ALBERT

Description: [1]  Different solutions have been proposed to solve the ‘faint young Sun problem’, defined by the fact that the Earth was not fully frozen during the Archean despite the fainter Sun. Most previous studies were performed with simple 1D radiative convective models and did not account well for the clouds and ice-albedo feedback or the atmospheric and oceanic transport of energy. We apply a Global Climate Model (GCM) to test the different solutions to the faint young Sun problem. We explore the effect of greenhouse gases (CO 2 and CH 4 ), atmospheric pressure, cloud droplet size, land distribution and Earth's rotation rate. We show that, neglecting organic haze, 100 mbars of CO 2 with 2 mbars of CH 4 at 3.8 Ga and 10 mbars of CO 2 with 2 mbars of CH 4 at 2.5 Ga allow a temperate climate (mean surface temperature between 10 °C and 20 °C). Such amounts of greenhouse gases remain consistent with the geological data. Removing continents produces a warming lower than +4 °C. The effect of rotation rate is even more limited. Larger droplets (radii of 17  µm versus 12  µm ) and a doubling of the atmospheric pressure produce a similar warming of around +7 °C. In our model, ice-free waterbelts can be maintained up to 25° N/S with less than 1 mbar of CO 2 and no methane. An interesting cloud feedback appears above cold oceans, stopping the glaciation. Such a resistance against full glaciation tends to strongly mitigate the faint young Sun problem.

Description: [1]  The Cross-track Infrared Sounder (CrIS) is the high spectral resolution spectroradiometer on the Suomi National Polar-Orbiting Partnership (NPP) satellite, providing operational observations of top-of-atmosphere thermal infrared radiance spectra for weather and climate applications. This paper describes the CrIS radiometric calibration uncertainty based on pre-launch and on-orbit efforts to estimate calibration parameter uncertainties, and provides example results of recent post-launch validation efforts to assess the predicted uncertainty. Pre-launch Radiometric Uncertainty (RU) estimates computed for the laboratory test environment are less than ~0.2 K 3-sigma for blackbody scene temperatures above 250 K, with primary uncertainty contributions from the calibration blackbody temperature, calibration blackbody reflected radiance terms and detector nonlinearity. Variability of the pre-launch RU among the longwave band detectors and midwave band detectors is due to different levels of detector nonlinearity. A methodology for on-orbit adjustment of nonlinearity correction parameters to reduce the overall contribution to RU and to reduce FOV-to-FOV variability is described. The resulting on-orbit RU estimates for Earth view spectra are less than 0.2 K 3-sigma in the midwave and shortwave bands, and less than 0.3 K 3-sigma in the longwave band. Post-launch validation efforts to assess the radiometric calibration of CrIS are underway; validation results to date indicate that the on-orbit RU estimates are representative. CrIS radiance products are expected to reach “Validated” status in early 2014.

Description: [1]  An extended Tracking Radar Echo by Correlation (TREC) technique, called T-TREC technique, has been developed recently to retrieve horizontal circulations within tropical cyclones (TCs) from single Doppler radar reflectivity ( Z ) and radial velocity ( V r , when available) data. This study explores, for the first time, the assimilation of T-TREC-retrieved winds for a landfalling typhoon, Meranti (2010), into a convection-resolving model, the WRF (Weather Research and Forecasting). The T-TREC winds or the original V r data from a single coastal Doppler radar are assimilated at the single time using the WRF 3DVAR, at 8, 6, 4 and 2 hours before the landfall of typhoon Meranti. In general, assimilating T-TREC winds results in better structure and intensity analysis of Meranti than directly assimilating V r data. The subsequent forecasts for the track, intensity, structure and precipitation are also better, although the differences becomes smaller as the V r data coverage improves when the typhoon gets closer to the radar. The ability of the T-TREC retrieval in capturing more accurate and complete vortex circulations in the inner-core region of TC is believed to be the primary reason for its superior performance over direct assimilation of V r data; for the latter, the data coverage is much smaller when the TC is far away and the cross-beam wind component is difficult to analyze accurately with 3DVAR method.

Description: [1]  Given its large population, vigorous and water-intensive agricultural industry, and important ecological resources, the western United States presents a valuable case study for examining potential near-term changes in regional hydroclimate. Using a high-resolution, hierarchical, five-member ensemble modeling experiment that includes a global climate model (CCSM), a regional climate model (RegCM), and a hydrological model (VIC), we find that increases in greenhouse forcing over the next three decades result in an acceleration of decreases in spring snowpack and a transition to a substantially more liquid-dominated water resources regime. These hydroclimatic changes are associated with increases in cold-season days above freezing and decreases in the cold-season snow-to-precipitation ratio. The changes in the temperature and precipitation regime in turn result in shifts toward earlier snowmelt, baseflow, and runoff dates throughout the region, as well as reduced annual and warm-season snowmelt and runoff. The simulated hydrologic response is dominated by changes in temperature, with the ensemble members exhibiting varying trends in cold-season precipitation over the next three decades, but consistent negative trends in cold-season freeze days, cold-season snow-to-precipitation ratio, and April 1 st snow water equivalent. Given the observed impacts of recent trends in snowpack and snowmelt runoff, the projected acceleration of hydroclimatic change in the western U.S. has important implications for the availability of water for agriculture, hydropower and human consumption, as well as for the risk of wildfire, forest die-off, and loss of riparian habitat.

Description: [1]  The consequences of different Quasi-Biennial Oscillation (QBO) nudging widths on stratospheric dynamics and chemistry are analyzed by comparing two model simulations with NCAR's WACCM model where the width of the QBO is varied between 22° and 8.5°N and S. The sensitivity to the nudging width is strongest in Northern Hemisphere (NH) winter where the Holton-Tan effect in the polar stratosphere, i.e., stronger zonal mean winds during QBO west phases, is enhanced for the wider compared to the narrower nudging case. The differences between QBO west and east conditions for the two model experiments can be explained with differences in wave propagation, wave-mean flow interaction and the residual circulation. In the wider nudging case a divergence anomaly in the mid-latitude upper stratosphere/lower mesosphere occurs together with an equatorward anomaly of the residual circulation. This seems to result in a strengthening of the meridional temperature gradient and hence a significant strengthening of the polar night jet (PNJ). In the narrower nudging case these circulation changes are weaker and not statistically significant, consistent with a weaker and less significant impact on the PNJ. Chemical tracers like ozone, water vapour and methane react accordingly. From a comparison of westerly minus easterly phase composite differences in the model to reanalysis and satellite data we conclude that the standard WACCM configuration (QBO22) generates more realistic QBO effects in stratospheric dynamics and chemistry during NH winter. Our study also confirms the importance of the secondary mean meridional circulation associated with the QBO for the Holton-Tan effect.

Description: [1]  The microphysical properties of ice fog were measured at two sites during a small IOP in January and February of 2012 in the Interior Alaska. The NCAR Video Ice Particle Sampler (VIPS) probe and formvar (polyvinyl formal) coated microscope slides were used to sample airborne ice particles at two polluted sites in the Fairbanks region. Both sites were significantly influenced by anthropogenic emission and additional water vapor from nearby open water power plant cooling ponds. Measurements show that ice fog particles were generally droxtal-shaped (faceted, quasi-spherical) for sub-10 µm particles, while plate shaped crystals were the most frequently observed particles between 10 and 50 µm. A visibility cutoff of 3 kilometers was used to separate ice fog events from other observations which were significantly influenced by larger (50–150 µm) diamond dust particles. [2]  The purpose of this study is to more realistically characterize ice fog microphysical properties in order to facilitate better model predictions of the onset of ice fog in polluted environments. Parameterizations for mass and projected area are developed and used to estimate particle terminal velocity. Dimensional characteristics are based on particle geometry and indicated that ice fog particles have significantly lower density values than water droplets as well as reduced cross-sectional areas, the net result being that terminal velocities are estimated to be less than half the value of those calculated for water droplets. Particle size distributions are characterized using gamma functions and have a shape factor (μ) of between −0.5 and −1.0 for polluted ice fog conditions.

Description: [1]  In our recent paper [6], the sensitivity of infrasound to the upper atmosphere is investigated using impulsive signals from the Tungurahua volcano in Ecuador. We reported on the coherent variability of thermospheric travel times, with periods equal to those of the tidal harmonics. Moreover, it was shown that the error in predicted thermospheric travel time is in accord with typical uncertainties in the upper atmospheric wind speed models. Given the observed response of the infrasound celerities to upper atmospheric tidal variability, it was suggested that infrasound observations may be used to reduce uncertainty in the knowledge of the atmospheric specifications in the upper atmosphere. In this paper, we discuss the estimation of upper atmospheric wind model updates from the infrasound data described in the aforementioned paper. The parameterization of the model space by empirical orthogonal functions is described; it is found that the wind model in the upper mesosphere and lower thermosphere can be described by a four-parameter model. Due to the small dimensionality of the model space, a grid search method can be used to solve the inverse problem. A Bayesian method is used to assess the uncertainty in the inverse solution given the a priori uncertainty in the data and model spaces and the non-linearity of the inverse problem at hand. We believe that this is the first study in which such methods are applied to real infrasound data, allowing for a rigorous analysis of this inverse problem. It is found that the complexity of the a posteriori model distribution increases for a larger dimensional model space and larger uncertainties in the data. A case study is presented in which the non-linear propagation from source to receiver is simulated using an updated wind model and non-linear ray theory. As non-linear propagation effects further constrain the propagation path, this is a way to check the physical self-consistency of the travel time inversion approach. We obtain excellent agreement between the simulated and observed waveforms.

Description: [1]  Thirty years of balloon-borne measurements over Boulder (40 ∘ N, 105 ∘ W) are used to investigate the water vapor trend in the tropopause region. This analysis extends previously published trends, usually focusing on altitudes greater than 16km, to lower altitudes. Two new concepts are applied: 1) Trends are presented in a thermal tropopause (TP) relative coordinate system from − 2km below to 10km above the TP. 2) Sonde profiles are selected according to TP height. Tropical (TP z  〉 14km), extratropical (TP z  〈 12km), and transitional air mass types (12km 〈 TP z  〈 14km), reveal three different water vapor reservoirs. The analysis based on these concepts reduces the dynamically–induced water vapor variability at the TP and principally favors refined water vapor trend studies in the upper troposphere and lower stratosphere. Nonetheless, this study showshow uncertain trends are at altitudes −2 to + 4km around the TP. This uncertainty in turn has an influence on the uncertainty and interpretation of water vapor radiative effects at the TP, which are locally estimated for the 30 year period to be of uncertain sign. The much discussed decrease in water vapor at the beginning of 2001 is not detectable between −2 to 2km around the TP. On lower stratospheric isentropes, the water vapor change at the beginning of 2001 is more intense for extratropical than for tropical air mass types. This suggests a possible link with changing dynamics above the jet stream such as changes in the shallow branch of the Brewer–Dobson circulation.

Description: [1]  Retrievals of sulfur dioxide (SO 2 ) from space-based spectrometers are in a relatively early stage of development. Factors such as interference between ozone and SO 2 in the retrieval algorithms often lead to errors in the retrieved values. Measurements from the Ozone Monitoring Instrument (OMI), SCanning Imaging Absorption spectroMeter for Atmospheric CHartographY (SCIAMACHY), and Global Ozone Monitoring Experiment −2 (GOME-2) satellite sensors, averaged over a period of several years, were used to identify locations with elevated SO 2 values and estimate their emission levels. About 30 such locations, detectable by all three sensors and linked to volcanic and anthropogenic sources, were found, after applying low- and high- spatial frequency filtration designed to reduce noise and bias and to enhance weak signals to SO 2 data from each instrument. Quantitatively, the mean amount of SO 2 in the vicinity of the sources, estimated from the three instruments is in general agreement. However, it's better spatial resolution makes it possible for OMI to detect smaller sources and with additional more detail as compared to the other two instruments. Over some regions of China, SCIAMACHY and GOME-2 data show mean SO 2 values that are almost 1.5 times higher than those from OMI but the suggested spatial filtration technique largely reconciles these differences.

Description: [1]  Dust is an important indicator of climate change. In paleo-climate research, sediments bearing signals of dust deposition offer a rich archive for climate-change history. However, the dust-climate link is very complex due to the various direct and indirect feedbacks in the Earth system. In this study, we examine two issues: (1) given the recent global warming, what are the dust variations, both globally and in key dust regions? and, (2) what are the climate drivers behind the variations? Using synoptic data for the period 1974-2012, we analyzed the global trend of dust frequency and visibility-derived dust concentrations and their characteristics in key dust regions, including North Africa, the Middle East, Southwest Asia, Northeast Asia, South America and Australia. We also examined the likely climate drivers for dust variations in the different regions by computing the correlations between the time series of dust and of major climate indices – the MEI (Multivariate El Niño/Southern Oscillation Index), NAO (North Atlantic Oscillation) and AMO (Atlantic Multi-decadal Oscillation). It was found that over the period 1984–2012, the global-mean (excluding North America and Europe) near-surface dust concentration decreased at 1.2% yr -1 . This decrease is mainly due to reduced dust activities in North Africa, accompanied by reduced activities in Northeast Asia, South America, and South Africa. A significant negative correlation between Saharan dust and AMO was detected and it seems reasonable to suggest that under present climate, the global dust trend is determined by the climate systems governing the Atlantic and North African regimes.

Description: [1]  We show measurements of middle atmospheric water vapor as measured by two ground-based Water Vapor Millimeter-wave Spectrometer (WVMS) instruments, and three satellite-based instruments: the Aura Microwave Limb Sounder (MLS), the Atmospheric Chemistry Experiment (ACE), and the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS). We also show CH 4 measurements from the MIPAS and ACE instruments and use these to help interpret the H 2 O variations. We find that interannual changes in stratospheric H 2 O of ~0.5 ppmv, observed from Table Mountain, California at 26 km and 40 km from 2010–2012, are caused primarily by dynamically driven changes in CH 4 during this period. The interannual variations in H 2 O observed over Mauna Loa, Hawaii, are shown to be quite similar to the average variations observed over 50 o S-50 o N in the lower mesosphere, thus we conclude that a single ground-based microwave instrument can provide a useful estimate of interannual globally averaged lower mesospheric H 2 O variations, even when such changes are as small as ~0.2-0.3 ppmv. We find that the increase of ~0.2-0.3 ppmv in H 2 O in the lower mesosphere since 2006 is qualitatively consistent with an increase in tropical tropopause temperature since ~2001.

Description: [1]  An adjoint version of the HARC neighborhood air quality model with 200 m horizontal resolution, coupled offline to the QUIC-URB fast response urban wind model, was used to perform 4D variational (4Dvar) inverse modeling of an industrial release of formaldehyde (HCHO) and sulfur dioxide (SO 2 ) in Texas City, Texas during the 2009 Study of Houston Atmospheric Radical Precursors (SHARP). The source attribution was based on real-time observations by the Aerodyne mobile laboratory and a high resolution 3D digital model of the emitting petrochemical complex and surrounding urban canopy. The inverse model estimate of total primary HCHO emitted during the incident agrees very closely with independent remote sensing estimates based on both Imaging and Multi-Axis Differential Optical Absorption Spectroscopy (DOAS). Whereas a previous analysis of Imaging DOAS data attributed the HCHO release to a Fluidized Catalytic Cracking Unit (FCCU), the HARC model attributed most of the HCHO and SO 2 release to a neighboring hydrotreater unit that desulfurizes the feed to the FCCU. Other facilities implicated by the source attribution were ultraformer and distillate desulfurization units and two flares. The inferred HCHO-to-SO 2 molar emission ratio was similar to that computed directly from ambient air measurements during the release. The model-estimated HCHO-to-CO molar emission ratio for combustion units with significant inferred emissions ranged from 2% to somewhat less than 7%, consistent with other observationally-based estimates obtained during SHARP. A model sensitivity study demonstrated that the inclusion of urban morphology has a significant, but not critical, impact on the source attribution.

