ALBERT

Description: [1]  Neutron monitors have recorded the flux of high energy Galactic cosmic rays for more than half a century. During the recent prolonged, deep minimum in solar activity many sources indicate that modulated Galactic cosmic rays have attained new space-age highs. However reported neutron monitor rates are ambiguous; some record new highs while others do not. This work examines the record of 15 long-running neutron monitors to evaluate cosmic ray fluxes during the recent extraordinary solar minimum in a long-term context. We show that ground-level neutron rates did reach a historic high during the recent solar minimum, and we present a new analysis of the cosmic ray energy spectrum in the year 2009 versus year 1987. To do this we define a reference as the average of eight high-latitude neutron monitors, four in the northern hemisphere (Apatity, Inuvik, Oulu, Thule) and four in the southern hemisphere (Kerguelen, McMurdo, Sanae, Terre Adelie). Most stations display changes in sensitivity, which we characterize by a simple linear trend. After correcting for the change in sensitivity, a consistent picture emerges. With our correction all stations considered display new highs at the recent solar minimum, approximately 3% above the previous record high. These increases are shown to be consistent with spacecraft observations.

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]  We present the occurrence frequency of downgoing auroral electron beams in magnetic local time and invariant latitude, and the dependence on solar cycle, as indicated by F10.7, on whether the ionospheric footpoint of the satellite is illuminated or dark, and on the energy flux carried by the electrons. As previously reported, we find that the occurrence of electron beams peaks in the pre-midnight local time sector and that solar illumination at the footpoint (solar zenith angle) reduces both the occurrence and energy of the electron beams. The effect of solar maximum conditions (indicated by F10.7) is almost as large as the effect of the solar zenith angle. The characteristic energy of the electron beams is dependent on the energy flux carried, in addition to both solar zenith angle and F10.7. The beam energy (and therefore the parallel potential drop) is ~1.6 times higher for during solar minimum than during solar maximum for both dark and illuminated footpoints. The beam energy during dark solar minimum conditions is a factor of ~3 more than during sunlit minimum conditions. The ‘area’ covered by intense aurora is also reduced during solar maximum, for both sunlit and dark conditions. There is no evidence that the statistical results are due to the fact that acceleration via parallel electric fields moves to lower latitudes during solar maximum.

Description: [1]  In this paper we describe and quantify the energy transfer, flow and distribution. Our high-resolution data-set covers 13 years of OMNI, SuperMAG and Kyoto data. We employ what we consider to be the best estimates for energy sinks, and relate these to SuperMAG indices for better coverage and spatial resolution. For the energy input we have used the method of dimensional analysis [ Vasyliunas et al ., 1982] that is presented in unit power and makes it appropriate for energy analysis. A cross-correlation analysis parameterizes the magnetospheric response on the solar wind parameters during a wide range of conditions, ranging from substorms and storms up to a decade. The determined functional form is then evaluated and scaled using superposed epoch analysis of geomagnetic storms, revealing that the effective area of interaction can not be considered static. Instead we present a dynamic area which depends to the first order on the cube of the IMF B z component. Also, we find that for longer time periods this area must be increased compared to the area used for geomagnetic storms. We argue that some of the terms in the energy coupling function are contributory to describing magnetosheath conditions, and discuss how our coupling function can be related to Maxwell stress components. Also, we quantify the relative importance of the different energy sinks during substorms, geomagnetic storms and long time series, and present the coupling efficiency of the solar wind. Our energy coupling functions is compared with the ɛ parameter [ Akasofu and Perreault , 1978] and performs better for almost any event.

Description: [1]  Plasma sheet flow bursts have been suggested to correspond to different types of auroral activity, such as poleward boundary intensifications (PBIs), ensuing auroral streamers, and substorms. The flow-aurora association leads to the important question of identifying the magnetotail source region for the flow bursts and how this region depends on magnetic activity. The present study uses the ARTEMIS spacecraft coordinated with conjugate ground-based auroral imager observations to identify flow bursts beyond 45 R E downtail and corresponding auroral forms. We find that quiet-time flows are directed dominantly earthward with a one-to-one correspondence with PBIs. Flow bursts during the substorm recovery phase, and during steady magnetospheric convection (SMC) periods are also directed earthward, and these flows are associated with a series of PBIs/streamers lasting for tens of minutes with similar durations to that of the series of earthward flows. Pre-substorm onset flows are also earthward and associated with PBIs/streamers. The earthward flows during those magnetic conditions suggest that the flow bursts, which lead to PBIs and streamers, originate from further downtail of ARTEMIS, possibly from the distant tail neutral line (DNL) or tailward-retreated near-Earth neutral line (NENL) rather than from the nominal NENL location in the mid-tail. We find that tailward flows are limited primarily to the substorm expansion phase. They continue throughout the period of auroral poleward expansion, indicating that the expansion-phase flows originate from the NENL and that NENL activity is closely related to the auroral expansion of the substorm expansion phase.

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 solar eclipse on 15 January 2010 traversed Asia and completed its travel on the Shandong Peninsula in China at sunset. Two vertical-incidence ionosondes at Wuhan and Beijing and the oblique-incidence ionosonde network in North China were implemented to record the ionospheric response to the solar eclipse. Following the initial electron density decrease caused by the eclipse, the ionosphere was characterized by a strong pre-midnight enhancement, and a subsequent ionospheric decay, and a ~10 hour later post-midnight enhancement. Neither geomagnetic disturbance occurred during the eclipse day, nor did obvious nighttime peak appear for the ten-day mean of the F2-layer critical frequency ( fo F2). The electron density profilogram of the Beijing ionosonde indicates that the two enhancements were the result of the plasma flux downward from the top ionosphere, possibly due to the steep decrease of the ionospheric electron density and plasma temperature during the solar eclipse. The two-dimensional differential fo F2 maps present the regional variations of the nighttime electron density peaks and decay. Both the pre- and post-midnight enhancements initially appeared in a belt almost in parallel with the eclipse track and then drifted southward. The different magnitudes of greatest eclipse in the umbra and outside tend to account for the different occurrence times of the plasma flux. The ionospheric decay following the pre-midnight enhancement is also considered as a consequence of the eclipse shade.

Description: [1]  We study a statistics of ∂  B z /∂  x in a thin stretching current sheet (substorm growth phases) observed by Cluster between 8 and 18 R E downtail. After 2005 spacecraft separation allowed to measure directly this derivative of B z along the tail axis. The near-tail events (within 14 R E ) exhibited a straight decrease of an initially large positive ∂  B z /∂  x to ∼ 1–2 nT/ R E . In the more stretched middle tail, usually the small | ∂  B z /∂  x | 〈 0.5 − 1 nT/ R E had no clear trend and fluctuated around zero with time scales 5–15 min. In general, negative ∂  B z /∂  x were ubiquitous. At some onsets larger negative ∂  B z /∂  x  〈 − 1 nT/ R E were associated with transient dipolarizations, propagating Earthward. There was no clear association of local plasma sheet activity onset with any value of ∂  B z /∂  x . We discuss relation of observations and recent modeling results.

