is not simply correlated with boundary-layer meteorological variables, such as temperature itself. This contribution quantifies spring-autumn lapse rate variability over 5 years at Vestari-Hagafellsjökull, a southerly outlet of Langjökull in Iceland. It is observed that summer lapse rates (0.57 °C 100 m −1 ) are significantly lower than non-summer rates, and are also lower than the Saturated Adiabatic Lapse Rate (SALR), which is often adopted in melt models. This is consistent with reduced near-surface temperature sensitivity to free-atmosphere temperature change during the occurrence of melting. A Variable Lapse Rate (VLR) regression model is calibrated with standardized, 750 hPa temperature anomalies derived from ERA-Interim climatology, which is shown to be highly significantly correlated with near-surface temperatures. The modelled VLR overestimates cumulative June–September Positive Degree Days (PDDs) by 3% when used to extrapolate temperatures from 1100 to 500 m a.s.l. on the glacier, whereas the SALR overestimates cumulative PDDs by 14%. ERA-Interim data therefore appear to offer a good representation of free-atmosphere temperature variability over Vestari-Hagafellsjökull, and the modelling approach offers a simple means of improving lapse rate parameterizations in melt models. Copyright © 2012 Royal Meteorological Society
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    An annual cycle of size-resolved aerosol hygroscopicity at a forested site in Colorado (2012)
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    Publication Date: 2012-03-16
    Description: The ability of particles composed wholly or partially of biogenic secondary organic compounds to serve as cloud condensation nuclei (CCN) is a key characteristic that helps to define their roles in linking biogeochemical and water cycles. In this paper, we describe size-resolved (14–350 nm) CCN measurements from the Manitou Experimental Forest in Colorado, where particle compositions were expected to have a large biogenic component. These measurements were conducted for 1 year as part of the Bio-hydro-atmosphere Interactions of Energy, Aerosols, Carbon, H2O, Organics, and Nitrogen program and determined the aerosol hygroscopicity parameter, κ, at five water supersaturations between ∼0.14% and ∼0.97%. The average κ value over the entire study and all supersaturations was κavg = 0.16 ± 0.08. Kappa values decreased slightly with increasing supersaturation, suggesting a change in aerosol composition with dry diameter. Furthermore, some seasonal variability was observed with increased CCN concentrations and activated particle number fraction, but slightly decreased hygroscopicity, during the summer. Small particle events, which may indicate new particle formation, were observed throughout the study period, especially in the summer, leading to increases in CCN concentration, followed by a gradual increase in the aerosol mode size. The condensing material appeared to be predominantly composed of organic compounds and led to a small decrease in κ at the larger activation diameters during and immediately after those events.
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    Process length scales and longitudinal damping in karst conduits (2012)
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    Publication Date: 2012-03-16
    Description: Simple mathematical models often allow an intuitive grasp of the function of physical systems. We develop a mathematical framework to investigate reactive or dissipative transport processes within karst conduits. Specifically, we note that for processes that occur within a characteristic timescale, advection along the conduit produces a characteristic process length scale. We calculate characteristic length scales for the propagation of thermal and electrical conductivity signals along karst conduits. These process lengths provide a quantitative connection between karst conduit geometry and the signals observed at a karst spring. We show that water input from the porous/fractured matrix is also characterized by a length scale and derive an approximation that accounts for the influence of matrix flow on the transmission of signals through the aquifer. The single conduit model is then extended to account for conduits with changing geometries and conduit flow networks, demonstrating how these concepts can be applied in more realistic conduit geometries. We introduce a recharge density function, ϕR, which determines the capability of an aquifer to damp a given signal, and cast previous explanations of spring variability within this framework. Process lengths are a general feature of karst conduits and surface streams, and we conclude with a discussion of other potential applications of this conceptual and mathematical framework.