Description: [1]  Carbonaceous aerosols have the potential to impact climate directly through absorption of incoming solar radiation and indirectly by affecting cloud and precipitation. To quantify this impact, recent modeling studies have made great efforts to simulate both the spatial and temporal distribution of carbonaceous aerosol's optical properties and associated radiative forcing. This study makes the first observationally constrained assessment of the direct radiative forcing of carbonaceous aerosols over California. By exploiting multiple observations (including ground sites and satellites), we constructed the distribution of aerosol optical depths and aerosol absorption optical depths (AAOD) over California for a ten-year period (2000–2010). We partitioned the total solar absorption into individual contributions from elemental carbon (EC), organic carbon (OC), and dust aerosols, using a newly developed scheme. Our results show that AAOD due to carbonaceous aerosols (EC and OC) at 440 nm was 50%–200% larger than natural dust, with EC contributing the bulk (70%–90%). Observationally constrained EC absorption agrees reasonably well with estimates from global and regional chemical transport models, but the models underestimate the OC AAOD by at least 50%. We estimated that the top of the atmosphere (TOA) forcing from carbonaceous aerosols was 0.7 W/m 2 and the TOA forcing due to OC was close to zero. The atmospheric heating of carbonaceous aerosol was 2.2–2.9 W/m 2 , of which EC contributed about 80–90%. We estimated the atmospheric heating of OC at 0.1–0.4 W/m 2 , larger than model simulations. EC reduction over the last two decades may have caused a surface brightening of 1.5–3.5 W/m 2 .

Description: [1]  The tropospheric seasonal cycles of N 2 O, CFC-11 (CCl 3 F), and CFC-12 (CCl 2 F 2 ) are influenced by atmospheric dynamics. The interannually varying summertime minima in mole fractions of these trace gases have been attributed to interannual variations in mixing of stratospheric air (depleted in CFCs and N 2 O) with tropospheric air with a few months lag. The amount of wave activity that drives the stratospheric circulation and influences the winter stratospheric jet and subsequent mass transport across the tropopause appears to be the primary cause of this interannual variability. We relate the observed seasonal minima of species at three Northern Hemisphere sites (Mace Head, Ireland; Trinidad Head, USA and Barrow, Alaska) with the behavior of the winter stratospheric jet. As a result, a good correlation is obtained between zonal winds in winter at 10 hPa, 58 o N-68 o N and the de-trended seasonal minima in the stratosphere-influenced tracers. For these three tracers, individual Pearson correlation coefficients (r) between 0.51 and 0.71 were found, with overall correlations of between 0.67 and 0.77 when ‘composite species’ were considered. Finally we note that the long-term observations of CFCs and N 2 O in the troposphere provide an independent monitoring method complementary to satellite data. Furthermore they could provide a useful observational measure of the strength of stratosphere-troposphere exchange and thus, could be used to monitor any long-term trend in the Brewer-Dobson Circulation which is predicted by climate models to increase over the coming decades.

Description: [1]  Monthly temperature and precipitation data from 41 global climate models (GCMs) of the Coupled Model Intercomparison Project Phase 5 (CMIP5) were compared to observations for the 20 th century, with a focus on the United States Pacific Northwest (PNW) and surrounding region. A suite of statistics, or metrics, was calculated, that included correlation and variance of mean seasonal spatial patterns, amplitude of seasonal cycle, diurnal temperature range, annual- to decadal- scale variance, long-term persistence, and regional teleconnections to ENSO. Performance, or credibility, was assessed based on the GCMs’ abilities to reproduce the observed metrics. GCMs were ranked in their credibility using two methods. The first simply treated all metrics equally. The second method considered two properties of the metrics: 1) redundancy of information (dependence) among metrics, and 2) confidence in the reliability of an individual metric for accurately ranking models. Confidence was related to how robust the estimate of the metric was to ensemble size, given that for most of the models only a small number of ensemble members (i.e. realizations of the 20 th century) were available. A cursory comparison with 24 CMIP3 models revealed few differences between the two generations of models with respect to the statistics analyzed.

Description: [1]  In this paper, the effects of temperature, pressure, winds, moisture and molecular content on the propagation of blast waves at high altitudes are investigated. These cause refractions and attenuations which modify the recorded ground overpressures from the ideal predictions. By coupling these effects together, the non-ideal corrections to the overpressures are estimated by applying approximations which are dependent on the angle of propagation of the blast wave.

Description: [1]  The most severe thunderstorms, producing extreme precipitation, occur over subtropical and midlatitude regions. Atmospheric conditions conducive to organized, intense thunderstorms commonly involve the coupling of a low-level jet (LLJ) with a synoptic short wave. The midlatitude synoptic activity is frequently modulated by the circumglobal teleconnection (CGT) – in which meridional gradients of the jet stream act as a guide for short Rossby waves. Previous research has linked extreme precipitation events with either the CGT or the LLJ, but has not linked the two circulation features together. In this study, a circulation-based index was developed by combining (a) the degree of the CGT and LLJ coupling, (b) the extent to which this CGT-LLJ coupling connects to regional precipitation, and (c) the spatial correspondence with the CGT (short-wave) trending pattern over the most recent 32 years (1979-2010). Four modern-era global reanalyses, in conjunction with four gridded precipitation datasets, were utilized to minimize spurious trends. The results are suggestive of a link between several recent extreme precipitation events and the CGT/LLJ trends, including those leading to the 2008 Midwest flood in U.S., the 2011 tornado outbreaks in southeastern U.S., the 2010 Queensland flood in northeastern Australia and the 2010 Pakistan flood. Moreover, an analysis of three CMIP5 models from the historical experiments points to the role of greenhouse gases in forming the CGT trends during the warm season.

Description: [1]  Mineral aerosols are produced during the erosion of soils by wind and are a common component of the total atmospheric particle load (dust) in arid and semi-arid regions. The size of these particles can vary widely from clay particles less than 2 µm to larger silt and sand-sized particles that can exceed 50 µm in diameter. In this study, we present two continuous records of total suspended particle (TSP) concentrations at sites in Mesa Verde and Canyonlands National Parks in Colorado and Utah, USA, respectively, and compare those values to measurements of fine and coarse particle concentrations made from nearby IMPROVE-network samplers. Average annual concentrations of TSP at Mesa Verde were 90 µg m -3 in 2011 and at Canyonlands were 171 µg m -3 in 2009, 113 µg m -3 in 2010, and 134 µg m -3 in 2011. In comparison, annual concentrations of fine (diameter of 2.5 µm and below) and coarse (2.5-10 µm diameter) particles at these sites were well below 10 µg m -3 in all years. The high concentrations of TSP appear to be the result of regional dust storms that result in elevated concentrations of very coarse particles greater than 10 µm in diameter. These conditions regularly occur from early spring until early fall with two-week mean TSP periodically in excess of 200 µg m -3 at Canyonlands. Measurement of particle sizes embedded on filters indicates that the median particle size of all particles varies between approximately 10 µm in winter and 40 µm during the spring dust-storm season. These persistently elevated concentrations of large particles indicate that regional dust emission as dust storms and events are important determinants of air quality in this region.

Description: [1]  Nitrogen oxide radicals (NO x ) produced by lightning are natural precursors for the production of the dominant tropospheric oxidants, OH and ozone. Observations of the interannual variability (IAV) of tropical ozone and of global mean OH (from the methylchloroform proxy) offer a window for understanding the sensitivity of ozone and OH to environmental factors. We present the results of simulations for 1998-2006 using the GEOS-Chem chemical transport model with IAV in tropical lightning constrained by satellite observations from the Lightning Imaging Sensor (LIS). We find that this imposed IAV in lightning NO x improves the ability of the model to reproduce observed IAV in tropical ozone and OH. Lightning is far more important than biomass burning in driving the IAV of tropical ozone, even though the IAV of NO x emissions from fires is greater than that from lightning. Our results indicate that the IAV in tropospheric OH is highly sensitive to lightning relative to other emissions, and suggest that lightning contributes an important fraction of the observed IAV in OH inferred from the methylchloroform proxy. Lightning affects OH through the HO 2 +NO reaction, an effect compounded by positive feedback from the resulting increase in ozone production and in CO loss. We can account in the model for the observed increase in OH in 1998-2004 and for its IAV, but the model fails to explain the OH decrease in 2004-2006. We find that stratospheric ozone plays little role in driving IAV in OH during 1998-2006, in contrast to previous studies that examined earlier periods.

Description: [1]  The gravity wave field in the lower stratosphere (between 15 km and 22 km altitude) above Mount Pleasant Airport (51°49’S, 58°26’W), on the Falkland Islands is studied using over 2100 high-resolution radiosonde soundings from 2002-2010. The seasonal variation in vertical direction of propagation shows a small decrease in numbers of upward propagating waves that is related to critical level filtering, however there is a very large increase in numbers of downward propagating waves between July and September; this is attributed to the proximity of the edge of the polar vortex. There is a seasonal variation in gravity wave energy density, with a large peak during the austral autumn equinox; this is markedly different to results in the literature both from Rothera, on the Antarctic Peninsula, and stations on the main Antarctic continent. This seasonal pattern has been shown to be linked to variations in the sources of upward propagating gravity waves. The seasonal variation in gravity wave characteristics above Mount Pleasant Airport seen in our results suggest that the gravity wave field in this region is determined by a combination of different gravity wave sources located above and below the lower stratosphere.

Description: [1]  Observed evaporative demand has decreased worldwide during the past several decades. This trend is also noted on the Tibetan Plateau, a region that is particularly sensitive to climate change. However, patterns and trends of evapotranspiration and their relationship to drought stress on the Tibetan Plateau are complex and poorly understood. Here, we analyze spatiotemporal changes in evapotranspiration and effective moisture (defined as the ratio of actual evapotranspiration ( ET a ) to reference crop evapotranspiration ( ET o )) based on the modified Lund–Potsdam–Jena Dynamic Global Vegetation Model (LPJ). Climate data from 80 meteorological stations on the Tibetan Plateau were compiled for the period 1981–2010 and future climate projections were generated by a regional climate model through the 21 st century. The results show regional trends towards decreasing ET o and statistically significant increases in ET a ( p  〈 0.05) and effective moisture during the period 1981–2010 ( p  〈 0.001). A transition from significant negative to positive ET o occurred in 1997. Additionally, a pronounced increase in effective moisture occurred during the period 1981–1997 because of significant decreased ET o before 1997. In the future, regional ET o and ET a are projected to increase, thus reducing drought stress, because of generally increased effective moisture. Future regional differences are most pronounced in terms of effective moisture, which shows notable increases in the northwestern plateau and decreases in the southeastern plateau. Moreover, the reduced magnitude of effective moisture is likely to intensify in the long term, due mainly to increased evaporative demand.

Description: [1]  Changes in the hydrological cycle being caused by human-induced global warming are triggering variations in observed spatiotemporal distributions of precipitation and temperature extremes, and hence in droughts and floods across China. Evaluation of future climate extremes based on General Circulation Models (GCMs) outputs will be of great importance in scientific management of water resources and agricultural activities. In this study, 5 precipitation extreme and 5 temperature extreme indices are defined. This study analyzes daily precipitation and temperature data for 1960-2005 from 529 stations in China and outputs of GCMs from the Coupled Model Intercomparison Project Phase 3 (CMIP3) and Phase 5 (CMIP5). Downscaling methods, based on QQ-plot and transfer functions, are used to downscale GCMs outputs to the site scale. Performances of GCMs in simulating climate extremes were evaluated using the Taylor diagram. Results showed that: (1) the multimodel CMIP5 ensemble performs the best in simulating observed extreme conditions; (2) precipitation processes are intensifying with increased frequency and intensity across entire China. The southwest China, however, is dominated by lengthening maximum consecutive dry days and also more heavy precipitation extremes; (3) warming processes continue with increasing warm nights, decreasing frost days and lengthening heat waves during the 21 st century; (4) changes in precipitation and temperature extremes exhibit larger changing magnitudes under RCP85 scenario; (5) for the evolution of changes in extremes, in most cases, the spatial pattern keeps the same, even though changing rates vary. In some cases, area with specific changing properties extends or shrinks gradually. The directions of trends may alter during the evolution; and (5) changes under RCP85 become more and more pronounced as time elapses. Under the peak-and-decline RCP26, changes in some cases don't decrease correspondingly during 2070-2099 even though the radiative forcing during 2070-2099 is less than during 2040-2069. The increase of radiative forcing triggers considerable regional variations in consecutive dry days, but causes only slight changes in the areal average in China. The results of this study imply higher flood risk across entire China but intensifying droughts in south China in the 21 st century, and also more heat-related losses in east coasts of China.

Description: [1]  Marine cloud brightening through sea spray injection has been proposed as a method of temporarily alleviating some of the impacts of anthropogenic climate change, as part of a set of technologies called geoengineering. We outline here a proposal for three coordinated climate modeling experiments to test aspects of sea spray geoengineering, to be conducted under the auspices of the Geoengineering Model Intercomparison Project (GeoMIP). The first, highly idealized, experiment ( G1ocean-albedo ) involves a uniform increase in ocean albedo to offset an instantaneous quadrupling of CO 2 concentrations from preindustrial levels. Results from a single climate model show an increased land-sea temperature contrast, Arctic warming, and large shifts in annual mean precipitation patterns. The second experiment ( G4cdnc ) involves increasing cloud droplet number concentration in all low-level marine clouds to offset some of the radiative forcing of an RCP4.5 scenario. This experiment will test the robustness of models in simulating geographically heterogeneous radiative flux changes and their effects on climate. The third experiment ( G4sea-salt ) involves injection of sea spray aerosols into the marine boundary layer between 30°S and 30°N to offset 2 W m -2 of the effective radiative forcing of an RCP4.5 scenario. A single model study shows that the induced effective radiative forcing is largely confined to the latitudes in which injection occurs. In this single model simulation, the forcing due to aerosol–radiation interactions is stronger than the forcing due to aerosol–cloud interactions.

Description: [1]  The problem of diurnal variation in surface emissivity over the Sahara Desert during non-raining days is studied and assessed with observations from the Infrared Atmospheric Sounding Interferometer (IASI). The analysis has been performed over a Sahara Desert dune target area during July 2010. Spinning Enhanced Visible and Infrared Imager observations from the European geostationary platform Meteosat-9 (Meteorological Satellite 9) have been also used to characterize the target area. Although the amplitude of this daily cycle has been shown to be very small, we argue that suitable nighttime meteorological conditions and the strong contrast of the reststrahlen absorption bands of quartz (8–14 μ m) can amplify its effect over the surface spectral emissivity. The retrieval of atmospheric parameters show that at nighttime an atmospheric temperature inversion occurs close to the surface yielding a thin boundary layer which acts like a lid, keeping normal convective overturning of the atmosphere from penetrating through the inversion. This mechanism traps water vapour close to the land and drives the direct adsorption of water vapour at the surface during the night. The diurnal variation in emissivity at 8.7 μ m has been found to be as large as 0.03 with high values at night and low values during the day. At 10.8 μ m and 12 μ m the variation has the same sign as that at 8.7 μ m, but with a smaller amplitude, 0.019 and 0.014, respectively. The impact of these diurnal variations on the retrieval of surface temperature and atmospheric parameters has been analyzed.