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]  ELF/VLF radio waves are difficult to generate with conventional antennas. Ionospheric HF heating facilities generate ELF/VLF waves via modulated heating of the lower ionosphere. HF heating of the ionosphere changes the lower ionospheric conductivity, which in the presence of natural currents such as the auroral electrojet, creates an antenna in the sky when heating is modulated at ELF/VLF frequencies. We present a summary of nearly 100 days of ELF/VLF wave generation experiments at the 3.6 MW HAARP facility near Gakona, Alaska, and provide a baseline reference of ELF/VLF generation capabilities with HF heating. Between February 2007 and August 2008, HAARP was operated on close to 100 days for ELF/VLF wave generation experiments, at a variety of ELF/VLF frequencies, seasons and times of day. We present comprehensive statistics of generated ELF/VLF magnetic fields observed at a nearby site, in the 500-3500 Hz band. Transmissions with a specific HF beam configuration (3.25 MHz, vertical beam, amplitude modulation) are isolated so the data comparison is self-consistent, across nearly 5 million individual measurements of either a tone or a piece of a frequency-time ramp. There is a minimum in the average generation close to local midnight. It is found that generation during local nighttime is on average weaker, but more highly variable, with a small number of very strong generation periods. Signal amplitudes from day to day may vary by as much as 20-30 dB. Generation strengthens by ~5 dB during the first ~30 minutes of transmission, which may be a signature of slow electron density changes from sustained HF heating. Theoretical calculations are made to relate the amplitude observed to the power injected into the waveguide and reaching250 km. The median power generated by HAARP and injected into the waveguide is ~0.05-0.1 W in this base-line configuration (vertical beam, 3.25 MHz, amplitude modulation), but may have generated hundreds of Watts for brief durations. Several efficiency improvements have improved the ELF/VLF wave generation efficiency further.

Description: [1]  An inversion technique for estimating the properties of the magnetospheric plasma from the harmonic frequencies of the toroidal standing Alfvén waves has been used to derive the global equatorial mass density covering radial distances from 4 to 9 Earth radii ( R E ), within the local time sector spanning from 0300 to 1900 hours. This broad range of L shell extending to the outer magnetosphere allows us to examine the local time and radial dependence of the quiet-time equatorial mass density during solar minimum and thereby construct a global distribution of the equatorial mass density. The toroidal Alfvén waves were detected with magnetometers on the Active Magnetospheric Particle Tracer Explorers (AMPTE)/Charge Composition Explorer (CCE) during the nearly 5 year interval from August 1984 to January 1989 and on the Geostationary Operational Environmental Satellites (GOES) (10, 11 and 12) for 2 years from 2007 to 2008, both of which were operating during solar minimum years. The derived equatorial mass density, ρ eq , at geosynchronous orbit (GEO) monotonically increases with increasing magnetic local time (MLT) from the nightside towards the dusk sector. At other radial distances, ρ eq has the same MLT variation as that of GEO, while the magnitude logarithmically decreases with increasing L value. An investigation of the Dst and Kp dependence shows that the median value of ρ eq varies little in the daytime sector during moderately disturbed times, which agrees with previous studies. ρ eq calculated from the F 10.7 dependent empirical model shows good agreement with that of CCE but overestimates that of GOES probably due to the extreme solar cycle minimum in years 2007–2008.

Description: [1]  In this paper we compare observations of the high latitude cusp from DMSP data to simulations conducted using the Lyon-Fedder-Mobarry (LFM) global magnetosphere simulation. The LFM simulation is run for the 31 Aug 2005 to 02 Sep 2005 moderate storm, from which the solar wind data exhibits a wide range of conditions that enable a statistical representation of the cusp to be obtained. The location of the cusp is identified using traditional magnetic depression and plasma density enhancement at high altitude. A new diagnostic using the parallel ion number flux is also tested for cusp identification. The correlation of the cusp latitude and various solar wind IMF coupling functions is explored using the three different cusp identification methods. The analysis shows 1) the three methods give approximately the same location and size of the simulated cusp at high altitude; 2) the variations of the simulated cusp are remarkably consistent with the observed statistical variations of the low-altitude cusp. In agreement with observations a higher correlation is obtained using other solar wind coupling functions such as the Kan-Lee electric field. The MLT position of the simulated cusp is found to depend upon the IMF By component, with a lower linear correlation. The width of the simulated cusp in both latitude and MLT is also examined. The size of the cusp is found to increase with the solar wind dynamic pressure with saturation seen when the dynamic pressure is greater than 3 nPa.

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]  Daily profiles of phase measurements as observed on fixed VLF-paths generally show a transient phase advance, followed by a phase delay, for about 90 minutes after sunrise hours. This is indicative of a reflecting ionospheric C-region developing along the terminator line at an altitude below the normal D-region. The suggested occurrence of a C-region is consistent with rocket measurements made in the 1960's, showing a maximum of the electron density between 64 and 68 km, and by radio sounding in the 1980's. In order to correctly describe the properties of the phase effect associated with the presence of a C-region, it is important to understand the subionospheric propagation characteristics of the VLF-paths. In this paper, we analyze the variations presented by the temporal properties of the VLF narrow-band phase effect, and determined a parameter associated with the appearance of the C-region at sunrise hours observed by receivers from the South America VLF Network (SAVNET). Periodic patterns emerge from the parameter curves. Two distinct temporal behavior regimes can be identified: one exhibiting slow variations between March and October, and another one exhibiting faster variations between October and March. Solar illumination conditions and the geometrical configuration of the VLF paths relative to the sunrise terminator partly explain the slow variation regime. During periods of faster variations, we have observed good association with atmospheric temperature variability found in the measurements of the TIMED-SABER satellite instrument, which we assume to be related to the Winter Anomaly atmospheric phenomenon. However, when comparing the parameter time series with temperature curves, no direct one-to-one correspondence was found for transient events.