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    A volatility basis set model for summertime secondary organic aerosols over the eastern United States in 2006 (2012)
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    Publication Date: 2012-03-16
    Description: A new secondary organic aerosol (SOA) parameterization based on the volatility basis set is implemented in a regional air quality model WRF-CHEM. Full meteorological and chemistry simulations are carried out for the United States for August–September 2006. Predicted organic aerosol (OA) concentrations are compared against surface measurements made by several networks and aircraft data from the TexAQS-2006 field campaign. Elemental carbon simulations are also evaluated in order to evaluate the model's ability to capture their emissions, transport, and removal. Certain measurement limitations, such as daily averaged OA concentrations, impose some difficulties on the model evaluation, and hourly averaged OA measurements provide more informative constraints compared to daily concentrations. The updated model demonstrates a significant improvement in simulating the OA concentrations compared to the standard WRF-CHEM, which predicts very little SOA. The improvement in organic carbon (OC) predictions is noticeable in correlations and model bias. The correlations of OC exceed that of the persistence forecasts for hourly concentrations in the southeast United States during daytime. The updated traditional SOA yields still lead to an underestimation of observed OA, while addition of the multigenerational volatile organic compound (VOC) oxidation drastically improves model performance. However, several key uncertainties remain in SOA formation and loss mechanisms, which are characterized through several perturbation simulations. Dry deposition of VOC oxidation products is an important factor in the atmospheric SOA budget. The combination of the biogenic VOC emissions, updated SOA yields, and aging mechanism result in biogenic SOA being the dominant OA component for much of the nonurban United States.
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    Shelf-basin exchange times of Arctic surface waters estimated from 228Th/228Ra disequilibrium (2012)
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    Publication Date: 2012-03-16
    Description: The transpolar drift is strongly enriched in 228Ra accumulated on the wide Arctic shelves with subsequent rapid offshore transport. We present new data of Polarstern expeditions to the central Arctic and to the Kara and Laptev seas. Because 226Ra activities in Pacific waters are 30% higher than in Atlantic waters, we correct 226Ra for the Pacific admixture when normalizing 228Ra with 226Ra. The use of 228Ra decay as age marker critically depends on the constancy in space and time of the source activity, a condition that has not yet adequately been tested. While 228Ra decays during transit over the central basin, ingrowth of 228Th could provide an alternative age marker. The high 228Th/228Ra activity ratio (AR = 0.8–1.0) in the central basins is incompatible with a mixing model based on horizontal eddy diffusion. An advective model predicts that 228Th grows to an equilibrium AR, the value of which depends on the scavenging regime. The low AR over the Lomonosov Ridge (AR = 0.5) can be due to either rapid transport (minimum age without scavenging 1.1 year) or enhanced scavenging. Suspended particulate matter load (derived from beam transmission and particulate 234Th) and total 234Th depletion data show that scavenging, although extremely low in the central Arctic, is enhanced over the Lomonosov Ridge, making an age of 3 years more likely. The combined data of 228Ra decay and 228Th ingrowth confirm the existence of a recirculating gyre in the surface water of the eastern Eurasian Basin with a river water residence time of at least 3 years.
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    Limits to reconstructing paleotopography from thermochronometer data (2012)
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    Publication Date: 2012-03-16
    Description: Recent studies suggest that orogens can achieve a topographic steady state whereby equilibrium is reached between tectonics and erosion. However, steady state topography may not be the norm in many orogens experiencing large changes in climate or tectonics, which can produce topographic transients. The quantification of transient topography over geologic timescales requires reconstructing paleotopography, but this has proven difficult in many cases. This study investigates the utility of bedrock thermochronometer data to reconstruct orogen paleotopography over million year timescales. Apatite (U-Th)/He and fission track ages are integrated with a thermokinematic model for a single-parameter inversion of paleotopography. An iterative scheme is used that minimizes the misfit between predicted and observed cooling ages to identify the range of paleotopographies that could produce observed ages within sample uncertainty. Two approaches are considered. First, synthetic 2-D topographies are used to test the robustness of the approach. The following topographic evolution scenarios are considered: (1) lateral ridge migration, (2) topographic relief change, and (3) valley widening and deepening from glaciation. Second, the method is applied in three dimensions to existing data from the Coast Mountains of British Columbia, Canada. Results from both applications of the model suggest that (1) paleotopographic reconstruction will typically underpredict the magnitude of topographic change, especially relief change; (2) paleotopography is most successfully reconstructed after lateral ridge migration in long-wavelength topographies; and (3) reconstructed paleotopography from the Coast Mountains, British Columbia, suggests that glacial erosion may have the potential to remove drainage divides and laterally shift topographic ridges and peaks.