Description: [1]  Retrieval of aerosol optical depth (AOD) from the Moderate Resolution Imaging Spectroradiometer (MODIS) using the Collection 5 (C005) algorithm provides large-scale (10x10 km) estimates that can be used to predict surface layer concentrations of particulate matter with aerodynamic diameter smaller than 2.5 microns (PM 2.5 ). However, these large-scale estimates are not suitable for identifying intra-urban variability of surface PM 2.5 concentrations during wildfire events when individual plumes impact populated areas. We demonstrate a method for providing high-resolution (2.5 km) kernel-smoothed estimates of AOD over California during the 2008 northern California fires. The method uses high-resolution surface reflectance ratios of the 0.66 µm and 2.12 µm channels, a locally derived aerosol optical model characteristic of fresh wildfire plumes, and a relaxed cloud filter. Results show that the AOD derived for the 2008 northern California fires outperformed the standard product in matching observed aerosol optical thickness (AOT) at three coastal AERONET sites and routinely explained more than 50% of the variance in hourly surface PM 2.5 concentrations observed during the wildfires.

Description: [1]  An observational study of nonlinear interaction between the quasi two-day wave (QTDW) and the diurnal and semidiurnal tides from meteor radar measurements at Maui is reported. The diurnal and semidiurnal tides show a short-term variation with the QTDW activity. The variation of amplitude of the semidiurnal tide is opposite to that of the QTDW. The minimum amplitudes of the diurnal tide appear several days later than the maximum amplitudes of the QTDW, and the diurnal tide obviously strengthens when the QTDW drops to small amplitudes. The bispectrum analysis shows significant nonlinear interactions among the QDTW and the tidal components. The two quasi 16-h modes with periods of 16.2 h and 15.8 h generated in the interactions of the QTDW with the diurnal and semidiurnal tides can clearly be distinguished because of the slight deviation of the QTDW period from 48 h. The bicoherence spectrum demonstrates that the QTDW and the semidiurnal tide have quite strong levels of coherence, indicating that the nonlinear interaction is a mechanism responsible for the variability of the semidiurnal tide. Although there is also some interaction between the QTDW and the diurnal tide, their coherence level is low. When the QTDW drops to very weak amplitudes, the background wind decreases and reverses. During this time, the diurnal tide holds large amplitudes. These results support the notion that the variability of the diurnal tide is mainly attributable to the strong QTDW-induced changes in the background atmosphere, which was shown in the modeling study by Chang et al . [2011]. Hence, both the nonlinear interaction and the background flow changes are responsible for the observed variation of the diurnal tide.

Description: [1]  We conducted three-member ensemble simulations using a global atmospheric model with a high-horizontal resolution of a 60-km grid size for the period 1872-2099 (228 years). Between 1872 and 2005, the model was forced with observed historical sea surface temperatures (SST), while between 2006 and 2099 the boundary SST data were estimated using the multi-model ensemble (MME) of the Coupled Model Intercomparison Project phase 3 (CMIP3) models and assuming A1B emission scenario. Annual mean precipitation (PAVE), the Simple Daily Precipitation Intensity Index (SDII), and the maximum 5-day precipitation total (R5d) averaged over East Asia increase almost monotonically through the 21st century. The statistically significant area of precipitation intensity increase is larger for 2080-2099 than for 2046-2065. In particular, intense rainfall will increase over northern and southern China during 2080-2099. The conversion rate from water vapor to precipitation per 1 °C rise in surface air temperature for SDII and R5D is much larger than that for PAVE during the 21st century. This suggests that extreme rainfall events will occur more frequently than moderate rainfall events even if the amount of temperature rise is same. Future changes in the horizontal transport of water vapor also leads to more intense precipitation over East Asia. In particular, the increase in clockwise water vapor transport due to intensification of the subtropical high contributes to increased intense precipitation over southern China.

Description: [1]  An aircraft field study [POST; Physics Of Stratocumulus Top] was conducted off the central California Coast in July and August of 2008 to deal with the known difficulty of measuring entrainment rates in the radiatively important stratocumulus [Sc] prevalent in that area. The CIRPAS Twin Otter research aircraft flew 15 quasi-Lagrangian flights in unbroken Sc and carried a full complement of probes including three high data-rate probes UFT-M [Ultra-Fast Temperature probe], PVM [Particulate Volume Monitor] probe, and gust probe. The probes’ co-location near the nose of the Twin Otter permitted estimation of entrainment fluxes and rates with an incloud resolution of 1 m. [2]  Results include the following: Application of the conditional-sampling variation of classical mixed layer theory for calculating the entrainment rate into cloud top for POST flights is shown to be inadequate for most of the Sc. Estimated rates resemble previous results after theory is modified to take into account both entrainment and evaporation at cloud top given the strong wind shear and mixing at cloud top. Entrainment rates show a tendency to decrease for large shear values, and the largest rates are for the smallest temperature jumps across the inversion. Measurements indirectly suggest that entrained parcels are primarily cooled by infrared flux divergence rather than cooling from droplet evaporation, while detrainment at cloud top causes droplet evaporation and cooling in the EIL [Entrainment Interface Layer] above cloud top.

Description: [1]  We present new measurements of the time-dependence of the ice nucleating ability of a wide range of materials including the minerals montmorillonite and kaolinite, the biological proxy ice nuclei Icemax, and flame soot generated from the incomplete combustion of ethylene gas. We also present time-dependence for ambient ice nuclei collected from rain water samples. Our data show that the time-dependence for all materials studied here is weak, suggesting that the modified singular approximation is valid over the range of times and temperatures encountered for mixed-phase clouds.

Description: [1]  Attributing changes in extreme daily precipitation to global warming is difficult, even when based on global climate model simulations or statistical trend analyses. The question about trends in extreme precipitation and their causes has been elusive because of climate models’ limited precision and the fact that extremes are both rare and occur at irregular intervals. Here a newly discovered empirical relationship between the wet-day mean and percentiles in 24-hr precipitation amounts was used to show that trends in the wet-day 95th percentiles world-wide have been influenced by the global mean temperature, consistent with an accelerated hydrological cycle caused by a global warming. A multiple regression analysis was used as a basis for an attribution analysis by matching temporal variability in precipitation statistics with the global mean temperature.

Description: [1]  We implement a new isoprene oxidation mechanism in a global 3-D chemical transport model (GEOS-Chem). Model results are evaluated with observations for ozone, isoprene oxidation products, and related species from the ICARTT aircraft campaign over the eastern United States in summer 2004. The model achieves an unbiased simulation of ozone in the boundary layer and the free troposphere, reflecting canceling effects from recent model updates for isoprene chemistry, bromine chemistry, and HO 2 loss to aerosols. Simulation of the ozone-CO correlation is improved relative to previous versions of the model and this is attributed to a lower and reversible yield of isoprene nitrates, increasing the ozone production efficiency (OPE) per unit of nitrogen oxides (NO x  ≡ NO + NO 2 ). The model successfully reproduces the observed concentrations of organic nitrates (∑ANs) and their correlations with HCHO and ozone. ∑ANs in the model is principally composed of secondary isoprene nitrates, including a major contribution from nighttime isoprene oxidation. The correlations of ∑ANs with HCHO and ozone then provide sensitive tests of isoprene chemistry and argue in particular against a fast isomerization channel for isoprene peroxy radicals. ∑ANs can provide an important reservoir for exporting NO x from the US boundary layer. We find that the dependence of surface ozone on isoprene emission is positive throughout the US, even if NO x emissions are reduced by a factor of 4. Previous models showed negative dependences that we attribute to erroneous titration of OH by isoprene.

Description: [1]  The extent to which the rain rate from shallow, liquid-phase clouds is microphysically influenced by aerosol, and therefore drop concentration N d perturbations, is addressed through analysis of the precipitation susceptibility, S o . Previously published work, based on both models and observations, disagrees on the qualitative behavior of S o with respect to variables such as liquid water path L or the ratio between accretion and autoconversion rates. Two primary responses have emerged: (i) S o decreases monotonically with increasing L and (ii) S o increases with L , reaches a maximum, and decreases thereafter. Here we use a variety of modeling frameworks ranging from box models of (size-resolved) collision-coalescence, to trajectory ensembles based on large eddy simulation to explore the role of time available for collision-coalescence t c in determining the S o response. The analysis shows that an increase in t c shifts the balance of rain production from autoconversion (a N d -dependent process) to accretion (roughly independent of N d ), all else (e.g., L ) equal. Thus with increasing cloud contact time warm rain production becomes progressively less sensitive to aerosol, all else equal. When the time available for collision-coalescence is a limiting factor, S o increases with increasing L whereas when there is ample time available, S o decreases with increasing L . The analysis therefore explains the differences between extant studies in terms of an important precipitation-controlling parameter, namely the integrated liquid water history over the course of an air parcel's contact with a cloud.

Description: [1]  In previous studies, the Yonsei University (YSU) planetary boundary layer (PBL) scheme implemented in the Weather Research and Forecasting (WRF) model was reported to perform less well at night, while performing better during the day. Compared to observations, predicted nocturnal low-level jets (LLJs) were typically weaker and higher. Also, the WRF model with Chemistry (WRF/Chem) with the YSU scheme was reported to sometimes overestimate near-surface ozone (O 3 ) concentration during the nighttime. The updates incorporated in WRF version 3.4.1, include modifications of the nighttime velocity scale used in the YSU boundary-layer scheme. The impacts of this update on the prediction of nighttime boundary layers and related implications for wind resource assessment and air quality simulations are examined in this study. The WRF/Chem model with the updated YSU scheme predicts smaller eddy diffusivities in the nighttime boundary layer, and consequently lower and stronger LLJs over a domain focusing on the southern Great Plains area, showing a better agreement with the observations. As a result, related overestimation problems for near-surface temperature and wind speeds appear to be resolved, and the nighttime minimum near-surface O 3 concentrations are better captured. Simulated vertical distributions of meteorological and chemical variables for weak wind regimes (e.g., in the absence of LLJ) are less impacted by the YSU updates.

Description: [1]  We introduce novel methodology to examine the ability of six regional climate models (RCMs) in the North American Regional Climate Change Assessment Program (NARCCAP) ensemble to simulate past extreme precipitation events seen in the observational record over two different regions and seasons. [2]  Our primary objective is to examine the strength of daily correspondence of extreme precipitation events between observations and the output of both the RCMs and the driving reanalysis product. To explore this correspondence, we employ methods from multivariate extreme value theory. These methods require that we account for marginal behavior, and we first model and compare climatological quantities which describe tail behavior of daily precipitation for both the observations and model output before turning attention to quantifying the correspondence of the extreme events. Daily precipitation in a west coast region of North America is analyzed in two seasons, and it is found that the simulated extreme events from the reanalysis-driven NARCCAP models exhibit strong daily correspondence to extreme events in the observational record. Precipitation over a central region of the United States is examined, and we find some daily correspondence between winter extremes simulated by reanalysis-driven NARCCAP models and those seen in observations, but no such correspondence is found forsummer extremes. Furthermore, we find greater discrepancies among the NARCCAP models in the tail characteristics of the distribution of daily summer precipitation over this region than seen in precipitation over the west coast region. We find that the models which employ spectral nudging exhibit stronger tail dependence to observations in the central region.

Description: [1]  Atmospheric organic aerosol concentrations depend in part on the gas-particle partitioning of primary organic aerosol (POA) emissions. Consequently, heating and dilution were used to investigate the volatility of biomass burning smoke particles from combustion of common North American trees/shrubs/grasses during the third Fire Lab at Missoula Experiment (FLAME-III). Fifty to eighty percent of the mass of biomass burning POA evaporated when isothermally diluted from plume (~1000 µg m -3 ) to ambient-like concentrations (~10 µg m -3 ), while roughly eighty percent of the POA evaporated upon heating to 100 °C in a thermodenuder with a residence time of ~14 seconds. Therefore, the majority of the POA emissions were semi-volatile. Thermodenuder measurements performed at three different residence times indicated that there were not substantial mass transfer limitations to evaporation (i.e., the mass accommodation coefficient appears to be between 0.1 and 1). An evaporation kinetics model was used to derive volatility distributions and enthalpies of vaporization from the thermodenuder data. A single volatility distribution can be used to represent the measured gas-particle partitioning from the entire set of experiments, including different fuels, organic aerosol concentrations, and thermodenuder residence times. This distribution, derived from the thermodenuder measurements, also predicts the dilution-driven changes in gas-particle partitioning. This volatility distribution and associated emission factors for each fuel studied can be used to update emission inventories and to simulate the gas-particle partitioning of biomass burning POA emissions in chemical transport models.

Description: [1]  We investigate the variations in tropospheric circulation over Asia associated with the stratospheric quasi-biennial oscillation (QBO) during Northern Hemisphere (NH) autumn (September-November) using the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) Reanalysis and NCEP-Department of Energy (DOE) Reanalysis II data sets for a 25-year period (1980-2004). The anomaly fields in this study are expressed as the easterly years minus the westerly years of the QBO. The zonal wind distribution at 200 hPa indicates easterly anomalies at low latitudes from India through Taiwan, statistically significant westerly anomalies at mid-latitudes including the Tibetan Plateau to northern China, and significant easterly anomalies in the high-latitude Eurasia north of 50°N. A momentum balance analysis indicates that these anomalies over Asia are attributable to atmospheric waves and residual mean meridional flow. Significant northward and descending anomalies are observed in the lower stratosphere over Asia, with detectable signals in the troposphere around Tibet. The adiabatic heating due to descending motions in the region centering on India and Bangladesh is consistent with the distribution of rainfall anomalies. The formation of zonal wind anomalies over South Asia seems to be associated with not only wave activities but also adiabatic heating and thermal advection. We suggest that the QBO may be linked to tropospheric circulation over Asia through wave activities and convective activities in autumn, and the wave activities in the low latitudes associated with the QBO seem to be connected with the rainfall distribution over the Asian monsoon region.

Description: [1]  Subtropical cyclogenesis and tropical transitions (TT) over the South Atlantic Ocean only received attention after the first documented Hurricane Catarina occurred close to the Southern Brazilian coast in March 2004. However, due to the lack of studies in this part of Atlantic Ocean, it is still unclear what the main environmental conditions and dynamical processes associated with TT or even subtropical cyclogenesis are over the region. This study presents a synoptic and dynamical analysis of the Subtropical Cyclone Anita which occurred in March 2010 near Brazilian coast. This system started as a pure subtropical cyclone, evolved to a condition favorable to TT, later developed a cold-cored structure and decayed as an extratropical cyclone. During the period favorable for TT, the turbulent heat fluxes (latent plus sensible) from the ocean decreased and Anita started interacting with another extratropical disturbance preventing the TT to happen. This interaction, in turn, increased the vertical wind shear, let the extratropical transition to occur and promoted the westward displacement of Anita to colder waters thus decreasing the turbulent heat fluxes. The results suggest that the combination of a dipole blocking pattern aloft, with contribution from barotropic energy conversions, and strong turbulent fluxes are important ingredients for tropical storms development. Hybrid storms in such environmental conditions can be one form of precursors of hurricanes over the South Atlantic.