Description: [1]  On 21 January 2005, one of the fastest interplanetary coronal mass ejections (ICME) of solar cycle 23, containing exceptionally dense plasma directly behind the sheath, hit the magnetosphere. We show from charge-state analysis that this material was a piece of the erupting solar filament, and further, based on comparisons to the simulation of a fast CME, that the unusual location of the filament material was a consequence of three processes. As the ICME decelerated, the momentum of the dense filament material caused it to push through the flux rope towards the nose. Diverging non-radial flows in front of the filament moved magnetic flux to the sides of the ICME. At the same time reconnection between the leading edge of the ICME and the sheath magnetic fields worked to peel away the outer layers of the flux rope creating a remnant flux rope and a trailing region of newly opened magnetic field lines. These processes combined to move the filament material into direct contact with the ICME sheath region. Within one hour after impact and under northward IMF conditions, a cold dense plasma sheet formed within the magnetosphere from the filament material. Dense plasma sheet material continued to move through the magnetosphere for more than 6 hours as the filament passed by the Earth. Densities were high enough to produce strong diamagnetic stretching of the magnetotail despite the northward IMF conditions and low levels of magnetic activity. The disruptions from the filament collision are linked to an array of unusual features throughout the magnetosphere, ionosphere and atmosphere. These results raise questions about whether rare collisions with solar filaments may, under the right conditions, be a factor in producing even more extreme events.

Description: [1]  We present observations from the Falkland Islands SuperDARN radar of the propagation of HF radio waves via the Weddell Sea ionospheric Anomaly (WSA), a region of enhanced austral summer nighttime ionospheric electron densities covering the southern Pacific and South Americas region. This anomaly is thought to be produced by uplift of the ionosphere by prevailing equatorwards thermospheric winds. Of particular interest are perturbations of the WSA-supported propagation, which suggest that during periods of geomagnetic disturbance the ionospheric layer can be lowered by several 10s of km and subsequently recover over a period of 1 to 2 hours. Perturbations can appear singly or as a train of 2 to 3 events. We discuss possible causes of the perturbations, and conclude that they are associated with equatorward-propagating large-scale atmospheric waves produced by magnetospheric energy deposition in the auroral or sub-auroral ionosphere. Changes in high/mid-latitude electrodynamics during geomagnetic storms may also account for the perturbations, but further modeling is required to fully understand their cause.

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]  Inner magnetosphere magnetic field and plasma flow data are examined during 228 steady magnetospheric convection events. We find that the B Z component of the magnetic field around geostationary orbit is weaker than during average conditions and the plasma flow speeds are higher than average in the dusk sector just beyond geostationary orbit. The SMC periods include more enhanced Earthward and tailward flow intervals than during average conditions. The steady convection period magnetic field is not steady: The near-geostationary nightside field grows increasingly taillike throughout the steady convection period. In the midtail, Earthward flows are enhanced in a wide region around the midnight sector, which leads to enhanced magnetic flux transport toward the Earth during the steady convection periods. Compared to well-known characteristics during magnetospheric substorms, the inner tail evolution resembles that during the substorm growth phase, while the midtail flow characteristics duringsteady convection periods are similar to those found during substorm recovery phases.

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]  A global total electron content (TEC) model response to geomagnetic activity described by the K p -index is built by using the Center for Orbit Determination of Europe (CODE) TEC data for full 13 years, January 1999 - December 2011. The model describes the most probable spatial distribution and temporal variability of the geomagnetically forced TEC anomalies assuming that these anomalies at a given modip latitude depend mainly on the K p -index, local time (LT) and longitude. The geomagnetic anomalies are expressed by the relative deviation of TEC from its 15-day median and are denoted as rTEC. The rTEC response to the geomagnetic activity is presented by a sum of two responses with different time delay constants and different sign of the cross-correlation function. It has been found that the mean dependence of rTEC on K p -index can be expressed by a cubic function. The LT dependence of rTEC is described by Fourier time series which includes the contribution of four diurnal components with periods 24, 12, 8 and 6 hours. The rTEC dependence on longitude is presented by Fourier series which includes the contribution of zonal waves with zonal wavenumbers up to 6. In order to demonstrate how the model is able to reproduce the rTEC response to geomagnetic activity three geomagnetic storms at different seasons and solar activity conditions are presented. The model residuals clearly reveal two types of the model deviation from the data: some underestimation of the largest TEC response to the geomagnetic activity and randomly distributed errors which are the data noise or anomalies generated by other sources. The presented TEC model fits to the CODE TEC input data with small negative bias of -0.204, root mean squares error RMSE  = 4.592 and standard deviation error STDE  = 4.588. The model offers TEC maps which depend on geographic coordinates (5 o x5 o in latitude and longitude), and universal time (UT) at given geomagnetic activity and day of the year. It could be used for both science and possible service (nowcasting and short-term prediction); for the latter a detailed validation of the model at different geophysical conditions has to be performed in order to clarify the model predicting quality.

Description: [1]  Magnetic reconnection (MR), a fundamental process in space plasmas that changes magnetic topology and converts magnetic energy into kinetic and thermal energies, is an ultimate driver of space weather. There exist two models of MR in the literatures, anti-parallel and component, associated with intensive studies of their generation and applications. In this paper we report an MR event observed by Cluster constellation in the geo-magnetotail where both types of MR were detected. By reconstructing the three-dimensional (3D) MR configuration we find that a pair of A-B nulls existed in both types of MR cases with two fan surfaces intersecting each other to form a separator line connecting the nulls. A weak or sizable magnetic field exists along the separator in the anti-parallel or component case, respectively. In the latter case, field strength is finite away from the two nulls and vanishes close to the nulls. Therefore, at least in the two cases observed, both anti-parallel and component MR geometries are local presentation of the separator MR configuration. This result supports the expectation that 3D nulls often occur as a crucial element of MR at least in the magnetotail and separator MR may play an important role in dynamics and reconfiguration of magnetic field in 3D MR processes.

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 development of equatorial plasma irregularity plumes can be well recorded by steerable backscatter radars operated at and off the magnetic equator due to the fact that the vertically extended plume structures are tracers of magnetically north-south aligned larger scale structures. From observations during March 2012, using two low latitude steerable backscatter radars in Southeast Asia, the Equatorial Atmosphere Radar (EAR) (0.2ºS, 100.3ºE; dip lat 10.4ºS) and the Sanya VHF radar (18.4ºN, 109.6ºE; dip lat 12.8ºN), the characteristics of backscatter plumes over the two sites separated in longitude by ~1000 km were simultaneously investigated. The beam steering measurements reveal frequent occurrences of multiple plumes over both radar sites, of which two cases are analyzed here. The observations on 30 March 2012 show plume structures initiated within the radar scanned area, followed by others drifting from the west of the radar beam over both stations. A tracing analysis on the onset locations of plasma plumes reveals spatially well-separated backscatter plumes, with a maximum east-west wavelength of about 1000 km, periodically generated in longitudes between 85ºE and 110ºE. The post-sunset backscatter plumes seen by the Sanya VHF radar are found to be due to the passage of sunset plumes initiated around the longitude of EAR. Most interestingly, the EAR measurements on the night of 21 March 2012 show multiple plume structures that developed successively in the radar scanned area with east-west separation of ~50 km, with however, no sunset plasma plume over Sanya. Co-located ionogram measurements show that spread F irregularities occurred mainly in the bottomside F-region at Sanya, whereas satellite traces in ionograms that are indications of large-scale wave structures, were observed on that night at both stations. Possible causes for the longitudinal difference in the characteristics of radar backscatter plumes are discussed.