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    Poleward coastal jets induced by westerlies in the Bay of Biscay (2012)
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    Publication Date: 2012-03-16
    Description: Two hydrodynamic surveys based on acoustic Doppler current profiler (ADCP) and drift buoys measurements taken in summer 2008 and 2009 revealed poleward coastal jets of up to 32 cm s−1 that lasted up to 22 d along the Aquitaine shelf in the southeastern area of the Bay of Biscay. A strong increase in bottom temperature was associated with these currents, up to 4°C in 5 d at 54 m depth. These observations occurred after a few days of westerlies, cross-shore winds which were thought to have only a limited impact on longshore circulation. Here, the MARS3D hydrodynamic model was used with a schematic bathymetry of the southeastern area of the Bay of Biscay to reproduce and analyze these coastal jets. Simulations revealed that the triggering mechanism of the poleward currents is unequivocally due to downwelling circulation induced along the Spanish coast. This downwelling induces an external longshore pressure gradient which generates a high-speed coastal-trapped wave that propagates along the French coast with a phase velocity of about 3 to 4 m s−1 and an internal baroclinic Kelvin wave with a phase velocity of about 1 m s−1. A sensitivity study of the role of stratification conditions, wind strength and duration was then carried out to determine the periods that are the most sensitive to this wind-induced circulation.
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    Critical state soil constitutive model for methane hydrate soil (2012)
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    Publication Date: 2012-03-16
    Description: This paper presents a new constitutive model that simulates the mechanical behavior of methane hydrate-bearing soil based on the concept of critical state soil mechanics, referred to as the “Methane Hydrate Critical State (MHCS) model”. Methane hydrate-bearing soil is, under certain geological conditions, known to exhibit greater stiffness, strength and dilatancy, which are often observed in dense soils and also in bonded soils such as cemented soil and unsaturated soil. Those soils tend to show greater resistance to compressive deformation but the tendency disappears when the soil is excessively compressed or the bonds are destroyed due to shearing. The proposed model represents these features by introducing five extra model parameters to the conventional critical state model. It is found that, for an accurate prediction of ground settlement, volumetric yielding plays an important role when hydrate soil undergoes a significant change in effective stresses and hydrate saturation, which are expected during depressurization for methane gas recovery.
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    Type II solar radio bursts: Modeling and extraction of shock parameters (2012)
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    Publication Date: 2012-03-16
    Description: This first paper in a two part series summarizes the current theory and the data-driven solar wind model for simulating dynamic spectra of type II radio bursts. It also introduces performance metrics and techniques for extraction of model shock parameters from these dynamic spectra. We use an iterative downhill simplex method which compares two dynamic spectra and quantitatively assesses and improves the agreement using two figures of merit: the first is based on the correlation function and the second is based on a normalized differences over the data set. By maximizing the agreement we are able to extract the input model shock parameters to within 30% or better when using model solar winds of increasing complexity. The effects on the spectra predicted and on the figures of merit from changing the model shock parameters and solar wind model are also investigated. The iterative downhill extraction method is then applied to the type II dynamic spectrum predicted using a realistic model solar wind and a shock model estimated for an observed type II event. The shock parameters are recovered to within 10% of the correct solution.
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    An updated examination of the Luzon Strait transport (2012)
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    Publication Date: 2012-03-16
    Description: Despite numerous previous estimates of Luzon Strait transport (LST), we attempt an update using a fine-resolution model. With these improvements, the circulation in and around Luzon Strait shows up rather realistically. Intrusion of a Kuroshio meander into the South China Sea (SCS) is seasonally varying. The LST, especially in the upper ocean, caused by a small difference between the large meander inflow and outflow, is also seasonally varying and subject to large standard deviation. The annual mean LST is estimated to be westward (−4.0 ± 5.1 Sv) along 120.75°E. We have also conducted process of elimination experiments to assess the relative importance of open ocean inflow/outflow, wind stress, and surface heat flux in regulating LST and its seasonality. The East Asian monsoon winds stand out as the predominant forcing. Without it, the upper ocean LST changes from westward to eastward (ranging up to 4 Sv) and, with misaligned seasonality, triggering an inflow from the Mindoro Strait to the SCS to replenish the water mass loss. Discounting monsoon winds, sea level in the Sulu Sea is generally higher because it receives the Indonesian Throughflow before the SCS, which causes an inflow from the Sulu Sea to the SCS. On the other hand, the annual mean wind from the northeast invites outflow from the SCS to the Sulu Sea (or inflow from the Luzon Strait). Weighing the two competing factors together, we see the cessation of northeast monsoon as a condition favorable for the Luzon Strait outflow or the Mindoro Strait inflow.