Description: [1]  The Suomi National Polar-orbiting Partnership (NPP) satellite was launched on October 28, 2011 and carries the Advanced Technology Microwave Sounder (ATMS) on board. ATMS is a cross-track scanning instrument observing in 22 channels at frequencies ranging from 23 to 183 GHz, permitting the measurements of the atmospheric temperature and moisture under most weather conditions. In this study, the ATMS radiometric calibration algorithm used in the operational system is first evaluated through independent analyses of prelaunch thermal vacuum data. It is found that the ATMS peak nonlinearity for all the channels are less than 0.5 K, which is well within the specification. For the characterization of the ATMS instrument sensitivity or noise equivalent differential temperatures (NEDT), both standard deviation and Allan variance of warm counts are computed and compared. It is shown that NEDT derived from the standard deviation is about three to five times larger than that from the Allan variance. It is shown that this difference results from a non-stationary component in the standard deviation of warm counts. The Allan variance is better suited than the standard deviation for describing NEDT. In the ATMS sensor brightness temperature data record (SDR) processing algorithm, the antenna gain efficiencies of main beam, cross-polarization beam and side lobes must be derived accurately from the antenna gain distribution function. However, uncertainties remain in computing the efficiencies at ATMS high frequencies. Thus, ATMS antenna brightness temperature data records (TDR) at channels 1 to 15 are converted to SDR with the actual beam efficiencies whereas those for channels 16 to 22 are only corrected for the near-field side-lobe contributions. The biases of ATMS SDR measurements to the simulations are consistent between GPS RO and NWP data and are generally less than 0.5 K for those temperature sounding channels where both the forward model and input atmospheric profiles are reliable.

Description: [1]  The operational ALADIN-France 3D-Var system is based on static background error covariances calculated off-line during a few-week past period. In this study, the impact of an on-line updated specification of background error covariances is evaluated in the ALADIN-France system. This evaluation is done by comparing three experiments, respectively based on (i) covariances calculated from a monthly average over a past period, (ii) covariances calculated from a monthly average over the period of study, and (iii) covariances calculated from a sliding daily average over the period of study. Firstly, it is shown through a comparison between experiments (i) and (ii) that updating the monthly average of error covariances has a positive impact on the short-range forecast quality. This is related to the specification of covariances which are more representative of average weather regimes at play during the period of study. Secondly, a comparison between experiments (ii) and (iii) indicates that additional positive impacts of a daily update of error covariances are also visible, although they tend to be somewhat localized and modest during this period. These impacts are illustrated by case studies for humidity during an anticyclonic situation, and for wind during a cyclonic event. These results support the idea to consider an on-line updated specification of background error covariances.

Description: [1]  On 5 August 2008, a localized heavy rainfall event caused a rapid increase in drainpipe discharge, which killed five people working in a drainpipe near Zoshigaya, Tokyo. This study compared the effects of artificial land cover and anthropogenic heat on this localized heavy rainfall event based on three ensemble experiments using a cloud-resolving model that includes realistic urban features. The first experiment (CTRL) considered realistic land cover and urban features, including artificial land cover, anthropogenic heat, and urban geometry. In the second experiment (NOAH), anthropogenic heat was ignored. In the third experiment (NOLC), urban heating from artificial land cover was reduced by keeping the urban geometry but with roofs, walls, and roads of artificial land cover replaced by shallow water. The results indicated that both anthropogenic heat and artificial land cover increased the amount of precipitation and that the effect of artificial land cover was larger than that of anthropogenic heat. However, in the middle stage of the precipitation event, the difference between the two effects became small. Weak surface heating in NOAH and NOLC reduced the near-surface air temperature and weakened the convergence of horizontal wind and updraft over the urban areas, resulting in a reduced rainfall amount compared with that in CTRL.

Description: [1]  Extratropical cyclones and their associated frontal systems are well known to be related to heavy precipitation events. Here an objective method is used to directly link extreme precipitation events with atmospheric fronts, identified using ECMWF Interim Reanalysis data, to quantify the importance of fronts for precipitation extremes globally. In some parts of the major midlatitude storm track regions, over 90% of precipitation extremes are associated with fronts, with slightly more events associated with warm fronts than cold fronts. On average 51% of global precipitation extremes are associated with fronts, with 75% in the midlatitudes and 31% in the tropics. A large proportion of extreme precipitation events occur in the presence of both a cyclone and a front, but remote fronts are responsible for many of the “front-only” events. The fronts producing extreme precipitation events are found to have up to 35% stronger frontal gradients than other fronts, potentially providing some improved forecasting capabilities for extreme precipitation events.

Description: [1]  The initial breakdown stage of 10 intracloud lightning flashes that may have produced terrestrial gamma-ray flashes (TGFs) are studied with wideband E-change, multi-band B-change, and VHF lightning mapping data; these flashes fit published criteria known to be associated with TGFs. The (x, y, z, t) locations of fast initial breakdown pulses (IBPs) were determined with E-change data using a time-of-arrival (TOA) technique. Each IBP includes one or more fast-rising sub-pulses. Previous research has shown that a typical intracloud flash initiates just above the main negative cloud charge (MNCC), then an initial negative leader propagates upward in 1 – 20 ms to the bottom of the upper positive cloud charge (UPCC), thereby establishing a conducting path between the MNCC and UPCC. TOA locations indicate that IBPs are directly related to the initial negative leader. The IBPs primarily occur in short (〈 750 µs) bursts of 2 – 5 pulses, and each burst produces a slow, monotonic E-change. Typically 1 – 3 IBP bursts are needed to span the vertical gap from the MNCC to the UPCC, with successive bursts separated by 1 – 5 ms. In the B-change data each IBP burst has an associated ULF pulse and several LF pulses, and these are caused by the same physical events that produce the slow, monotonic E-change and fast-rising IBP sub-pulses, respectively. Based on similarities with known TGF-associated signals, we speculate that a relativistic electron avalanche causes each LF pulse/IBP sub-pulse pair; thus each pair has the potential to cause a TGF.

Description: [1]  Stratospheric Sudden Warming (SSW) events are typically, but not always, accompanied by negative Northern Annular Mode anomalies in the troposphere. However, large uncertainties remain as to which dynamical processes are responsible for those anomalies.In order to highlight sources of variability in stratosphere-troposphere coupling amongst SSW events, we present a case study of three selected events and show detailed Transformed Eulerian Mean diagnostics for momentum changes in the stratosphere and troposphere in the course of those events. Our results suggest that planetary-scale waves, especially the zonal wavenumber two component, may not only play an important role for the onset of tropospheric anomalies in response to SSW events, but also for introducing variability in the vertical coupling, i.e., whether the tropospheric circulation anomalies lag, lead or occur simultaneous to the weakening of the vortex. Particularly, the meridional propagation of those waves in the upper troposphere could be an important factor that determines whether SSW events lag or lead tropospheric anomalies.

Description: [1]  Three years of gravity wave observations from the HIRDLS instrument on NASA's Aura satellite are examined. We produce estimates of the global distribution of gravity wave momentum flux as a function of individual observed wave packets. The observed distribution at the 25 km altitude level is dominated by the small proportion of wave packets with momentum fluxes greater than ~0.5 mPa. Depending on latitude and season, these wave packets only comprise ~7–25% of observations, but are shown to be almost entirely responsible for the morphology of the observed global momentum flux distribution. Large-amplitude wave packets are found to be more important over orographic regions than over flat ocean regions, and to be especially high in regions poleward of 40S during austral winter. The momentum flux carried by the largest packets relative to the distribution mean is observed to decrease with height over orographic wave generation regions, but to increase with height at tropical latitudes; the mesospheric intermittency resulting is broadly equivalent in both cases. Consistent with previous studies, waves in the top 10% of the extratropical distribution are observed to carry momentum fluxes more than twice the mean and waves in the top 1% more than 10× the mean, and the Gini coefficient is found to characterise the observed distributions well. These results have significant implications for gravity wave modelling.

Description: [1]  Aerosol optical properties simulated by the global 3-D tropospheric chemistry and transport model GEOS-Chem (GC) from 2008 to 2010 over the contiguous United States were evaluated with ground observations from Aerosol Robotic Network (AERONET) sites and aerosol products reported by the Multi-angle Imaging SpectroRadiometer (MISR). Overall, the correlation coefficient (r) and regression slope between AERONET and GC 2° x 2.5° (2 ° latitude x 2.5 ° longitude) daily total column aerosol optical depth (AOD) was 0.6 and 0.51, respectively. After using the nested GC 0.5° x 0.667° model to control for spatial variability, removing several outliers, and averaging over a monthly timescale, the agreement was significantly improved to an r of 0.84 and a slope of 0.75. Seasonal, hourly, and geographical statistics for GC 0.5° x 0.667° and AERONET AODs show a similar data range and variation, with higher mean values in the summer, the evening, and in the eastern U.S. Smaller correlation coefficients are seen in the summer and winter, in the evening, and in the western U.S. To investigate the optical properties of major GC tracers, MISR Level 2 aerosol products were used to calculate inorganic aerosol, dust, and absorbing non-dust AOD. Both GC and MISR suggest that, on average, inorganic aerosol has the highest AOD (GC: 0.071, MISR: 0.089) nationally, followed by absorbing non-dust species (GC: 0.025, MISR: 0.041), and dust (GC: 0.013, MISR: 0.014). The large discrepancies in our inter-comparison are due to GC underestimation of inorganic aerosol levels during all four seasons in the western U.S., and dust during summer in the eastern U.S., along with overestimation of summertime absorbing non-dust species over the northwestern U.S. These uncertainties are attributed to underestimation of inorganic aerosol emissions in more polluted western regions, the transport of Sahara dust in the summer, misuse of the fire files, and MISR retrieval uncertainties in the surface and choice of aerosol models.

Description: [1]  The eruption of the Icelandic volcano Eyjafjallajökull in the spring of 2010 lasted for 39 days with an explosive phase (14–18 April), an effusive phase (18 April–4 May) and a phase with renewed explosive activity (5–17 May). Images every 5 seconds from a camera mounted 34 km from the volcano are available for most of the eruption. Applying the maximum cross-correlation method (MCC) on these images, the velocity structure of the eruption cloud has been mapped in detail for four time intervals covering the three phases of the eruption. The results show that on average there are updrafts in one part of the cloud, and lateral motion or downdrafts in another. Even within the updraft part, there are alternating motions of strong updrafts, weak updrafts and downward motion. These results show a highly variable plume driven by intermittent explosions. The results are discussed in the context of integral plume models, and in terms of elementary parcel theory.

Description: [1]  The Madden–Julian oscillation (MJO) is the dominant mode of intraseasonal variability in tropical rainfall on the large scale, but its signal is often obscured in individual station data, where effects are most directly felt at the local level. The Fly River system, Papua New Guinea, is one of the wettest regions on Earth and is at the heart of the MJO envelope. 16-year time series of daily precipitation at 15 stations along the river system exhibit strong MJO modulation in rainfall. At each station, the difference in rainfall rate between active and suppressed MJO conditions is typically 40% of the station mean. The spread of rainfall between individual MJO events was small enough such that the rainfall distributions between wet and dry phases of the MJO were clearly separated, at the catchment level. This implies that successful prediction of the large-scale MJO envelope will have a practical use for forecasting local rainfall. In the steep topography of the New Guinea Highlands, the mean and MJO signal in station precipitation is twice that in the satellite TRMM 3B42HQ product, emphasising the need for ground truthing satellite-based precipitation measurements. A clear MJO signal is also present in the river level, which peaks simultaneously with MJO precipitation input in its upper reaches, but lags the precipitation by approximately 18 days on the flood plains.

Description: [1]  Many challenges remain for estimating the Antarctic ice sheet surface mass balance (SMB), which represents a major uncertainty in predictions of future sea-level rise. Validating continental scale studies is hampered by the sparse distribution of in-situ data. Here we present a 26-year mean SMB of the Fimbul ice shelf in East Antarctica between 1983–2009, and recent interannual variability since 2010. We compare these data to results of large-scale SMB studies for similar time periods, obtained from regional atmospheric modeling and remote sensing. Our in-situ data include ground penetrating radar, firn cores and mass balance stakes, and provide information on both temporal and spatial scales. The 26-year mean SMB on the Fimbul ice shelf varies between 170 and 620 kg m -2 a -1 giving a regional average value of 310 ±70 kg m -2 a -1 . Our measurements indicate higher long-term accumulation over large parts of the ice shelf compared to the large-scale studies. We also show that the variability of the mean annual SMB, which can be up to 90 %, can be a dominant factor in short-term estimates. The results emphasize the importance of using a combination of ground based validation data, regional climate models and remote sensing over a relevant time period in order to achieve a reliable SMB for Antarctica.

Description: [1]  A global model of sodium in the mesosphere and lower thermosphere has been developed within the framework of the National Center for Atmospheric Research's Whole Atmosphere Community Climate Model (WACCM). The standard fully-interactive WACCM chemistry module has been augmented with a chemistry scheme that includes 9 neutral and ionized sodium species. Meteoric ablation provides the source of sodium in the model and is represented as a combination of a meteoroid input function (MIF) and a parameterized ablation model. The MIF provides the seasonally and latitudinally varying meteoric flux which is modeled taking into consideration the astronomical origins of sporadic meteors and considers variations in particle entry angle, velocity,mass and the differential ablation of the chemical constituents. WACCM simulations show large variations in the sodium constituents over timescales from days to months. Seasonality of sodium constituents is strongly affected by variations in the MIF and transport via the mean meridional wind. In particular, the summer to winter hemisphere flow leads to the highest sodium species concentrations and loss rates occurring over the winter pole. In the Northern Hemisphere, this winter maximumcan be dramatically affected by stratospheric sudden warmings. Simulations of the January 2009 major warming event show that it caused a short term decrease in the sodium column over the polar cap that was followed by a factor of three increase in the following weeks. Overall, the modeled distribution of atomic sodium in WACCM agrees well with both ground-based and satellite observations. Given the strong sensitivity of the sodium layer to dynamical motions, reproducing its variability providesa stringent test of global models, and should help to constrain key atmospheric variables in this poorly sampled region of the atmosphere.

Description: [1]  The hydrological impact of enhancing Earth's albedo by solar radiation management is investigated using simulations from 12 Earth System models contributing to the Geoengineering Model Intercomparison Project (GeoMIP). We contrast an idealized experiment,G1, where the global mean radiative forcing is kept at pre-industrial conditions by reducing insolation while the CO 2 concentration is quadrupled, to a 4xCO 2 experiment. The reduction of evapotranspiration over land with instantaneously increasing CO 2 concentrations in both experiments largely contributes to an initial reduction in evaporation. A warming surface associated with the transient adjustment in 4xCO 2 generates an increase of global precipitation by around 6.9% with large zonal and regional changes in both directions, including a precipitation increase of 10% over Asia and a reduction of 7% for the North American summer monsoon. Reduced global evaporation persists in G1 with temperatures close to pre-industrial conditions. Global precipitation is reduced by around 4.5% and significant reductions occur over monsoonal land regions: East Asia (6%), South Africa (5%), North America (7%) and South America (6%). The general precipitation performance in models is discussed in comparison to observations. In contrast to the 4xCO 2 experiment, where the frequency of months with heavy precipitation intensity is increased by over 50% in comparison to the control, a reduction of up to 20% is simulated in G1. These changes in precipitation in both total amount and frequency of extremes, point to a considerable weakening of the hydrological cycle in a geoengineered world.