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]  The solar wind impacting the Earth varies over a wide range of time scales, driving a corresponding range of geomagnetic activity. Past work has strongly indicated the rate of merging on the frontside magnetosphere is the most important predictor for magnetospheric activity, especially over a few hours. However the magnetosphere exhibits variations on other time scales, including UT, seasonal, and solar cycle variations. Much of this geomagnetic variation cannot be reasonably attributed to changes in the solar wind driving – that is, it is not created by the original Russell-McPherron effect or any generalization thereof. In this paper we examine the solar cycle, seasonal, and diurnal effects based upon the frequency of substorm onsets, using a data set of 53,000 substorm onsets. These were identified through the SuperMAG collaboration and span three decades with continuous coverage. Solar cycle variations include a profound minima in 2009 (448 substorms) and peak in 2003 (3727). The magnitude of this variation (a factor of 8.3) is not explained through variations in estimators of the frontside merging rate (such as d Φ MP / dt ), even when the more detailed probability distribution functions are examined. Instead, v , or better, n 1/2 v 2 seems to be implicated in the dramatic difference between active and quiet years, even beyond the role of velocity in modulating merging. Moreover, we find that although most substorms are preceded by flux loading (78.5% are above the mean and 83.8% above median solar wind driving) a high solar wind v is almost as important (68.3% above mean, 74.8% above median). This and other evidence suggests either v or n 1/2 v 2 (but probably not p ) plays a strong secondary role in substorm onset. As for the seasonal and diurnal effects, the elliptical nature of the Earth's orbit, which is closest to the Sun in January, leads to a larger solar wind driving (measured by Bs , vBs , or d Φ MP / dt ) in November, as is confirmed by 22 years of solar wind observations. However substorms peak in October and March, and have a UT dependence best explained by whether a conducting path established by solar illumination exists in at least one hemisphere in the region where substorm onsets typically occur.

Description: [1]  The primary sources of energetic electron precipitation (EEP) which affect altitudes 〈100 km (〉30 keV) are expected to be from the radiation belts, and during substorms. EEP from the radiation belts should be restricted to locations between L  = 1.5-8, while substorm produced EEP is expected to range from L  = 4-9.5 during quiet geomagnetic conditions. Therefore, one would not expect any significant D-region impact due to electron precipitation at geomagnetic latitudes beyond about L  = 10. In this study we report on large unexpectedly high latitude D-region ionization enhancements, detected by an incoherent scatter radar at L  ≈ 16, which appear to be caused by electron precipitation from substorms. We go on to reexamine the latitudinal limits of substorm produced EEP using data from multiple low-Earth orbiting spacecraft, and demonstrate that the precipitation stretches many hundreds of kilometers polewards of the previously suggested limits. We find that a typical substorm will produce significant EEP over the IGRF L -shell range L  = 4.6 ± 0.2-14.5 ± 1.2, peaking at L  = 6-7. However, there is significant variability from event to event; in contrast to the median case, the strongest 25% of substorms have significant EEP in the range spaning L  = 4.1 ± 0.1-20.7 ± 2.2, while the weakest 25% of substorms have significant EEP in the range spaning L  = 5.5 ± 0.1-10.1 ± 0.7. We also examine the occurrence probability of very large substorms, focusing on those events which appear to be able to disable geostationary satellites when they are located near midnight MLT. On average these large substorms occur approximately 1-6 times per year, a significant rate given the potential impact on satellites.

Description: [1]  This paper presents new observations of the behaviour of simulated dust particles in space plasma based on a 3D particle in cell code. Multistep Monte Carlo collision is employed to simulate the dust charging process which is validated for the cases of charging of isolated dust particle and ensemble dust particles, where results indicate good agreement between simulation and theories. The code is then used to investigate plasma properties near a charged surface in a vicinity of a cloud of dust particles. The simulation reveals that a cloud of dust particle close to a spacecraft surface affects plasma densities around the spacecraft as well the spacecraft's surface potential. It is suggested that dust cloud causes the surface to charge to higher negative potential. The simulation also suggests that the combination of surface potential and dust cloud potential produces a region of trapped low energy electrons.

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 here document with magnetic field observations a passage of the MESSENGER spacecraft through Mercury's magnetosphere under conditions of a quasi-parallel bow shock, i.e., when the direction of the upstream interplanetary magnetic field (IMF) was within 45° of the bow shock normal. The spacecraft's fast transition of the magnetosheath and the steady solar wind conditions during the period analyzed allow both spatial and temporal properties of the shock crossing to be investigated. The observations show that the shock reformation process can be nearly periodic under stable solar wind conditions. Throughout the 25-min-long observation period, the pulsation duration deviated by at most ~10% from the average 10 s period measured. This quasi-periodicity allows us to study all aspects of the shock reconfiguration, including ultra-low-frequency waves in the upstream region and large-amplitude magnetic structures observed in the vicinity of the magnetosheath − solar wind transition region and inside the magnetosheath. We also show that bow shock reformation can be a substantial source of wave activity in the magnetosphere, on this occasion having given rise to oscillations in the magnetic field with peak-to-peak amplitudes of 40–50 nT over large parts of the dayside magnetosphere. The clean and cyclic behavior observed throughout the magnetosphere, the magnetosheath, and the upstream region indicates that the subsolar region was primarily influenced by a cyclic reformation of the shock front, rather than by a spatial and temporal patchwork of short large-amplitude magnetic structures (SLAMS), as is generally the case at the terrestrial bow shock under quasi-parallel conditions.

Description: [1]  The entry of solar wind into the magnetosphere is strongly influenced by kinetic-scale boundary layers where the rapid variation in the magnetic field and/or velocity can drive transport. In current layers with strong Alfvénic velocity shear, the generation of vortices from the Kelvin-Helmholtz instability can drive magnetic reconnection even in broader current sheets by locally compressing these layers as the vortices develop. Previous two-dimensional (2D) fully kinetic simulations of this vortex-induced reconnection process have demonstrated the copious formation of magnetic islands in regions of strongly compressed current between the vortices. Here we describe the first three-dimensional (3D) fully kinetic simulations of this process and demonstrate that the compressed current sheets give rise to magnetic flux ropes over a range of oblique angles and along the entire extent of the compressed current layer around the periphery of the vortex. These flux ropes propagate with the shear flow and eventually merge with the vortex. Over longer time scales, this basic scenario is repeated as the vortices drive new compressed current sheets. In the final stage, the vortices undergo a merging process that drives new compressed current sheets and flux ropes. Based on these simulations, a simple model is proposed that predicts the size of these flux ropes relative to their parent vortex. Both the relative sizes as well as the structure of the profiles across the vortex are in reasonable agreement with THEMIS observations at the Earth's low-latitude magnetopause.