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    Beating HF waves to generate VLF waves in the ionosphere (2012)
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    Publication Date: 2012-03-16
    Description: Beat-wave generation of very low frequency (VLF) waves by two HF heaters in the ionosphere is formulated theoretically and demonstrated experimentally. The heater-induced differential thermal pressure force and ponderomotive force, which dominate separately in the D and F regions of the ionosphere, drive an electron current for the VLF emission. A comparison, applying appropriate ionospheric parameters shows that the ponderomotive force dominates in beat-wave generation of VLF waves. Three experiments, one in the nighttime in the absence of D and E layers and two in the daytime in the presence of D and E layers, were performed. X mode HF heaters of slightly different frequencies were transmitted at CW full power. VLF waves at 10 frequencies ranging from 3.5 to 21.5 kHz were generated. The frequency dependencies of the daytime and nighttime radiation intensities are quite similar, but the nighttime radiation is much stronger than the daytime one at the same radiation frequency. The intensity ratio is as large as 9 dB at 11.5 kHz. An experiment directly comparing VLF waves generated by the beat-wave approach and by the amplitude modulation (AM) approach was also conducted. The results rule out the likely contribution of the AM mechanism acting on the electrojet and indicate that beat-wave in the VLF range prefers to be generated in the F region of the ionosphere through the ponderomotive nonlinearity, consistent with the theory. In the nighttime experiment, the ionosphere was underdense to the HF heaters, suggesting a likely setting for effective beat-wave generation of VLF waves by the HF heaters.
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    Dehydration-induced damage and deformation in gypsum and implications for subduction zone processes (2012)
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    Publication Date: 2012-03-10
    Description: Experimental heating tests were performed on Volterra gypsum to study the micromechanical consequences of the dehydration reaction. The experimental conditions were drained at 5 MPa fluid pressure and confining pressures ranging from 15 to 55 MPa. One test was performed with a constantly applied differential stress of 30 MPa. The reaction is marked by (1) a porosity increase and homogeneous compaction, (2) a swarm of acoustic emissions, (3) a large decrease in P and S wave velocities, and (4) a decrease in VP/VS ratio. Wave velocity data are interpreted in terms of crack density and pore aspect ratio, which, modeling pores as spheroids, is estimated at around 0.05 (crack-like spheroid). Complementary tests performed in an environmental scanning electron microscope indicate that cracks first form inside the gypsum grains and are oriented preferentially along the crystal structure of gypsum. Most of the visible porosity appears at later stages when grains shrink and grain boundaries open. Extrapolation of our data to serpentinites in subduction zones suggest that the signature of dehydrating rocks in seismic tomography could be a low apparent Poisson's ratio, although this interpretation may be masked by anisotropy development due to preexisting crystal preferred orientation and/or deformation-induced cracking. The large compaction and the absence of strain localization in the deformation test suggests that dehydrating rocks maybe seen as soft inclusions and could thus induce ruptures in the surrounding, nonreacting rocks.
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    Airborne lidar measurements of wave energy dissipation in a coral reef lagoon system (2012)
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    Publication Date: 2012-03-10
    Description: Quantification of the turbulent kinetic energy dissipation rate in the water column, ε, is very important for assessing nutrient uptake rates of corals and therefore the health of coral reef lagoon systems. However, the availability of such data is limited. Recently, at Lady Elliot Island (LEI), Australia, we showed that there was a strong correlation between in situ measurements of surface-wave energy dissipation and ε. Previously, Reineman et al. (2009), we showed that a small airborne scanning lidar system could measure the surface wavefield remotely. Here we present measurements demonstrating the use of the same airborne lidar to remotely measure surface wave energy fluxes and dissipation and thereby estimate ε in the LEI reef-lagoon system. The wave energy flux and wave dissipation rate across the fore reef and into the lagoon are determined from the airborne measurements of the wavefield. Using these techniques, observed spatial profiles of energy flux and wave energy dissipation rates over the LEI reef-lagoon system are presented. The results show that the high lidar backscatter intensity and point density coming from the high reflectivity of the foam from depth-limited breaking waves coincides with the high wave-energy dissipation rates. Good correlations between the airborne measurements and in situ observations demonstrate that it is feasible to apply airborne lidar systems for large-scale, long-term studies in monitoring important physical processes in coral reef environments. When added to other airborne techniques, the opportunities for efficient monitoring of large reef systems may be expanded significantly.