Description: [1]  Atmospheric circulation in a Snowball Earth is critical for determining cloud behavior, heat export from the tropics, regions of bare ice, and sea glacier flow. These processes strongly affect Snowball Earth deglaciation and the ability of oases to support photosynthetic marine life throughout a Snowball Earth. Here we establish robust aspects of the Snowball Earth atmospheric circulation by running six general circulation models with consistent Snowball Earth boundary conditions. The models produce qualitatively similar patterns of atmospheric circulation and precipitation minus evaporation. The strength of the Snowball Hadley circulation is roughly double modern at low CO 2 and greatly increases as CO 2 is increased. We force a 1D axisymmetric sea glacier model with GCM output and show that, neglecting zonal asymmetry, sea glaciers would limit ice thickness variations to O (10%). Global mean ice thickness in the 1D sea glacier model is well-approximated by a 0D ice thickness model with global mean surface temperature as the upper boundary condition. We then show that a thin-ice Snowball solution is possible in the axysymmetric sea glacier model when forced by output from all the GCMs if we use ice optical properties that favor the thin-ice solution. Finally, we examine Snowball oases for life using analytical models forced by the GCM output and find that conditions become more favorable for oases as the Snowball warms, so that the most critical time for the survival of life would be near the beginning of a Snowball Earth episode.

Description: [1]  Extensive measurements of aerosol number size distributions (in the size range 10 to 875 nm) carried out over the oceanic regions of Bay of Bengal and Arabian Sea during two large cruise experiments (one during pre-monsoon and the other during winter) are used to investigate the spatial distribution of aerosol size distributions in general, and that of fine particles in particular, within the marine atmospheric boundary layer. The size distributions, over northwestern Bay of Bengal, lying downwind of the continental outflow from Indo-Gangetic Plain, and over the Eastern Bay of Bengal, under the influence of East Asian outflow, showed a bimodal structure with prominent mode (100–125 nm) in the accumulation regime and weak mode (30–40 nm) in the Aitken regime during both pre-monsoon and winter seasons. While the Aitken mode was found to be relatively quite weak in the Indo Gangetic Plain outflow during both the seasons, it was prominent in the East Asian outflow regions, especially during the winter season. The distributions over the northern Arabian Sea, a region quite prone to advection of dust during pre-monsoon and summer season, showed a prominent Aitken mode (~45 nm) followed by a weaker accumulation mode during pre-monsoon season. Analysis of SeaWiFS data revealed a systematic collocation of Aitken mode aerosols and the high chlorophyll concentration in the northern Arabian Sea implying the role of ocean biogeochemistry in influencing the aerosol size distributions. Such a feature implying biogeochemical influence was not seen over eastern and northern Bay of Bengal during pre-monsoon and winter season.

Description: [1]  Understanding future changes in the frequency, intensity and duration of extreme events in response to increased greenhouse gas forcing is important for formulating adaptation and mitigation strategies that minimize damages to natural and human systems. We quantify transient changes in daily-scale seasonal extreme precipitation events over the U.S. using a 5-member ensemble of nested, high-resolution climate model simulations covering the 21 st century in the IPCC SRES A1B scenario. We find a strong drying trend in annual and seasonal precipitation over the Southwest in autumn, winter and spring, and over the central U.S. in summer. These changes are accompanied by statistically significant increases in dry day frequency and dry spell lengths. Our results also show substantial increases in the frequency of extreme wet events over the northwestern U.S. in autumn, winter and spring, and the eastern U.S. in spring and summer. In addition, the average precipitation intensity increases relative to the extreme precipitation intensity in all seasons and most regions, with the exception of the Southeast. Therefore, most regions receive a greater fraction of total seasonal precipitation from extreme events. These results imply fewer but heavier precipitation events in the future, leading to more frequent wet and dry extremes in most regions of the U.S. Our simulations suggest that many of these changes are likely to become statistically significant by the mid-21 st century. Given current vulnerabilities, such changes in extreme precipitation could be expected to increase stress on water resources in many areas of the U.S., including during the near-term decades.

Description: [1]  Two data assimilation (DA) methods, a simple rule-based direct insertion (DI) approach and an one-dimensional ensemble Kalman filter (EnKF) method, are evaluated by assimilating snow cover fraction observations into the Community Land surface Model (CLM). The ensemble perturbation needed for the EnKF resulted in negative snowpack biases. Therefore, a correction is made to the ensemble bias using an approach that constrains the ensemble forecasts with a single unperturbed deterministic LSM run. This is shown to improve the final snow state analyses. The EnKF method produces slightly better results in higher elevation locations, whereas results indicate that the DI method has a performance advantage in lower elevation regions. In addition, the two DA methods are evaluated in terms of their overall impacts on the other land surface state variables (e.g., soil moisture) and fluxes (e.g., latent heat flux). The EnKF method is shown to have less impact overall than the DI method, and causes less distortion of the hydrological budget. However, the land surface model adjusts more slowly to the smaller EnKF increments, which leads to smaller but slightly more persistent moisture budget errors than found with the DI updates. The DI method can remove almost instantly much of the modeled snowpack, but this also allows the model system to quickly revert to hydrological balance for non-snowpack conditions.

Description: [1]  Airborne Multi-AXis Differential Optical Absorption Spectroscopy (AMAX-DOAS) measurements of NO 2 tropospheric vertical columns were performed over California for two months in summer 2010. The observations are compared to the NASA Ozone Monitoring Instrument (OMI) tropospheric vertical columns (data product v2.1) in two ways: (1) Median data was compared for the whole time period for selected boxes and the agreement was found to be fair (R = 0.97, slope = 1.4 ± 0.1, N = 10). (2) A comparison was performed on the mean of coincident AMAX-DOAS measurements within the area of the corresponding OMI pixels with the tropospheric NASA OMI NO 2 assigned to that pixel. The effects of different data filters were assessed. Excellent agreement and a strong correlation (R = 0.85, slope = 1.05 ± 0.09, N = 56) was found for (2) when the data was filtered to eliminate large pixels near the edge of the OMI orbit, the cloud radiance fraction was 〈 50 %, the OMI overpass occurred within 2 hr of the AMAX-DOAS measurements, the flight altitude was 〉 2 km, and a representative sample of the footprint was taken by the AMAX-DOAS instrument. The AMAX-DOAS and OMI data sets both show a reduction of NO 2 tropospheric columns on weekends by 38 ± 24 % and 33 ± 11 %, respectively. The assumptions in the tropospheric satellite air mass factor simulations were tested using independent measurements of surface albedo, aerosol extinction and NO 2 profiles for Los Angeles for July 2010 indicating an uncertainty of 12 %.

Description: [1]  This study presents a nonlinear spatio-temporal analysis of 1167 station temperature records from the United States Historical Climatology Network covering the period from 1898 through 2008. We use the Empirical Mode Decomposition (EMD) method to extract the generally nonlinear trends of each station. The statistical significance of each trend is assessed against three null models of the background climate variability, represented by stochastic processes of increasing temporal correlation length. We find strong evidence that more than 50 percent of all stations experienced a significant trend over the last century with respect to all three null models. A spatio-temporal analysis reveals a significant cooling trend in the South-East and significant warming trends in the rest of the contiguous US. It also shows that the warming trend appears to have migrated equatorward. This shows the complex spatio-temporal evolution of climate change at local scales.

Description: [1]  Despite growing interest in the visible light absorbing organic component of atmospheric aerosols, referred to as “brown” carbon, our knowledge of its chemical composition remains limited. It is well accepted that biomass burning is one important source of “brown” carbon in the atmosphere. In this study, cloud water samples heavily affected by biomass burning were collected at Mount Tai (1534 m, ASL), located in Shandong province in the North China Plain in summer 2008. The samples were analyzed with high performance liquid chromatography equipped with a UV/Vis absorbance detector immediately followed by electrospray ionization and analysis using a time-of-flight (ToF) mass spectrometer. The high mass resolution and accuracy provided by the ToF mass spectrometer allows determination of the elemental composition of detected ions. Using this approach the elemental compositions of 16 major light absorbing compounds, which together accounted for approximately half of measured sample absorption between 300 and 400 nm, were determined. The most important classes of light absorbing compounds were found to be nitrophenols and aromatic carbonyls. Light absorption over this wavelength range by reduced nitrogen compounds was insignificant in these samples.

Description: [1]  Remote sensing techniques offer the unique possibility to continuously and automatically monitor the atmospheric state from ground and space. Ground based microwave radiometers (MWRs), for example, are frequently used for temperature and humidity profiling of the lower troposphere. In order to improve the profiles in the middle and upper troposphere, further information is needed. In this respect, satellite measurements are expected to be very useful. In this study, the synergy benefit in temperature and humidity clear-sky profiling using different combinations of state-of-the-art microwave and infrared ground and satellite based instruments is assessed. The synergy benefit is regarded as the information gain in light of ground based MWR observations together with some climatological a priori knowledge. The maximum information content for this kind of synergy is estimated by assuming optimum conditions, e.g., no forward model uncertainties and a horizontal homogeneous atmosphere. For a mid-latitude site, the ground based MWR gives about 4.4 and 2.4 independent pieces of information on the temperature and humidity profile, respectively. For the temperature profile, the combination with IASI and AMSU-A/MHS increases the information by a factor of about 1.8 and 1.5, respectively, with highest benefit in warm and/or humid conditions. The vertical information on humidity is significantly improved by highly spectrally resolved IR observations from ground or space, when the atmosphere is cold and dry; the vertical information is more than tripled. If measurements from AMSU-A/MHS, IASI or SEVIRI are included, retrieval uncertainties in the middle and upper troposphere are significantly reduced by up to 68%.

Description: [1]  Detailed tracing of an exhaust plume from a rocket's initial trajectory is a scientifically and diagnostically useful technique. It can provide detailed information on the atmosphere's mean winds, wind shears, turbulent regime and physical state over a wide altitude range from 50 to 200 km. We analyze Soyuz rocket exhaust plumes from Plesetsk on 21 May 2009 and 27 June 2011, which uncovered significantly different atmospheric states and underlying dynamics. The first case showed highly dynamical conditions in the mesosphere, characterized by vortex structures, wind shears and small-scale turbulent eddies. The estimated turbulent energy dissipation rates ranged 330-460 mW kg −1 . A characteristic balloon-shaped trail was observed at altitudes between 105 and 160 km, having rapid expansion rates of 500-800 m s −1 over the time period of 2 min which can be explained by complex gas dynamic processes in the rocket wake involving the collision of shock waves. In the second case, we show evidence that the rocket exhaust trail persisted without any changes during its motion from Plesetsk via Denmark to the UK for 9 hours, indicating extremely stable atmospheric conditions. This case introduces a new state of the summer mesosphere – remarkably quiet conditions, probably never observed before. The rocket plumes studied, related to the initial rocket trajectory, are essentially twilight phenomena as seen from the ground using wideband spectrum cameras, that is the Sun should be below the horizon by 6°. For the first time, we analyze the dynamics of rocket exhaust products at the initial trajectory in the mesosphere and lower thermosphere using detailed photographic imaging taken from the ground.

Description: [1]  Rust and bunt spores that act as ice nuclei could change the formation characteristics and properties of ice-containing clouds. In addition, ice nucleation on rust and bunt spores, followed by precipitation, may be an important removal mechanism of these spores from the atmosphere. Using an optical microscope, we studied the ice nucleation properties of spores from four rust species ( Puccinia graminis , Puccinia triticina , Puccinia allii , Endocronartium harknesssii ) and two bunt species ( Tilletia laevis , Tilletia tritici ) immersed in water droplets. We show that the cumulative number of ice nuclei per spore is 5 × 10 -3 , 0.01 and 0.10 at temperatures of roughly -24 °C, -25 °C and -28 °C, respectively. Using a particle dispersion model, we also investigated if these rust and bunt spores will reach high altitudes in the atmosphere where they can cause heterogeneous freezing. Simulations suggest that after 3 days and during periods of high spore production, between 6-9 % of 15  μ m particles released over agricultural regions in Kansas (U.S.), North Dakota (U.S.), Saskatchewan (Canada) and Manitoba (Canada) can reach at least 6 km in altitude. An altitude of 6 km corresponds to a temperature of roughly -25 °C for the sites chosen. The combined results suggest that (a) ice nucleation by these fungal spores could play a role in the removal of these particles from the atmosphere and (b) ice nucleation by these rust and bunt spores are unlikely to compete with mineral dust on a global and annual scale at an altitude of approximately 6 km.

Description: [1]  In this paper we outline a new objective dust source detection method for the central and western Sahara (CWS), based on the automated tracking of individual dust plumes in data from the Spinning Enhanced Visible and Infrared Imager (SEVIRI), available every 15 minutes at ~0.03 o spatial resolution. The method is used to map the origin of summertime dust storms in the CWS for June to August 2004 – 2010. It reveals the sources of these events in unprecedented detail, allowing for the identification of specific, highly active source areas. The study of collocated surface features reveals that many of the dominant sources are likely associated with palaeo-lakes and outwash plains, many in close proximity to the Saharan mountains. Extensive non-source areas are associated with low albedo and elevated terrain, pointing to the mountainous regions of the Sahara. Additionally, sand seas are not identified as important source areas, but their margins sometimes are. The automated tracking method also facilitates analysis of the transport direction of dust plumes from key source regions, and the inference of emission mechanisms. It is found that there are two broad domains within the CWS: one in southwest Algeria and northwest Mali, characterised primarily by transport towards the southwest and very likely dominated by low-level jets embedded in the northeasterly Harmattan winds; and a second in southern Algeria, northwest Niger and northeast Mali where there is no preferred transport direction and a strong potential association between dust events and deep convection, pointing towards cold pool outflows as the likely deflation mechanism.

Description: [1]  The Role of aerosols effect on two tropical cyclones over Bay of Bengal are investigated using a convection permitting model with two-moment mixed-phase bulk cloud microphysics scheme. The simulation results show the role of aerosol on the microphysical and dynamical properties of the cloud and bring out the change in efficiency of the clouds in producing precipitation. The tracks of the TCs are hardly affected by the changing aerosol concentrations, but the intensity exhibits significant sensitivity due to the change in aerosol concentration. It is also clearly seen from the analyses that higher heating in the middle troposphere within the cyclone center is in [2]  response to latent heat release as a consequence of greater graupel formation. Greater heating in the middle level is particularly noticeable for the clean aerosol regime which causes enhanced divergence in the upper level which, in turn, forces the lower level convergence. As a result, the cleaner aerosol perturbation is more unstable within the cyclone core and produces a more intense cyclone as compared to the other two aerosol perturbations. This study along with previous simulations show the robustness of the concept of TC weakening by storm ingestion of high concentrations of CCN. The consistency of these model results gives us confidence in stating there is a high probability that ingestion of high CCN concentrations in a TC will lead to weakening of the storm but has little impact on storm direction . Moreover, as pollution is increasing over the Indian sub-continent, this study suggests pollution may be weakening TCs over the Bay of Bengal.

Description: [1]  Land surface hydrological modeling is sensitive to near-surface air temperature, which is especially true for the cryosphere. The lapse rate of near-surface air temperature is a critical parameter when interpolating air temperature from station data to gridded cells. To obtain spatially distributed, fine-resolution near-surface (2 m) air temperature in the mainland China, monthly air temperature from 553 Chinese national meteorological stations (with continuous data from 1962 to 2011) are divided into 24 regional groups to analyze spatio-temporal variations of lapse rate in relation to surface air temperature and relative humidity. The results are as follows: (1) Evaluation of estimated lapse rate shows that the estimates are reasonable and useful for temperature-related analyses and modeling studies. (2) Lapse rates generally have a banded spatial distribution from southeast to northwest, with relatively large values on the Tibetan Plateau and in northeast China. The greatest spatial variability is in winter with a range of 0.3–0.9 °C·100 m -1 , accompanied by an inversion phenomenon in the northern Xinjiang Province. In addition, the lapse rates show a clear seasonal cycle. (3) The lapse rates maintain a consistently positive correlation with temperature in all seasons, and these correlations are more prevalent in the north and east. The lapse rates exhibit a negative relationship with relative humidity in all seasons, especially in the east. (4) Substantial regional differences in temporal lapse rate trends over the study period are identified. Increasing lapse rates are more pronounced in northern China, and decreasing trends are found in southwest China, which are more notable in winter. An overall increase of air temperature and regional variation of relative humidity together influenced the change of lapse rate.