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]  We survey the properties of electron pitch angle distributions in the magnetotail plasma sheet at a distance between 15 and 19 R E from the Earth, using data from the Cluster PEACE instrument. We limit our survey to those pitch angle distributions measured when the IMF had been steadily northward or steadily southward for the previous three hours. We find that, at sub-keV energies the plasma sheet electron pitch angle distribution has an anisotropy such that there is a higher differential energyflux of electrons in the (anti-) field-aligned directions. Fitting the measured pitch angle distributions with both a single and two component kappa distribution reveals that this anisotropy is the result of the presence of a second, cold, component of electrons that is observed more often than not, and occurs during both the northward and southward IMF intervals. We present evidence that suggests the cold electron component has an ionospheric, rather than magnetosheath, source and is linked tothe large scale field aligned current systems that couple the magnetosphere and ionosphere.

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]  Driver functions for the Earth's magnetosphere-ionosphere system are derived from physical principles. Two processes act simultaneously: a reconnection-coupled MHD generator G and a viscous interaction. G accounts for the dayside reconnection rate, the length of the reconnection X-line, and current-saturation limits for the solar-wind generator. Two viscous drivers are derived: Bohm viscosity B and the freestream-turbulence effect F. A problematic proxy effect is uncovered wherein the viscous driver functions also describe the strength of reconnection. Two magnetospheric-driver functions written in terms of upstream-solar-wind parameters are constructed: G+B and G+F. The driver functions are tested against 7 geomagnetic indices. The reaction of the geomagnetic indices to G+B and G+F is nonlinear: nonlinear versions of the driver functions are supplied. Applying the driver functions at multiple timesteps yields correlation coefficients of ~85% with the AE and Kp indices; it is argued that multiple timestepping removes high-frequency uncorrelated signal from the drivers. Autocorrelation-function analysis shows strong 1-d and 1-yr periodicities in the AE index that are not in the solar-wind driver functions; correspondingly, highpass and lowpass filtering finds uncorrelated signal at 1-d and 1-yr timescales. Residuals (unpredicted variance) between the geomagnetic indices and the driver functions are analyzed: the residuals are anti-correlated with the solar-wind velocity, the solar F 10.7 radio flux, and the solar-wind current-saturation parameter. Removing diurnal, semiannual, and annual trends from the indices improves their correlation with the solar-wind driver functions. Simplified versions of the driver functions are constructed: the simplified drivers perform approximately as well as the full drivers.

Description: [1]  Plasma properties of Saturn's pre-midnight tail region are surveyed using Cassini/CAPS ion observations from 2010. Only low-latitude (|lat| 〈 6°) intervals in which the CAPS viewing was roughly symmetric inward and outward around the corotation direction are used. Our numerical moments algorithm returns nonzero ion density for 70% (999) of the intervals selected. Of these, 642 had detectable water-group ion densities, and the remainder were dominantly, if not entirely, light ions. The derived plasma parameters are similar to those found in an earlier study for the post-midnight tail region, except that we find little evidence for the systematic outflows identified in that study, and we do find numerous significant inflow events. One such inflow is identified as a dipolarization event, the first reported plasma properties of such a structure at Saturn. A second, long-lasting event may be evidence for the existence at times of a quasi-steady reconnection region in the pre-midnight tail. The large majority of the plasma flows are found to be within 20° of the corotation direction, though with flow speeds significantly lower than full corotation. While the inflow events represent plausible evidence for internally-driven mass loss in the pre-midnight region, the absence of significant outflow events suggests that in the region surveyed here, tail reconnection has not yet proceeded to involve lobe field lines, so the disconnected plasma continues its general motion in the corotation direction.

Description: [1]  Following the first-time ionospheric imaging of a seismic fault, here we perform a case-study on retrieval of parameters of the extended seismic source ruptured during the great M9.0 Tohoku-oki earthquake. Using 1Hz ionospheric GPS data from the Japanese network of GPS receivers (GEONET) and several GPS satellites, we analyze spatio-temporal characteristics of co-seismic ionospheric perturbations and we obtain information on the dimensions and location of the sea surface uplift (seismic source). We further assess the criterion for the successful determination of seismic parameters from the ionosphere: the detection is possible when the line-of-sights from satellites to receivers cross the ionosphere above the seismic fault region. Besides, we demonstrate that the multi-segment structure of the seismic fault of the Tohoku-oki earthquake can be seen in high-rate ionospheric GPS-data. Overall, our results show that, under certain conditions, ionospheric GPS-derived TEC measurements could complement the currently working systems, or independent ionospherically-based system might be developed in the future.

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]  The non-dipolar portions of the Earth's main magnetic field constitute substantial differences between the two hemispheres. Beside the magnetic flux densities and patterns being different in the Northern Hemisphere (NH) and Southern Hemisphere (SH), also the offset between the invariant magnetic and the geographic poles is larger in the SH than in the NH. We investigated the effects of this magnetic field asymmetry on the high-latitude thermosphere and ionosphere using global numerical simulations, and compared our results with recent observations. While the effects on the high-latitude plasma convection are small, the consequences for the neutral wind circulation are substantial. The cross-polar neutral wind and ion drift velocities are generally larger in the NH than the SH, and the hemispheric difference shows a semidiurnal variation. The neutral wind vorticity is likewise larger in the NH than in the SH, with the difference probably becoming larger for higher solar activity. In contrast, the spatial variance of the neutral wind is considerably larger in the SH polar region, with the hemispheric difference showing a strong semidiurnal variation. Its phase is similar to the phase of the semidiurnal variation of the hemispheric magnitude differences. Hemispheric differences in ion drift and neutral wind magnitude are most likely caused partly by the larger magnetic flux densities in the near-polar regions of the SH, and partly by the larger offset between the invariant and geographic pole in the SH, while differences in spatial variance are probably just caused by the latter. We conclude that the asymmetry of the magnetic field, both in strength and in orientation, establishes substantial hemispheric differences in the neutral wind and plasma drift in the high-latitude upper atmosphere, which can help to explain observed hemispheric differences with EDI/Cluster and CHAMP.