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    Stick, slip, and opening of wavy frictional faults: A numerical approach in two dimensions (2012)
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    Publication Date: 2012-03-10
    Description: We use a two-dimensional displacement discontinuity method (DDM) for quasi-static boundary value problems to investigate sinusoidal faults of finite length in an otherwise homogeneous and isotropic elastic material. The DDM incorporates a complementarity algorithm to enforce appropriate contact boundary conditions along the model fault. The numerical solution for the model sinusoidal fault converges to the analytical solution for a straight fault of finite length as the ratio amplitude/wavelength goes to zero. It does not converge to the analytical solution for an infinite sinusoidal interface as the ratio distance/wavelength goes to zero. We provide stick, slip, and opening distributions along wavy faults with a range of uniform coefficients of friction, amplitude/wavelength ratios, and wave numbers. As the number of sinusoidal waves or the amplitude/wavelength is increased, mean slip decreases. Additionally, the fault geometry causes slip to deviate significantly from the elliptical distribution of a planar fault. We demonstrate that the displacement discontinuity of wavy faults cannot be prescribed a priori. This necessitates implementation of the complementarity algorithm and precludes an analytical solution. We employ the terms lee and stoss instead of releasing and restraining bends because a local minimum in slip may occur along lee sides, as well as stoss sides. In some cases, lee sides stick while stoss sides slip. Trends in the slip perturbation can be explained by the angular relationship between the local fault trace and the orientation of the remote principal stresses; however, the displacement discontinuity along a wavy model fault cannot be explained by this relationship alone.
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    Reply to comment by S. A. Shapiro on “Role of seepage forces on seismicity triggering” (2012)
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    Publication Date: 2012-03-10
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    Correction to “Dusty plasma in the vicinity of Enceladus” (2012)
    Wiley
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    Publication Date: 2012-03-10
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
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    Overcooling in the upper thermosphere during the recovery phase of the 2003 October storms (2012)
    Wiley
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    Publication Date: 2012-03-10
    Description: Infrared radiative emissions by carbon dioxide (CO2) and nitric oxide (NO) are the major cooling mechanisms of the lower thermosphere. During geomagnetically active periods, the NO density and cooling rate in the auroral regions increase significantly as a result of particle precipitation and Joule heating. Previous studies have shown that the time for NO density to recover to quiet time levels is longer than that of the thermosphere temperature or density recovery. This study explores the implications of these different recovery rates for the post-storm thermosphere. Thermosphere densities retrieved from the CHAMP and GRACE accelerometer measurements and NO cooling rates measured by TIMED/SABER are used to examine their variations during the post-storm period of the October 2003 geomagnetic storms. It was found that thermosphere densities at both CHAMP and GRACE altitudes recovered rapidly and continuously decreased below the quiet time densities during the post-storm period, especially at middle latitudes. Compared with the quiet time values, the maximum depletion in the CHAMP and GRACE densities after the storm is about 23–36%, and the estimated decrease of thermospheric temperature is as large as 70–110 K. Our analysis suggests that the elevated NO cooling rate, resulting from the slower recovery of NO densities in the post-storm period, is a plausible cause for this apparent post-storm overcooling of the thermosphere.
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  • 97
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    On the nature of radar backscatter and 250 MHz scintillation linked with an intense daytime Es patch (2012)
    Wiley
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    Publication Date: 2012-03-10
    Description: In this paper we present and analyze the Gadanki radar observations of an unusual event of daytime radar echoes from the E region, which spread over a range of 105–150 km and displayed a “U shape” in the range-time SNR map. The U shape echoing structure was an added feature to the commonly observed lower E region echoes with slowly descending features and 150 km echoes with range migration displaying a forenoon descent and afternoon ascent. Distinctly different Doppler velocities were observed in the two arms of the U shape structure with velocities increasing with height and surprisingly exceeding the Doppler velocities of the 150 km echoes. The Doppler spectra display features very similar to those observed below 100 km indicating turbulence as the underlying process. A collocated ionosonde observed unusually strong sporadic E (Es) activity with maximum reflected/scattered frequency (ftEs) reaching 16 MHz in close correspondence with the U shape structure. During the same duration, a collocated 250.6 MHz scintillation receiver revealed scintillation activity, not observed before from Gadanki. The unusual radar observations, strong Es activity, and daytime scintillation are first of its kind from Gadanki. Detailed analysis suggests that the U shape radar echoes extending to 150 km range were due to the sidelobe detection of the E region irregularities with special features that were also responsible for the daytime scintillation. The genesis and implication of the irregularities are discussed.