Description: [1]  A laboratory investigation of the electric charge separated in collisions between vapor grown ice crystals and a target growing by riming is presented in this work, with the goal of studying the performance of the non–inductive mechanism under microphysical conditions similar to some of those which occur in the stratiform regions of the Mesoscale Convective Systems. A series of experiments was conducted by using a target 2 mm in diameter, for ambient temperatures between −7 °C and −13 °C, effective liquid water content between 0.05 and 0.5 g m -3 and air speeds between 1 and 3 m s -1 . Charge diagrams of the sign of the electric charge transfer on the rimer as a function of the ambient temperature and the effective liquid water content for each velocity are presented. The results show that the riming target charges positive for temperatures above −10 °C. For temperatures below −10 °C the charging is positive for high liquid water content, and negative for low liquid water content. The magnitude of the charge transfer per collision under the studied conditions ranges from 0.01 to 0.2 fC. The implications of these results to the electrification processes are discussed.

Description: [1]  We describe a method to evaluate cloud microphysics simulated with a global cloud resolving model against CloudSat and CALIPSO satellite data. Output from the Nonhydrostatic Icosahedral Atmospheric Model (NICAM) is run through a satellite-sensor simulator (Joint Simulator for satellite sensors), then directly compared to the radar and lidar signals from CloudSat and CALIPSO. The forward approach allows for consistency in cloud microphysical assumption involved in the evaluation. To investigate the dependence of the signals on the temperature, we use temperature extensively as the vertical coordinate. The global statistical analysis of the radar reflectivity shows that the simulation overestimates all the percentiles above –50 °C, and that snow category contributes significantly to low reflectivity values between –80 and –40 °C. The simulated lidar signals have two modes associated with cloud ice and snow categories, though the observations have only one mode. The synergetic use of radar reflectivity and lidar backscatter enables us to determine the relative magnitudes of ice/liquid water contents and effective radii without use of retrievals. The radar-and-lidar diagnosis for cloud tops shows that, due to snow category, NICAM overestimates the mass-equivalent effective radius and underestimates ice water content. Also, the diagnosis was shown to be useful to investigate sensitivities of the parameters of bulk microphysical schemes on the water contents and sizes. The non-spherical scattering of ice particles was shown to affect the above radar-and-lidar diagnosis for large reflectivity ranges, but not to alter most of the other diagnoses for this simulation.

Description: [1]  Tonle Sap Lake is the largest freshwater lake in Southeast Asia. During the post-monsoon season, a small linear cloud system has been observed over this lake in early morning, while the sky above the surrounding land is clear. Although this cloud system is apparently generated by land breezes, previous studies on land–lake (sea) circulation have suggested that environmental factors at low latitudes inhibit development of nocturnal land breezes. In this study, we investigate the mechanism of these early-morning clouds through numerical simulation. The simulations show a linear updraft system over the lake, forming along the southwest lakeshore around 22:00 and moving northeast to the middle of the lake. The heavier air mass from the land meets the extraordinarily warm and humid air mass over the lake, triggering updrafts under the conditionally convective instability. The characteristic high surface water temperature was favorable for generation of the land breeze and updraft systems. That high surface water temperature of the lake is produced by the tropical climate along with efficient energy absorption, because of shallowness of the water body. This unique feature can generate a clear nocturnal land breeze circulation accompanying a migrating updraft system over the lake, despite its low latitude.

Description: [1]  This paper explores the use of a parametric geostatistical model for combining rainfall characteristics derived from raingauge data with the same characteristics derived from remote-sensed data-sets. Hypotheses can then be tested about which predictors significantly increase precision of an estimated characteristic. Although applicable wherever ground-level data and remote-sensed data are to be combined, the statistical procedure set out in the paper is developed for two examples of rainfall characteristics: (i) G , the mean annual rainfall at an ungauged site, conditional on knowledge of two predictor variables T (the mean annual rainfall calculated from the TRMM 3B42 data-set for 1998–2009 ), and C (mean annual rainfall derived from the CMORPH data-set for 2003–2009 ); (ii) the mean annual maximum one-day rainfall H , interpolated using the same modelling procedure with predictor variables T and C derived from annual maximum one-day rainfalls in the same remote-sensed data-sets. Prediction errors showed no bias, skewness of distribution, or spatial heterogeneity. The model's generality means that it could be used with any predictors other than T and C , possibly derived from other satellite data-sets or radar. Provided that predictor variables are correlated with the variable to be predicted, it is not necessary for the model relating them to be fitted using data from identical periods, nor for the grid spacing of T and C to be identical. Model performance was evaluated by using a “leave-one-site-out” procedure, which showed that the RMSE of model predictions at omitted sites was smaller than RMSEs obtained from five other well-known spatial predictors.

Description: [1]  Version 6 ozone profiles for 1978-1979 from the Limb Infrared Monitor of the Stratosphere experiment on the NIMBUS 7 satellite (or LIMS v6) are assimilated into an updated version of the GEOS-5 model of NASA. First, an assimilation study is carried out using GEOS-5 version 7.2 (v7.2) and Solar Backscatter UltraViolet (SBUV) version 8.6 ozone profiles. Then, a second study is conducted that ingests both the LIMS and SBUV ozone, as weighted by their estimated absolute error vectors. Ozone from this second study compares well with independent observations from the Stratospheric Aerosol and Gas Experiment (SAGE I) and from time series of ozonesonde data at Hohenpeissenberg and at Wallops Island. Assimilation of the LIMS data gives improved ozone distributions in the upper stratosphere (pressure 〈 5 hPa) and in the polar night region—the latter where Solar Backscatter UltraViolet (SBUV) does not observe. The LIMS ozone leads to improved total column ozone analyses in winter/spring outside of the tropics, based on independent comparisons with total ozone from the Total Ozone Mapping Spectrometer (TOMS). The LIMS ozone also adds information in the tropics on coherent structural features at 20 to 30 hPa, related to the phase transition of the quasi-biennial oscillation (QBO) wind field. It is affirmed that the process of data assimilation represents a cost effective way of characterizing new and/or reprocessed satellite ozone datasets. It is concluded that the GEOS-5 v7.2 model with the addition of the LIMS data can improve analyses of ozone in 1978-1979.

Description: [1]  Most estimates of the global mean indirect effect of anthropogenic aerosol on the Earth's energy balance are from simulations by global models of the aerosol lifecycle coupled with global models of clouds and the hydrologic cycle. Extremely simple models have been developed for integrated assessment models, but lack the flexibility to distinguish between primary and secondary sources of aerosol. Here a simple but more physically-based model expresses the aerosol indirect effect using analytic representations of cloud and aerosol distributions and processes. Although the simple model is able to produce estimates of aerosol indirect effects that are comparable to those from some global aerosol models using the same global mean aerosol properties, the estimates by the simple model are sensitive to preindustrial cloud condensation nuclei concentration, preindustrial accumulation mode radius, width of the accumulation mode, size of primary particles, cloud thickness, primary and secondary anthropogenic emissions, the fraction of the secondary anthropogenic emissions that accumulates on the coarse mode, the fraction of the secondary mass that forms new particles, and the sensitivity of liquid water path to droplet number concentration. Estimates of present day aerosol indirect effects as low as −5 W m -2 and as high as −0.3 W m -2 are obtained for plausible sets of parameter values. Estimates are surprisingly linear in emissions. The estimates depend on parameter values in ways that are consistent with results from detailed global aerosol-climate simulation models, which adds to understanding of the dependence on aerosol indirect effect uncertainty on uncertainty in parameter values.

Description: [1]  A continuous 5-year time series of tropopause altitude made by the SOUSY VHF radar on Svalbard (78°N, 16°E) is examined for diurnal variation. The data have a nominal time resolution of 1 hour, allowing for investigation of intra-day timescales not normally possible using radiosonde observations. Periodicities identified in tropopause altitude are found to vary with season. Surface air temperature measurements naturally exhibit diurnal variations which depend on season: however it is demonstrated that, using a simple scenario of constant adiabatic lapse rate in the troposphere, the tropopause altitude variation that might be anticipated from that of the surface air temperature is not in agreement with the radar observation. Of several mechanisms that could be postulated for this discrepancy, it is demonstrated that a likely candidate is intra-day top-down forcing from the stratosphere.

Description: [1]  The Optical Spectrograph and Infrared Imaging System (OSIRIS) on the Odin satellite currently has an eight-year dataset of nighttime Antarctic nitric oxide densities, [NO], in the mesosphere-lower thermosphere (MLT) region. In this work, the OSIRIS data are compared with a similar dataset from the Sub-Millimetre Radiometer (SMR), also on the Odin satellite. Both of the Odin datasets are compared with twilight [NO] from the Atmospheric Chemistry Experiment – Fourier Transform Spectrometer (ACE-FTS) on the SciSat-I satellite. Direct comparisons of OSIRIS and SMR profiles show large differences, indicating that the individual [NO] profiles of one or both datasets may not be valid. However, when comparing averaged [NO], variations on timescales of weeks-years in all three datasets are in good agreement and correspond to the 27-day and 11-year solar cycles. The averaged OSIRIS values are typically 10% greater than SMR and 30% greater than ACE-FTS, which are within the estimated OSIRIS systematic uncertainties. These results suggest that the satellite-derived datasets can be used for determining polar-mean NO climatology and variations on timescales of weeks-years. The OSIRIS and SMR nighttime datasets show that the [NO] peak height in the MLT decreases throughout the autumn, from an altitude near or above 100 km to a minimum altitude ranging from 90-95 km around winter solstice. A similar decrease in [NO] peak height is observed in modelled NO climatology from the Specified Dynamics – Whole Atmosphere Community Climate Model (SD-WACCM), although the SD-WACCM climatology exhibits a decrease throughout autumn from 107 km down to 102 km. The results suggest that global climate models require more sophisticated auroral forcing simulations in order to reproduce observed NO variations in this region.

Description: [1]  We use formaldehyde (HCHO) vertical column measurements from the SCanning Imaging Absorption spectroMeter for Atmospheric CHartographY (SCIAMACHY) and Ozone Monitoring Instrument (OMI), and a nested-grid version of the GEOS-Chem chemistry transport model, to infer an ensemble of top-down isoprene emission estimates from tropical South America during 2006, using different model configurations and assumptions in the HCHO air-mass factor (AMF) calculation. Scenes affected by biomass burning are removed on a daily basis using fire count observations, and we use the local model sensitivity to identify locations where the impact of spatial smearing is small, though this comprises spatial coverage over the region. We find use of the HCHO column data more tightly constrains the ensemble isoprene emission range from 27–61 Tg C to 31–38 Tg C for SCIAMACHY, and 45–104 Tg C to 28–38 Tg C for OMI. Median uncertainties of the top-down emissions are about 60–260% for SCIAMACHY, and 10–90% for OMI. We find the inferred emissions are most sensitive to uncertainties in cloud fraction and cloud top pressure (differences of ±10%), the a priori isoprene emissions (±20%), and the HCHO vertical column retrieval (±30%). Construction of continuous top-down emission maps generally improves GEOS-Chem's simulation of HCHO columns over the region, with respect to both the SCIAMACHY and OMI data. However, if local time top-down emissions are scaled to monthly mean values, the annual emission inferred from SCIAMACHY are nearly twice those from OMI. This difference cannot be explained by the different sampling of the sensors, or uncertainties in the AMF calculation.

Description: [1]  Estimation of rainfall intensities from radar measurements relies to a large extent on power-laws relationships between rain rates R and radar reflectivities Z , i.e. Z  =  a * R ^ b . This relationship is generally applied unawarely of the scale, which is questionable since the nonlinearity of this relation could lead to undesirable discrepancies when combined with scale aggregation. Since the parameters ( a , b ) are expectedly related with drop size distribution properties, they are often derived at disdrometer scale, not at radar scale, which could lead to errors at the latter. We propose to investigate the statistical behaviour of Z-R relationships across scales both on theoretical and empirical sides. Theoretically, it is shown that claimed multifractal properties of rainfall processes could constrain the parameters ( a , b ) such that the exponent b would be scale-independent but the prefactor a would be growing as a (slow) power-law of time or space scale. In the empirical part (which may be read independently of theoretical considerations), high resolution disdrometer (Dual-Beam Spectropluviometer) data of rain rates and reflectivity factors are considered at various integration times comprised in the range 15 s – 64 min. A variety of regression techniques is applied on Z-R scatterplots at all these time scales, establishing empirical evidence of a behaviour coherent with theoretical considerations: a grows as a 0.1-power law of scale while b decreases more slightly. The properties of a are suggested to be closely linked to inhomogeneities in the DSDs since extensions of Z-R relationships involving (here, strongly nonconstant) normalization parameters of the DSDs seem to be more robust across scales. [2]  The scale-dependence of simple Z  =  a * R ^ b relationships is advocated to be a possible source of overestimation of rainfall intensities or accumulations. Several ways for correcting such scaling biases (which can reach 〉 15-20% in terms of relative error) are suggested. Such corrections could be useful in some practical cases where Z-R scale biases are significant, which is especially expected for convective rainfall.

Description: [1]  HCFC-22 (CHClF2, chlorodifluoromethane) is an ozone-depleting substance, the consumption of which is controlled under the Montreal Protocol. Within a Bayesian inversion framework, we use measurements of HCFC-22 atmospheric concentrations to constrainestimates of HCFC-22 emissions, at the grid-point 3.75° × 2.5° and 8-day resolution, from January 1995 to December 2010. Starting from a new gridded bottom-up inventory which is then optimized, our method shows continuously rising global emissions, from 182 ± 11 Gg in 1995, to the maximum of 410 ± 9 Gg in 2009. This is mainly due to an increase of emissions in developing regions, particularly in Eastern Asia, and occurs despite the current phase-out in developed countries. The high temporal resolution of our inversion (8-day) allows to reveal some of the emission seasonality, the global posterior sources ranging from 25 Gg/month in November to 42 Gg/month in July, for example, in 2010.

Description: [1]  The concurrent variation features between the East Asian subtropical jet and polar front jet were investigated during persistent snow storm period in 2007/08 winter over southern China. The East Asian subtropical jet was divided into two parts: 1) the plateau jet, located along the southern side of the Tibetan Plateau, and 2) ocean jet, situated at the southeastern Japan Island. The concurrent intensity variation among the polar front jet, plateau jet and ocean jet, and the associated atmospheric anomalous signals were examined. Possible mechanism for concurrent variation among the three jets was also investigated from a perspective of synoptic-scale transient eddy activities (STEA). The enhanced plateau jet was simultaneously correlated with the weakened polar front jet, while the variation of the ocean jet lagged the variation of the plateau jet (polar front jet) about 5 days. The concurrent variation between the plateau jet and polar front jet acted as an important bridge that linked the snow storm to the atmospheric anomalous signals associated with the cold and warm air activities. Due to the opposite trends of STEA variation over the southern and northern sides of Tibetan Plateau, the plateau jet and polar front jet exhibited a significant concurrent variation feature. The STEA anomalies over the plateau jet and polar front jet regions propagated downstream to the East Asian coast as a wave train along the southern and northern sides of Tibetan Plateau, respectively, resulting in a 5-day lag variation relationship between the ocean jet and plateau jet (polar front jet).