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]  We have identified 8 events with double-belt structure in the outer radiation belt from 110 CME-driven magnetic storms and 223 CIR-driven storms during 1994 to 2003 based on the SAMPEX data sets. Among them, 3 cases are related to CME-driven magnetic storms and 5 cases are related CIR-driven storms. All double-belt structure events in the outer radiation belt are found during the recovery phase of a magnetic storm for both CME- and CIR-related events—they usually start to form within 3–4 days after the onset of the magnetic storm. The pre-conditions needed to form a double-belt structure, for all the CME-related events, are found to be high solar wind dynamic pressure ( P dy ) and southward IMF Bz; Nevertheless, for the CIR-related events, they are found to be associated with high speed stream with southward interplanetary magnetic field, which is enhanced by a suitably orientated B y component.It is further found that the flux distributions of the double-belt structure can be fitted well with a simply exponential decay function of L ∗ . Based on the RBC index, the proportion of the total number of 1.5-6.0 MeV electrons inside the position of maximum fluxes to that outside the maximum fluxes keeps rising during the double-belt period, which implies that the acceleration mainly occurs at regions inside the location of maximum fluxes. We suggest that the plasmapause and the strong wave-particle interactions with VLF and ULF waves near it, play an important role in the development of the double-belt structures.

Description: [1]  The solar minimum period during 2008–2009 was characterized by lower thermospheric density than the previous solar minimum, and lower than any previously measured. Recent work [ Solomon et al ., 2010; 2011] used the NCAR Thermosphere-Ionosphere-Electrodynamics General Circulation Model to show that the primary cause of density changes from 1996 to 2008 was a small reduction in solar extreme-ultraviolet irradiance (EUV), causing a decrease in thermospheric temperature and hence a contracted thermosphere. There are similar effects in the ionosphere, with most measurements showing an F -region ionosphere that is unusually low in density, and in peak altitude. This paper addresses the question of whether model simulations previously conducted, and their solar, geomagnetic, and anthropogenic inputs, produce ionospheric changes commensurate with observations. We conducted a 15-year model run and obtained good agreement with observations of the global mean thermospheric density at 400 km throughout the solar cycle, with a reduction of ~30% from the 1996 solar minimum to 2008–2009. We then compared ionosonde measurements of the mid-day peak density of the ionospheric F -region ( N m F 2 ) to the model simulations at various locations. Reasonable agreement was obtained between measurements and the model, supporting the validity of the neutral density comparisons. The global average N m F 2 was estimated to have declined between the two solar minima by ~15%. In these simulations, a 10% reduction of solar EUV plays the largest role in causing the ionospheric change, with a minor contribution from lower geomagnetic activity, and a very small additional effect from anthropogenic increase in CO 2 .

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]  We report the statistical features of sporadic sodium layers (SSLs) and the thermospheric enhanced sodium layers (TeSLs) observed by a lidar chain located at Beijing (40.2°N, 116.2°E), Hefei (31.8°N, 117.3°E), Wuhan (30.5°N, 114.4°E), and Haikou (19.5°N, 109.1°E). The average SSL occurrence rate was approximately 46.0, 12.3, 13.8, and 15.0 hr per SSL at Beijing, Hefei, Wuhan, and Haikou, respectively. However, the TeSLs occurred relatively infrequently and were more likely to appear at low and high latitudinal sites. Both the SSLs and TeSLs at four lidar sites showed evident summer enhancements and correlated well with Es ( f o E s  〉  4  MHz ). The co-observations of SSLs at three lidar site pairs, i.e., Hefei – Beijing, Hefei – Wuhan and Hefei – Beijing, indicated that a large-scale SSL extended horizontally for at least a few hundred kilometers and exhibited a tidal-induced modulation. Moreover, the SSLs were better correlated for the Hefei – Wuhan and Hefei – Haikou pairs than the Hefei – Beijing pair, which suggested a difference in the dynamical/chemical process in mesosphere and lower thermosphere (MLT) between the Beijing site and the other sites.

Description: [1]  A highly sensitive all-sky EMCCD airglow imager has been operative in Longyearbyen, Norway (78.1°N, 15.5°E) since October 2011. The imager obtains the 630.0 nm all-sky images with an exposure time of 4 sec, which is about 10 times shorter than the conventional cooled CCD airglow imagers. This new equipment allows us to image the on-going structuring of polar cap patches in 2D fashion. Here, we report a case in which faint undulations appeared along the trailing edge of patches propagating in the central polar cap. The separation between the fingers in the undulations was about 50–100 km and the e -folding time of their growth was ~5 min. We suggest that the gradient-drift instability (GDI) is one of the possible generation mechanisms of the undulating structures. The reasons for this interpretation are 1) the asymmetry in the preference of structuring between the leading and trailing edges is qualitatively consistent with the GDI mechanism, and 2) the linear growth rate of GDI calculated by using electron density estimates from simultaneous EISCAT Svalbard radar observations is roughly consistent with the observed growth time of the fingers. Such “unstable polar cap patches" could be important sources of seed irregularities, which would eventually be broken down to smaller-scale density perturbations affecting the trans-ionospheric satellite communications in the central polar cap.

Description: [1]  As the Polar spacecraft apogee precessed through the magnetic equator in 2001, Polar encountered numerous substorm events in the region between geosynchronous orbit and 10 Re geocentric distance; most of them in the plasma sheet boundary layers. Of these, a small number were recorded near the neutral sheet in the evening sector. Polar/TIDE provides a unique perspective on the lowest energy ion plasma, showing that these events exhibited a damped wavelike character, initiated by a burst of radially outward flow transverse to the local magnetic field at ~ 80 km/sec. They then exhibit strongly damped cycles of inward/outward flow with a period of several minutes. After one or two cycles, they culminated in a hot plasma electron and ion injection, quite similar to those observed at geosynchronous orbit. Cold plasmaspheric plasmas comprise the outward flow cycles, while the inward flow cycles contain counter-streaming field-parallel polar wind like flows. The observed wave-like structure, preceding the arrival of an Earthward moving substorm injection front, suggests an outward displacement driven by the inward motion at local times closer to midnight, that is, a “snowplow” effect. The damped in/out flows are consistent with interchange oscillations driven by the arrival at the observed local time by an injection originating at greater radius and local time.

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]  Accurate knowledge of the global distribution of magnetospheric chorus waves is essential for radiation belt modeling because it provides a direct link to understanding radiation belt losses and acceleration processes. In this paper, we report on newly developed models of the global distribution of chorus amplitudes based on in-situ measurements of IMF and solar wind parameters as well as geomagnetic indices using an artificial neural network technique. We find that solar wind speed and IMF B Z are the most influential parameters that affect the evolution of the magnetospheric chorus. The variations of chorus amplitudes in the outer ( L  ≥ 7) and in the inner (5 ≤  L  〈 7) regions, respectively, are well correlated with the variations of solar wind speed and IMF B Z . In addition, the solar wind parameter-based chorus model generally results in a slightly higher correlation between measured and modeled chorus amplitudes than any other models including geomagnetic indices AE, Kp, and Dst. The developed model shows that the chorus is amplified near the pre-noon sector during the geomagnetically disturbed conditions. With increasing southward IMF B Z the location of peak chorus amplitude moves from the pre-noon sector to the midnight sector, which is due to the enhanced electron injection near midnight. We also present a comparison of diffusive transport simulations for radiation belt electrons interacting with two newly developed chorus models, solar wind parameter-based and geomagnetic index-based chorus models. The comparison between two models shows that the modeling outside the plasmapause can affect the dynamic even inside the plasmasphere because the populations outside the plasmapause can act as seed population to radiation belt particles inside the plasmapause. One weakness of our chorus modeling is that it is trained during the early phase of solar cycle 24 where very few strong storms occurred. Therefore, our model might not be valid in reproducing the chorus activity under extremely disturbed conditions, which should be updated in the future once chorus measurements for such conditions become available.