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  • 98
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    A pure marine aerosol model, for use in remote sensing applications (2012)
    Wiley
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    Publication Date: 2012-03-15
    Description: Retrievals of aerosol optical depth (AOD) and related parameters from satellite measurements typically involve prescribed models of aerosol size and composition, and are therefore dependent on how well these models are able to represent the radiative behavior of real aerosols. This study uses aerosol volume size distributions retrieved from Sun-photometer measurements at 11 Aerosol Robotic Network (AERONET) island sites, spread throughout the world's oceans, as a basis to define such a model for pure (unpolluted) maritime aerosol. Volume size distributions are observed to be bimodal and approximately lognormal, although the coarse mode is skewed with a long tail on the low-radius end. The relationship of AOD and size distribution parameters to meteorological conditions is also examined. As wind speed increases, so do coarse-mode volume and radius. The AOD and Ångström exponent show linear relationships with wind speed, although with considerable scatter. Links between aerosol properties and near-surface relative humidity, columnar water vapor, and sea surface temperature are also explored. A recommended bimodal maritime model, which is able to reconstruct the AERONET AOD with accuracy of order 0.01–0.02, is presented for use in aerosol remote sensing applications. This accuracy holds at most sites and for wavelengths between 340 nm and 1020 nm. Calculated lidar ratios are also provided, and are in the range of other studies, although differ more strongly from those currently used in Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) processing.
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    Topics: Geosciences , Physics
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  • 99
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    Complex fluid flow revealed by monitoring CO2 injection in a fluvial formation (2012)
    Wiley
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    Publication Date: 2012-03-16
    Description: At Cranfield, Mississippi, United States, a large-scale carbon dioxide (CO2) injection through an injection well (∼3,080 m deep) was continuously monitored using U-tube samplers in two observation wells located 68 and 112 m east of the injector. The Lower Tuscaloosa Formation injection zone, which consists of amalgamated fluvial point-bar and channel-fill deposits, presents an interesting environment for studying fluid flow in heterogeneous formations. Continual fluid sampling was carried out during the first month of CO2 injection. Two subsequent tracer tests using sulfur hexafluoride (SF6) and krypton were conducted at different injection rates to measure flow velocity change. The field observations showed significant heterogeneity of fluid flow and for the first time clearly demonstrated that fluid flow evolved with time and injection rate. It was found the wells were connected through numerous, separate flow pathways. CO2 flowed through an increasing fraction of the reservoir and sweep efficiency improved with time. The field study also first documented in situ component exchange between brine and gas phases during CO2 injection. It was found that CH4 degassed from brine and is enriched along the gas–water contact. Multiple injectate flow fronts with high CH4 concentration arrived at different times and led to gas composition fluctuations in the observation wells. The findings provide valuable insights into heterogeneous multiphase flow in rock formations and show that conventional geological models and static fluid flow simulations are unable to fully describe the heterogeneous and dynamic flow during fluid injection.
    Print ISSN: 0148-0227
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  • 100
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    Laboratory spectroscopic analyses of electron irradiated alkanes and alkenes in solar system ices (2012)
    Wiley
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    Publication Date: 2012-03-16
    Description: We report results from laboratory experiments of 10 keV electron irradiation of thin ice films of water and short-chain hydrocarbons at ∼10−8 Torr and temperatures ranging from 70–100 K. Hydrocarbon mixtures include water with C3H8, C3H6, C4H10 (butane and isobutane), and C4H8, (1-butene and cis/trans-2-butene). The double bonds of the alkenes in our initial mixtures were rapidly destroyed or converted to single carbon bonds, covalent bonds with hydrogen, bonds with −OH (hydroxyl), bonds with oxygen (C-O), or double bonds with oxygen (carbonyl). Spectra resulting from irradiation of alkane and alkene ices are largely indistinguishable; the initial differences in film composition are destroyed and the resulting mixture includes long-chain, branched aliphatics, aldehydes, ketones, esters, and alcohols. Methane was observed as a product during radiolysis but CO was largely absent. We find that while some of the carbon is oxidized and lost to CO2 formation, some carbon is sequestered into highly refractory, long-chain aliphatic compounds that remain as a thin residue even after the ice film has been raised to standard temperature and pressure. We conclude that the high availability of hydrogen in our experiments leads to the formation of the formyl radical which then serves as the precursor for formaldehyde and polymerization of longer hydrocarbon chains.
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