Description: [1]  The Upper Mesosphere-Lower Thermosphere (UMLT) region of the atmosphere is known to vary on many temporal and spatial scales. However, this region of the atmosphere is very difficult to measure and monitor continuously. In this paper we demonstrate an intriguing connection between mesopause temperatures and the intensity of Very Low Frequencies (VLF) narrowband (NB) signals reflected off the lower ionosphere. The temperature data used are from the SABER instrument on-board the TIMED satellite, while the VLF data are obtained from various ground-based receiving systems. The results of the analysis show a high anti-correlation between temperature and VLF amplitude. It is shown that the variability of the UMLT temperatures and VLF amplitudes can be explained by global seasonal solar irradiance changes (~72% of the variability), while the remaining variability has its origins from other sources (~28%). High resolution mesopause temperature estimates might be achieved in the future by combining VLF NB observations and calculated solar irradiance variability (as a function of hour, day, and location, i.e., latitude).

Description: [1]  Level1 measurements, including cross-polarized backscatter, from the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) lidar have been used to document the vertical structure of the cloud thermodynamic phase at global scale. We built a cloud phase identification (liquid, ice, undefined) in the GCM-Oriented Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) Cloud Product (GOCCP) and analysed the spatial distribution of liquid and ice clouds in 5 JFM seasons of global scale observations (2007-2011). We developed a cloud phase diagnosis in the Cloud Feedback Model Intercomparison Program Observation Simulator Package (COSP) to evaluate the cloud phase description in the LMDZ5B climate model. The diagnosis in the simulator is fully consistent with the CALIPSO-GOCCP observations to ensure differences between the observations and the “model + simulator” ensemble outputs can be attributed to model biases. We compared the liquid and ice cloud vertical distributions simulated by the model with and without the simulator.to quantify the impact of the simulator. The model does not produce liquid clouds above 3 km, and produces ice instead of liquid at low- and mid-altitudes in polar regions, as well as along the Inter-Tropical Convergence Zone. The model is unable to replicate the observed co-existence of liquid and ice cloud between 0 and -40 °C. Liquid clouds dominate for T 〉 -21 °C in the observations, T 〉 -12 °C in the model + simulator, and T 〉 -7.5 °C in the model parameterization. Even if the simulator shifts the model cloud phase parameterization to colder temperature because of the lidar instrument peculiarities, the cloud phase transition remains too warm compared to observations.

Description: [1]  The variability of the precipitating cloud population of the Madden-Julian Oscillation (MJO) is represented by statistics of echo features seen by the Tropical Rainfall Measuring Mission's Precipitation Radar over the central Indian and West Pacific Oceans. Echo features include isolated shallow echoes, deep convective cores, wide convective cores, and broad stratiform regions. Isolated shallow echoes are ever-present but most numerous during suppressed MJO phases. Broad stratiform regions dominate variability in a real coverage and maximize during active phases. Deep convective and wide convective cores are more common and variable in number than broad stratiform regions. The magnitude of variability is similar in both regions. In the central Indian Ocean, active MJO phases have synchronous maximization of deep convective entities. In the West Pacific, broad stratiform regions maximize prior to wide convective cores. Reanalysis indicates that isolated shallow echoes are most numerous dry mid-tropospheric conditions and strong low-level (1000–750 hPa) shear. Mid-tropospheric moisture increases before deeper convective features increase in number, maximizes as deep convective features maximize, and decreases as wide convective cores and broad stratiform regions decline in population. Active-stage deep and wide convective cores occur preferentially with moist mid-tropospheric conditions and strong low-level shear. Acute shear may favor downdraft momentum transport and consequently more robust gust-front convective triggering. Broad stratiform regions maximize with a moist mid-troposphere, strong low-level shear, and moderate upper-level (750–500 hPa) shear that is not so strong that the stratiform region disconnects from its convective moisture source.

Description: [1]  Radiocarbon ( 14 C) has proven to be a powerful tool in distinguishing modern and fossil fuel sources contributing to organic aerosols. By applying this concept to ice core records of the dissolved organic carbon (DOC) fraction, we developed a setup dedicated to the extraction of DOC from Alpine ice core samples for 14 C micro-analysis. With respect to the difficulties and limitations of this analytical method, it is shown that a total process blank mass of (6 ± 3) μgC with a 14 C signature of (0.71 ± 0.17) can be obtained, corresponding to a minimum sample size between 200 g for industrial and 600 g for pre-industrial ice. Radiocarbon analyses of eight DOC ice core samples from the high accumulation glacier Col du Dôme (European Alps) were mainly performed over the bomb peak period. These data, being associated with snow deposition over the summer half-years, show an overall mean fossil contribution of (25 ± 9) %. Adaptation of the DO 14 C values to the atmospheric 14 CO 2 record revealed that the biogenic input to ice core DOC is associated with a fast recycling biospheric component, likely linked to a turnover time of less than three years.

Description: [1]  There is an acid deposition conundrum in China: contrary to conventional wisdom, extremely high ambient sulfate concentrations in northeastern China are not always accompanied by correspondingly high acidities. To investigate this discrepancy, data from two independent sets of in situ field measurements were analyzed along with SCIAMACHY satellite observations and MOZART chemical transport model calculations. The field measurements included soluble aerosol ion concentrations and pH and particulate data from 11 cities as well as pH measurement data from 74 sites in China. [2]  This study explores the basis for and the impacts of the large discrepancy in northeastern China between the major acidity precursors (SO 2 and NOx) and measured acidity levels as indicated by pH values. There are extremely high SO 2 emissions and ambient concentrations in northeastern China, while the corresponding acidity is unusually low (high pH) in this region. This is inconsistent with the usual situation where high-acidity precursor pollutants result in low pH (high acidity) values and acid rain conditions. In other regions, such as southern China and the United States, high SO 2 concentrations are typically well correlated with high acidities. Using measured soluble particle measurements (including both positively and negatively charged ions), it is seen that there are high values of alkaline ions in northeastern China that play an important role in neutralizing acidity in this region. This result strongly suggests that the high alkaline concentrations, especially Ca 2+ , increase warm season pH values by about 0.5 in northern China, partially explaining the inconsistency between sulfate concentrations and acidity. This has a very important implication for acid rain mitigation -- especially in northeastern China. [3]  However, there are additional issues pertaining to the precursor-acidity relationship that need further investigation. Why is it that the reduction in acidity due to the alkaline ions is only significant in summer? During winter, the measured alkaline ions play a much smaller role in explaining the discrepancy. The measured alkaline ions in this study were mostly obtained from particles in the PM 2.5 range. However, the size of calcium particles is typically much larger -- extending well beyond 2.5 µm -- and so a significant amount of calcium may be underestimated by PM 2.5 measurements alone. The under-sampling of calcium particles is further exacerbated in that the sampling protocol excluded particle (and soluble ion and pH) measurements during dust storms. This all leads to the need for an improved understanding of pollutant-ion-particulate interactions in China, and their role in explaining the counter-intuitive conclusion that dust mitigation strategies in China could have the unintended consequence of exacerbating acid rain conditions.

Description: [1]  Transformation and distribution of mercury (Hg) species play an important role in the biogeochemical cycling of mercury in aquatic systems. Measurements of water/air exchange fluxes of Hg, reactive mercury (RHg), and dissolved gaseous mercury (DGM) concentrations were conducted at 14 sites in five reservoirs on the Wujiang River, Guizhou, Southwest China. Clear spatial and temporal variations in Hg fluxes, RHg and DGM concentrations were observed in the study area. Hg fluxes and RHg concentrations exhibited a consistent diurnal variation in the study area, with maximum fluxes and concentrations during daytime. A typical diurnal trend of DGM with elevated concentration at night was observed in a eutrophic reservoir with elevated bacteria abundance, suggesting a bacteria-induced production of DGM in this reservoir. For other reservoirs, a combination of sunlight-stimulated production and loss via photo-induced oxidation and evaporation regulated the diurnal trends of DGM. Seasonal variations with elevated Hg fluxes and RHg concentrations in warm season were noticeable in the study area, which highlighted the combined effect of interrelationships between Hg species in water and environmental parameters. Hg fluxes exhibited much more significant correlations with RHg and THg concentrations and air temperature compared to DGM concentrations and solar radiation. The measured fluxes were significantly higher than those simulated using the water/air thin film Hg 0 gradient model. Aside from the potential limitations of dynamic flux chamber method, this may also suggest the thin film gas exchange model is not capable of predicting water/air Hg flux under low wind speed conditions. Additionally, it is speculated that DGM concentrations might vary significantly in surface waters with depth, and measurements of DGM at a depth of 2 – 4 cm below the water surface probably underestimated the DGM concentration that should be taken into account in simulations of water/air flux using the thin film gas exchange model. An empirical model of water/air Hg flux was developed and the simulated fluxes compared well with measurements. The model yields a mean annual Hg emission of 3.2 ± 1.0 kg in the study area.

Description: [1]  The impact of the global sea surface temperature (SST) warming trend, which is the leading mode of SST variability, on late summer Asian rainfall is analyzed based on the simulations of five atmospheric general circulation models (AGCMs), which are performed by the U. S. Climate Variability and Predictability (CLIVAR) Drought Working Group. Our evaluations of the model outputs indicate that these models roughly capture the main features of climatological rainfall and circulations over Asia and the western North Pacific (WNP), but they simulate a too strong monsoon trough and a too northward shifted in the subtropical anticyclone in the WNP, and fail to reproduce the rain belt over East Asia. [2]  It is found that all of the models simulate an intensified WNP subtropical high (WNPSH) in late summer, and an enhanced precipitation in the tropical Indian Ocean and the maritime continent, and a suppressed precipitation in the South Asian monsoon region, the South China Sea and the Philippine Sea, when the models are forced with the SST trend, which is characterized by a significant increase in the Indian Ocean and western Pacific. All these changes are suggested to be dynamically coherent. The warmer SST trend in the Indian Ocean and western Pacific may suppress precipitation over the Philippine Sea, and thus result in a lower-tropospheric anticyclonic circulation over the subtropical WNP. The warmer SSTs in the Indian Ocean may also be responsible for the anomalous easterlies and resultant less rainfall over the South Asian monsoon region. The precipitation changes forced by the SST trend are similar in the maritime continent, but show an apparent difference over East Asia, in comparison with the observed rainfall trend over lands. The possible reasons for this difference are discussed.

Description: ABSTRACT [1]  The location and shift in the position of the maximum lightning activity with respect to the position of monsoon trough has been investigated in the region of 20 0  N–32 0  N and 75 0 E – 95 0 E during the Indian summer monsoon seasons of 2002 and 2003 from the Lightning Imaging Sensor (LIS) data from the TRMM satellite. The polynomial curve fitting the maximum lightning activity followed the position of the monsoon trough in both, the normal monsoon and drought years. Observations of the maximal lightning activity in the TRMM-LIS data collected during a period of 13 years from 1998 to 2010, confirm these findings in the monsoon season. When the seasonal position of the monsoon trough moved from the plains to the submontane region of the Himalayas during a monsoon break period, the polynomial fitting of the maximum lightning activity also moved from the central Indian plains to that region. An empirical orthogonal function (EOF) analysis is applied to two different data sets of precipitation and observed high lightning flash rates over the monsoon trough region for the period of 1995–2005. The first and second components, EOF1 and EOF2, of precipitation values are out–of–phase with those of the flash rates. Our results imply that deep electrified convective systems during monsoon periods are responsible for the lightning activity in the monsoon trough region and the lightning activity in this region can be used as a proxy for the location and shift of the seasonal monsoon trough.

Description: [1]  Interannual variability in tropical sea-surface temperatures (SSTs) associated with the El Niño-Southern Oscillation is linked to teleconnections with the Northern Annular Mode (NAM). Previous work highlighted that the sign and amplitude of the NAM response to tropical SSTs is controlled by the total wave activity entering the subpolar stratosphere, which depends on the linear interference of planetary wave anomalies with the climatological stationary wave field. This study uses multiple configurations ofatmospheric general circulation models to assess the robustness of these linkages to details of the tropical SST forcing and model configuration. Across 23 cases with idealized SST forcing the amplitudes of the tropical and extratropical wave responses are found to scale approximately linearly with forcing strength. But wave amplitude alone is not sufficient to predict the NAM response. Instead, the spatial structure of the wave response (and hence the linear interference) provides the best explanation of the NAM response in all cases. Linear interference explains most of the total wave activity response even in cases with stronger nonlinear contributions, due to consistent cancellation between quasi-stationary wave nonlinearity and nonlinearity arising from transient waves. Within this limited set of experiments, there is no evidence for a consistent sensitivity of the NAM response to horizontal resolution or to vertical resolution in the stratosphere. These findings reveal that linear interference provides a robust and reproducible mechanism linking midlatitude wave responses to zonal mean circulation (NAM) responses across a wide variety of forcing cases.

Description: [1]  In the absence of tangible progress in reducing greenhouse gas emissions, the implementation of solar radiation management has been suggested as measure to stop global warming. Here we investigate the impacts on northern mid-latitude cirrus from continuous SO 2 emissions of 2–10 Mt/a in the tropical stratosphere. Transport of geoengineering aerosols into the troposphere was calculated along trajectories based on ERA Interim reanalyses using ozone concentrations to quantify the degree of mixing of stratospheric and tropospheric air termed “troposphericity”. Modelled size distributions of the geoengineered H 2 SO 4 -H 2 O droplets have been fed into a cirrus box model with spectral microphysics. The geoengineering is predicted to cause changes in ice number density by up to 50 %, depending on troposphericity and cooling rate. We estimate the resulting cloud radiative effects from a radiation transfer model. Complex interplay between the few large stratospheric and many small tropospheric H 2 SO 4 -H 2 O droplets gives rise to partly counteracting radiative effects: local increases in cloud radiative forcing up to +2 W/m 2 for low troposphericities and slow cooling rates, and decreases up to −7.5 W/m 2 for high troposphericities and fast cooling rates. The resulting mean impact on the northern mid-latitudes by changes in cirrus is predicted to be low, namely 〈1% of the intended radiative forcing by the stratospheric aerosols. This suggests that stratospheric sulphate geoengineering is unlikely to have large microphysical effects on the mean cirrus radiative forcing. However, this study disregards feedbacks, such as temperature and humidity changes in the upper troposphere, which must be examined separately.

Description: [1]  Previous studies using satellite measurements showed evidence that subtropical upper troposphere/lower stratosphere ozone (O 3 ) can be modulated by tropical intraseasonal variability, the most dominant form of which is the Madden–Julian oscillation (MJO) with a period of 30–60 days. Here we further study the MJO modulation in the upper troposphere/lower stratosphere O 3 over the northern extratropics and the Arctic. Significant MJO-related O 3 signals (13–20 Dobson units) are found over the northern extratropics (north of 30°N). The O 3 anomalies change their magnitude and patterns depending on the phase of the MJO. Over the Arctic, the MJO-related O 3 anomalies are dominated by a wavenumber-2 structure and are anticorrelated with the geopotential height (GPH) anomalies at 250 hPa. The latter is similar to the findings in the previous studies over subtropics and indicates that the Arctic upper troposphere/lower stratosphere O 3 anomalies are associated with dynamical motions near the tropopause. The teleconnection from the tropics to the Arctic is likely through propagation of planetary waves generated by the equatorial heating that affects the tropopause height and O 3 at high latitudes.