Description: [1]  We investigate the storm-scale morphology of the magnetospheric magnetic field as well as underlying distributions of electric currents, equatorial plasma pressure and entropy for four Steady Magnetospheric Convection (SMC) events that occurred during the May 2000 and October 2011 magnetic storms. The analysis is made using the empirical geomagnetic field model TS07D, in which the structure of equatorial currents is not predefined and it is dictated by data. The model also combines the strengths of statistical and event-oriented approaches in mining data for the reconstruction of the magnetic field. The formation of a near-Earth minimum of the equatorial magnetic field in the midnight sector is inferred from data without ad hoc assumptions of a special current system postulated in earlier empirical reconstructions. In addition, a new SMC class is discovered where the minimum equatorial field is substantially larger and located closer to Earth. The magnetic field tailward of the minimumis also much larger, and the corresponding area of accumulated magnetic flux may occupy a very short tail region. The equatorial current and plasma pressure are found to be strongly enhanced far beyond geosynchronous orbit and in a broad local time interval covering the whole nightside region. This picture is consistent with independent recent statistical studies of the SMC pressure distributions, global MHD and kinetic RCM-E simulations. Distributions of the flux tube volume and entropy inferred from data reveal different mechanisms of the magnetotail convection crisis resolution for two classes of SMC events.

Description: [1]  Drifts in the Parker spiral interplanetary magnetic field are known to be an important component in the propagation of galactic cosmic rays, while they are thought to be negligible for Solar Energetic Particles (SEPs). As a result they have so far been ignored in SEP propagation modelling and data analysis. We examine drift velocities in the Parker spiral within single particle first-order adiabatic theory, in a local coordinate system with an axis parallel to the magnetic field. We show that, in the presence of scattering in interplanetary space, protons at the high end of the SEP energy range experience significant gradient and curvature drift. In the scatter-free case, drift due to magnetic field curvature is present. The magnitude of drift velocity increases by more than an order of magnitude at high heliographic latitudes compared to near the ecliptic; it has a strong dependence on radial distance r from the Sun, reaching a maximum at r ~1 AU at low heliolatitudes and r ~10 AU at high heliolatitudes. Due to the mass over charge dependence of drift velocities, the effect of drift for partially ionised SEP heavy ions is stronger than for protons. Drift is therefore likely to be a considerable source of cross field transport for high energy SEPs.

Description: [1]  We present a novel method for the automatic retrieval of local plasma density measurements from the Mars advanced radar for sub-surface and ionospheric sounding (MARSIS) active ionospheric sounder (AIS) instrument. The resulting large data set is then used to study the configuration of the Martian ionosphere at altitudes above ~300 km. An empirical calibration routine is used, which relates the local plasma density to the measured intensity of multiple harmonics of the local plasma-frequency oscillation, excited in the plasma surrounding the antenna in response to the transmission of ionospheric sounding pulses. Enhanced accuracy is achieved in higher-density ( n e  〉 150 cm − 3 ) plasmas, when MARSIS AIS is able to directly measure the fundamental frequency of the local plasma oscillation. To demonstrate the usefulness of this data set, the derived plasma densities are binned by altitude and solar zenith angle (SZA) in regions over weak ( nT) and strong ( nT) crustal magnetic fields, and we find clear and consistent evidence for a significant asymmetry between these two regions. We show that within the ~300-1200 km altitude range sampled, the median plasma density is substantially higher on the dayside in regions of relatively stronger crustal fields than under equivalent illuminations in regions of relatively weaker crustal fields. Conversely, on the nightside, median plasma densities are found to be higher in regions of relatively weaker crustal fields. We suggest that the observed asymmetry arises as a result of the modulation of the efficiency of plasma transport processes by the irregular crustal fields, and the generally horizontal draped interplanetary magnetic field (IMF).

Description: [1]  Global models of the Van Allen radiation belts usually include resonant wave-particle interactions as a diffusion process, but there is a large uncertainty over the diffusion rates. Here we present a new diffusion matrix for whistler mode chorus waves that can be used in such models. Data from seven satellites are used to construct 3,536 power spectra for upper and lower band chorus for 1.5 ≤  L ∗  ≤ 10, MLT, magnetic latitude 0 o  ≤ | λ m | ≤ 60 o and five levels of K p . Five density models are also constructed from the data. Gaussian functions are fitted to the spectra and capture typically 90% of the wave power. The frequency maxima of the power spectra vary with L ∗ and are typically lower than that used previously. Lower band chorus diffusion increases with geomagnetic activity and is largest between 21:00 and 09:00 MLT. Energy diffusion extends to a few MeV at large pitch angles 〉 60 o and at high energies exceeds pitch angle diffusion at the loss cone. Most electron diffusion occurs close to the geomagnetic equator (〈 12 o ). Pitch angle diffusion rates for lower band chorus increase with L ∗ and are significant at L ∗  = 8 even for low levels of geomagnetic activitywhile upper band chorus is restricted to mainly L ∗  〈 6. The combined drift and bounce averaged diffusion rates for upper and lower band chorus extend from a few keV near the loss cone up to several MeV at large pitch angles indicating loss at low energies and net acceleration at high energies.

Description: [1]  We report that radio science (RS) experiment onboard Mars Express (MEX) have observed three plasma layers in the nighttime ionosphere of Mars at altitudes ~80-100 km, ~120 km and ~160 km, which are reproduced by model calculation due to impact of meteoroid, solar wind proton and electron respectively. The densities of 21 ions (Mg + , Fe + , Si + , MgO + , MgCO 2 + , MgO 2 + , MgN 2 + , FeO + , FeO 2 + , FeN 2 + , FeCO 2 + , SiO + , SiCO 2 + , SiN 2 + , SiO 2 + , CO 2 + , N 2 + , O + , O 2 + , CO + , and NO + ) have been computed between altitude 50 km and 200 km. The model shows that all atmospheric ions (CO 2 + , N 2 + , O + , CO + , O 2 + and NO + ) are produced above 100 km due to solar wind electron and proton impact ionizations. The metallic ions are formed between 50 km and 100 km due to ablation of micrometeoroids. It is found that mass ~3.0 × 10 -4  g of incoming meteoroid is sufficient for meteor ablation and its characteristic flux ~4.0 × 10 -15  cm -2  s -1 could produce the nighttime metallic layer observed by MEX. The calculated electron densities are also compared with the occultation measurements made by Mars 4/5 in the nighttime ionosphere of Mars.