Description: [1]  Knowledge of the variability of climate in the past is essential for understanding current climatic changes. Therefore, we investigated two temperature indices and seven rainfall time series of North-Western Europe since the 17 th century. Trends and multi- to interdecadal variability are similar in England and Northern France for temperature, whereas a strong regional contrast is evident between the two regions for rainfall. Multi- and interdecadal variabilities display several periods of enhanced amplitude for both temperature and rainfall that may be related to large-scale climate control. On these scales, temperatures in both England and France display phase opposition with the Atlantic Multidecadal Oscillation (AMO) before 1800, while they are in phase afterwards, as determined by wavelet coherence. On the other hand, the relationships between temperature and the North Atlantic Oscillation (NAO) are weak across multi- and interdecadal scales for the whole period under study. For rainfall, coherence with AMO is observed for scales at around 30–60-year, whereas coherence with NAO is detected on 50–80-year and interdecadal 16–23-year scales. However, relationships between rainfall variability and North Atlantic climate indices are highly contrasted depending on the region considered. Finally, the results of a mixed spectral/EOF analysis of mean sea-level pressure on these co-oscillation time scales highlight not only NAO regimes, but also other patterns explaining a non-negligible amount of variance during certain time periods.

Description: [1]  Precipitable water (PW) retrievals from FORMOSAT-3/Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) Global Positioning System (GPS) radio occultation (RO) measurements were analyzed and compared with those derived from Special Sensor Microwave/Imager (SSM/I) and Advanced Microwave Scanning Radiometer for Earth Observation System (AMSR-E) during the El Niño-Southern Oscillation (ENSO) events from 2007 to 2011. For the three ENSO events in 2007-2011, monthly mean binned COSMIC PW results are in a very high correlation (up to 0.98) with those of SSM/I and AMSR-E over the ocean, generally with root-mean-square differences less than 4 mm. PW retrievals from the three satellites are also of similar latitudinal variations. However, the PW is slightly underestimated by GPS RO, in particular, from 10°S to 30°N. This underestimate may be caused partially by the fact that not all RO measurements can reach the surface. Inter-satellite PW anomaly comparisons for the winter months in the ENSO events, with respect to those during the neutral (non-ENSO) months, show consistent ENSO signals with major PW anomaly near the central Pacific in the warm event and near the Indonesian region and east of Australia in the two cold events. However, the 2007/2008 La Niña is somewhat less correlated for COSMIC with AMSR-E and SSM/I. For the stronger 2010/2011 La Niña, their PW anomalies are in higher correlations of about 0.8.

Description: ABSTRACT [1]  We provide statistical evidence of the effect of the solar wind dynamic pressure (P sw ) on the northern winter and spring circulations. We find that the vertical structure of the Northern Annular Mode (NAM), the zonal mean circulation and Eliassen-Palm (EP)-flux anomalies show a dynamically consistent pattern of downward propagation over a period of ~45 days in response to positive P sw anomalies. When the solar irradiance is high, the signature of P sw is marked by a positive NAM anomaly descending from the stratosphere to the surface during winter. When the solar irradiance is low, the P sw signal has the opposite sign, occurs in spring, and is confined to the stratosphere. The negative P sw signal in the NAM under low solar irradiance conditions is primarily governed by enhanced vertical EP-flux divergence and a warmer polar region. The winter P sw signal under high solar irradiance conditions is associated with positive anomalies of the horizontal EP-flux divergence at 55-75°N and negative anomalies at 25-45°N, which corresponds to the positive NAM anomaly. The EP-flux divergence anomalies occur ~15 days ahead of the mean-flow changes. A significant equatorward shift of synoptic-scale Rossby wave breaking (RWB) near the tropopause is detected during Jan-Mar, corresponding to increased anti-cyclonic RWB and a decrease in cyclonic RWB. We suggest that the barotropic instability associated with asymmetric ozone in the upper stratosphere and the baroclinic instability associated with the polar vortex in the middle and lower stratosphere play a critical role for the winter signal and its downward propagation.

Description: [1]  A radiative transfer model (RTM) that provides a link between model states and satellite observations (e.g. brightness temperature) can act as an observation operator in land data assimilation to directly assimilate brightness temperatures. In this study, a microwave Land Data Assimilation System (LDAS) was developed with three RTMs (QH, LandEM and CMEM) as its multi-observation operators (LDAS-MO). Assimilation experiments using the Advanced Microwave Scanning Radiometer for the Earth Observing System (AMSR-E) satellite brightness temperature data from July 2005 to December 2008 were then conducted to investigate the impact of the RTMs on the assimilated results over China. It was found that the assimilated volumetric soil-water content using each of the three observation operators improved the estimation of soil moisture content in the top soil layer (0–10 cm), with reduced root mean square errors (RMSEs), and increased correlation coefficients with field observations (OBS) as compared to a control run with no assimilation for the absence of frozen or snow-covered conditions. The assimilated soil moisture for the QH model, which was more sensitive to dry soil than the other models, produced closer correlations with OBS in arid and semi-arid regions while smaller RMSEs were observed for LandEM. CMEM agreed most closely with OBS over the middle and lower reaches of the Yangtze River due to its better simulation of the brightness temperature over densely vegetated areas. To improve assimilation accuracy, a Bayesian model averaging (BMA) scheme for the LDAS-MO was developed. The BMA scheme was found to significantly enhance assimilation capability producing the soil moisture analysis, showing the lowest RMSEs and highest correlations with OBS over all areas. It was demonstrated that the BMA scheme with LDAS-MO has the potential to estimate soil moisture with high accuracy.

Description: [1]  We evaluate the annual mean Radiative Shortwave flux Downward at the Surface (RSDS) and reflected shortwave (RSUT) and Radiative Longwave flux Upward at Top of atmosphere (RLUT) from the 20th century Coupled Model Intercomparison Project Phase 5 (CMIP5) and Phase 3 (CMIP3) simulations as well as from the NASA GEOS5 model and MERRA analysis. [2]  The results show that a majority of the models have significant regional biases in the annual means of RSDS, RLUT, and RSUT, with biases from -30 to 30 Wm -2 . While the global average CMIP5 ensemble mean biases of RSDS, RLUT and RSUT are reduced compared to CMIP3 by about 32% (e.g., -6.9 to 2.5 Wm -2 ), 43%, and 56%, respectively. This reduction arises from a more complete cancellation of the pervasive negative biases over oceans and newly larger positive biases over land. In fact, based on these biases in the annual mean, Taylor diagram metrics, and RMSE, there is virtually no progress in the simulation fidelity of RSDS, RLUT, and RSUT fluxes from CMIP3 to CMIP5. [3]  A persistent systematic bias in CMIP3 and CMIP5 is the underestimation of RSUT and overestimation of RSDS and RLUT in the convectively active regions of the tropics. The amount of total ice and liquid atmospheric water content in these areas is also underestimated. We hypothesize that at least a part of these persistent biases stems from the common global climate model (GCM)practice of ignoring the effects of precipitating and/or convective core ice and liquid in their radiation calculations.

Description: [1]  A combined field and laboratory study was conducted to improve our understanding of the chemical and hygroscopic properties of organic compounds in aerosols sampled in the background continental atmosphere. PM 2.5 (particles with aerodynamic diameters smaller than 2.5 µm) aerosols were collected from 24 June to 28 July, 2010 at Storm Peak Laboratory in the Park Range of northwestern Colorado. New particle formation was frequent at Storm Peak Laboratory during this campaign, and the samples were not influenced by regional dust storms. Filter samples were analyzed for organic carbon (OC) and elemental carbon (EC), water soluble OC (WSOC), major inorganic ions, and detailed organic speciation. WSOC was isolated from inorganic ions using solid phase absorbents. Hygroscopic growth factors and cloud condensation nucleus (CCN) activity of the WSOC were measured in the laboratory. Organic compounds comprised the majority (average of 64% with a standard deviation of 9%) of the mass of measured species and WSOC accounted for an average of 89% (with a standard deviation of 21%) of OC mass. Daily samples were composited according to back-trajectories. On average, organic acids, sugars, and sugar alcohols accounted for 12.5 ± 6.2% (average ± standard deviation) of WSOC. Based on the composition of these compounds and that of high molecular weight compounds identified using ultra high resolution mass spectrometry, the organic mass to organic carbon ratio of the WSOC is estimated to be 2.04. The average hygroscopic growth factors at RH = 80% (GF 80 ) were 1.10 ± 0.03 for particles derived from isolated WSOC and 1.27 ± 0.03 for particles derived from the total water-soluble material (WSM). CCN activity followed a similar pattern. The critical diameters at a super-saturation of 0.35% were 0.072 ± 0.009 and 0.094 ± 0.006 µm for particles derived from WSM and isolated WSOC, respectively. These growth factor results compare favorably with estimates from thermodynamic models, which explicitly relate the water activity (RH) to concentration for the total soluble material identified in this study.

Description: [1]  To assess horizontal and vertical transports of methane (CH 4 ) concentrations at different heights within the troposphere, we analyzed simulations by12 chemistry-transport models (CTMs) that participated in the TransCom-CH 4 intercomparison experiment. Model results are compared with aircraft measurements at 13 sites in Amazon/Brazil, Mongolia, Pacific Ocean, Siberia/Russia and United States during the period of 2001 to 2007.The simulations generally show good agreement with observations for seasonal cycles and vertical gradients. The correlation coefficients of the daily averaged model and observed CH 4 time series for the analyzed years are generally larger than 0.5, and the observed seasonal cycle amplitudes are simulated well at most sites, considering the between-model variances. However, larger deviations show up below 2 km for the model-observation differences in vertical profiles at some locations, e.g., at Santarem, Brazil, and in the upper troposphere, e.g., at Surgut, Russia. Vertical gradients and concentrations are underestimated at Southern Great Planes, United States and Santarem, and overestimated at Surgut. Systematic over- and under-estimation of vertical gradient are mainly attributed to inaccurate emission and only partly to the transport uncertainties. However, large differences in model simulations are found over the regions/seasons of strong convection, which is poorly represented in the models. Overall, the zonal and latitudinal variations in CH 4 are controlled by surface emissions below 2.5 km, and transport patterns in the middle and upper troposphere. We show that the models with larger vertical gradients, coupled with slower horizontal transport, exhibit greaterCH 4 interhemisphericgradients in the lower troposphere. These findings have significant implications for the future development of more accurate CTMs with the possibility of reducing biases in estimated surface fluxes by inverse modeling.

Description: [1]  Statistical cloud schemes with prognostic probability distribution functions have become more important in atmospheric modeling, especially since they are in principle scale adaptive and capture cloud physics in more detail. While in theory the schemes have a great potential, their accuracy is still questionable. High resolution three-dimensional observational data of water vapor and cloud water, which could be used for testing them, are missing. We explore the potential of ground-based remote sensing such as lidar, microwave and radar to evaluate prognostic distribution moments using the “perfect model approach”. This means that we employ a high resolution weather model as virtual reality and retrieve full three-dimensional atmospheric quantities and virtual ground-based observations. We then use statistics from the virtual observation to validate the modeled 3D statistics. Since the data are entirely consistent any discrepancy occurring is due to the method. Focusing on total water mixing ratio we find that the mean ratio can be evaluated decently, but that it strongly depends on the meteorological conditions as to whether the variance and skewness are reliable. Using some simple schematic description of different synoptic conditions we show how statistics obtained from point or line measurements can be poor at representing the full three-dimensional distribution of water in the atmosphere. We argue that a careful evaluation of measurement data and detailed knowledge of the meteorological situation is necessary to judge whether we can use the data for an evaluation of higher moments of the humidity distribution used by a statistical cloud scheme.

Description: [1]  A space-time interpolator for creating average geophysical fields from satellite measurements is presented and tested. It is designed for optimal spatio-temporal averaging of heterogeneous data. While it is illustrated with satellite surface wind measurements in the tropics, the methodology can be useful for interpolating, analyzing, and merging a wide variety of heterogeneous and satellite data in the atmosphere and ocean over the entire globe. The spatial and temporal ranges of the interpolator are determined by averaging satellite and in situ measurements over increasingly larger space and time windows and matching the corresponding variability at each scale. This matching provides a relationship between temporal and spatial ranges, but does not provide a unique pair of ranges as a solution to all averaging problems. The pair of ranges most appropriate for a given application can be determined by performing a spectral analysis of the interpolated fields and choosing the smallest values that remove any or most of the aliasing due to the uneven sampling by the satellite. The methodology is illustrated with the computation of average divergence fields over the equatorial Pacific Ocean from SeaWinds-on-QuikSCAT surface wind measurements, for which 72 h and 510 km are suggested as optimal interpolation windows. It is found that the wind variability is reduced over the cold tongue and enhanced over the Pacific warm pool, consistent with the notion that the unstably stratified boundary layer has generally more variable winds and more gustiness than the stably stratified boundary layer. It is suggested that the spectral analysis optimization can be used for any process where time-space correspondence can be assumed.

Description: [1]  To investigate the major sources of summertime organic aerosol (OA) and provide insights into secondary organic aerosol (SOA) formation, positive matrix factorization (PMF) analysis was performed on a large set of organic species measured during the California at the Nexus of Air Quality and Climate Change (CalNex) campaign in Bakersfield, CA. Six OA source factors were identified, including one representing primary organic aerosol (POA), four different types of SOA representing local, regional, and nighttime production, and one representing a complex mixture of additional OA sources that were not further resolvable. POA accounted for an average of 15% of measured OA. The complex mixture of additional OA sources contributed an average of 13% of measured OA. The combined contribution of four types of SOA to measured OA averaged 72% and varied diurnally from 78% during the day to 66% at night. Both regional and local SOA were significant contributors to measured OA during the day, but regional SOA was the larger one, especially in the afternoon. Although contributions to SOA from oxidation of biogenic gas-phase compounds were less constrained, they were evident and dominantly occurred at night. The formation of SOA is indicated to be mainly through gas-to-particle condensation of gas-phase oxidation products during the day. Our results indicate that effective control measures to reduce summertime OA in Bakersfield should focus on reducing sources of gas-phase organics that serve as SOA precursors during the day, and it is more effective to reduce SOA precursors at the regional scale in the afternoon.

Description: [1]  The new particle formation (NPF)-explicit version of the WRF-chem model, which we developed recently, can calculate the growth and sink of nucleated clusters explicitly with 20 aerosol size bins from 1 nm to 10 µm. In this study, the model is used to understand spatial and temporal variations of the frequency of NPF events and the concentrations of aerosols (condensation nuclei, CN) and cloud condensation nuclei (CCN) within the boundary layer in East Asia in spring 2009. Model simulations show distinct north–south contrast in the frequency and mechanism of NPF in East Asia. NPF mostly occurred over limited periods and regions between 30° and 45°N, such as northeast China, Korea, and Japan, including regions around active volcanoes (Miyakejima and Sakurajima). At these latitudes, NPF was considerably suppressed by high concentrations of preexisting particles under stagnant air conditions associated with high-pressure systems, while nucleation occurred more extensively on most days during the simulation period. Conversely, neither nucleation nor NPF occurred frequently south of 30°N because of lower SO 2 emissions and H 2 SO 4 concentrations. The period-averaged NPF frequency was 3 times higher at latitudes of 30° – 45°N than at latitudes of 20° – 30°N. The north–south contrast of NPF frequency is validated by surface measurements in outflow regions in East Asia. The period- and domain-averaged contribution of secondary particles is estimated to be 44% for CN (〉 10 nm) and 26% for CCN at a supersaturation of 1.0% in our simulation, though the contribution is highly sensitive to the magnitudes and size distributions of primary aerosol emissions and the coefficients in the nucleation parameterizations.