Description: [1]  Ultra Low Frequency (ULF) waves transfer energy in the Earth's magnetosphere through a variety of mechanisms that impact the Earth's ionosphere, radiation belts, and other plasma populations. Measurements of the electromagnetic portion of the energy transfer rate are an important source of information for assessing the importance of ULF waves relative to other energy transfer mechanisms as well as a diagnostic for studying the behavior of ULF waves. Using THEMIS satellite data, we examine the time averaged electromagnetic energy transfer rate, or Poynting vector, as a function of frequency and region of the magnetosphere; for this study, we focus on the direction and rate of energy transfer relative to the background magnetic field, comparing perpendicular and parallel transfer rates. This study extends earlier studies of the ULF wave Poynting vector that focused on narrower frequency ranges or specific regions of the magnetosphere; here, we consider the 3–50 mHz frequency range, all local time sectors, radial distances from 3 to 13 Re, and magnetic latitudes close to the equatorial plane. We measure time averaged Poynting vectors that range from  10 − 11 to 10 − 5 W / m 2 , with larger Poynting vector magnitudes occurring at largerradial distances and smaller frequencies. In every spatial region and frequency we examined, we found a large degree of scatter in both the Poynting vector magnitude and direction. The Poynting vector tends to be anisotropic at all frequencies,with more energy transferred along rather than across the background magnetic field. This preference for parallel energy transfer near the magnetic equator suggests that Joule dissipation in the ionosphere and the acceleration of auroral electrons are the largest sinks of ULF wave energy in the magnetosphere.

Description: [1]  Previous statistical studies have found a close relationship between high speed streams and high latitude geomagnetic activity. The speed by itself would increase the geoeffectivity of the solar wind. But it is also believed that pure Alfvénic fluctuations, often found in the trailing part of the streams, play a role in the solar wind driving of geomagnetic activity by amplifying the north-south component of the magnetic field ( B Z ), and thereby the dayside reconnection electric field. By automatically identifying slow and fast solar wind streams and by analysing them for more than one solar cycle, we aimed to study the relation between speed, Alfvénicity and B Z in the solar wind. It was found out that streams whose trailing parts are dominated by pure Alfvénic fluctuations, are the most geoeffective streams, on average. However, it is not the pure Alfvénic fluctuations themselves which cause the streams to be more geoeffective. There is only a variation of about 10 % in B Z due to the Alfvénicity of the fluctuations. Instead the streams are more geoeffective because the pure Alfvénic fluctuations tend to occur during high solar wind speed and strong IMF. There is a substantial variation within the solar cycle of how Alfvénic the solar wind streams are, and years with many extremely Alfvénic streams tends to have more days with moderately large geoeffectivity. The list of solar wind streams is included as extra material to this paper.

Description: [1]  Data Interpolating Empirical Orthogonal Functions (DINEOFs) are a data-based method for determining a few orthogonal basis functions that optimally reproduce a given data set. This technique is applied to meridional drift measurements performed by the Coupled Ion Neutral Dynamics Investigation (CINDI) onboard the Communication/Navigation Outage Forecasting System (C/NOFS) as well as electron density profiles derived from GPS Radio Occulations (RO) performed by COSMIC. The low densities of the equatorial ionosphere spanning 2009 - 2010 restricted quality drift measurements by CINDI to altitudes near perigee, limiting the local time coverage of measurements. Full local time descriptions may be obtained as perigee moves through all local times though this requires a minimum 67 day season. To increase the data coverage of the ionosphere CINDI data is supplemented with COSMIC GPS RO data. DINEOFs are applied to median meridional drift measurements as well as COSMIC measurements spanning 2009-10 and are used to make a best estimate of the equatorial ionosphere at locations not observed. The scattered distribution of COSMIC profiles as well as the physical relationship between meridional ion drifts and the distribution of density with altitude improve thequality of the reconstructions compared to using CINDI alone. The DINEOF reconstructions demonstrate that the annual anomomly of reduced ionospheric densities in June compared to December measured by COSMIC is coincident with a change in the meridional ion drifts at the geomagnetic equator measured by CINDI.

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]  It is well accepted that the propagation of electromagnetic ion cyclotron (EMIC) waves are bidirectional near their source regions and unidirectional when away from these regions. The generally believed source region for EMIC waves is around the magnetic equatorial plane. Here we describe a series of EMIC waves in the Pc1 (0.2-5 Hz) frequency band above the local He + cyclotron frequency observed in situ by all four Cluster spacecraft on 9 April 2005 at mid-magnetic latitudes (MLAT = ~33°-49°) with L = 10.7-11.5 on the dayside (MLT = 10.3-10.4). A Poynting vector spectrum shows that the wave packets consist of multiple groups of packets propagating bidirectionally, rather than unidirectionally, away from the equator, while the local plasma conditions indicate that the spacecraft are entering into a region sufficient for local wave excitation. One possible interpretation is that, while part of the observed waves are inside their source region, the others are either close enough to the source region, or mixed with the wave packets from multiple source regions at different latitudes.

Description: [1]  Using mesospheric temperature information obtained by Rayleigh lidar observations over Gadanki (13.5°N, 79.2°E) and temperature, chemical constituents and their exothermic chemical reaction heating rates obtained by Sounding of Atmosphere by Broadband Emission Radiometry (SABER) instrument onboard Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) satellite over and near to Gadanki location, it is demonstrated that, the dominant role of chemical heating for the occurrence of a few large mesospheric inversion layer (MIL) events during January-February 2011. The heating rates due to exothermic chemical reactions among atomic oxygen (O), atomic hydrogen (H), molecular oxygen (O 2 ), ozone (O 3 ) and hydroxyl (OH) species are the key factors involved in the MIL formation. As relatively larger (lower) ozone mixing ratios are observed at the top (bottom) of the inversion layers, large heating rate (~ 15 K/day) is found to occur between 80 and 85 km due to the exothermic reaction between atomic hydrogen and ozone H + O 3  → OH + O 2 , which dominates all other important exothermic chemical reactions during these MIL events and the large volume emission rates of energy of O 2 , OH are found to be mostly due to the reaction between H and O 3. It is also demonstrated that dynamics plays a negligible role in the formation of these MILs as conditions for the convective and dynamic instability are found to be not satisfied during these MIL events.

Description: [1]  Bursty bulk flows (BBFs) play an important role for the mass, energy and magnetic flux transport in the plasma sheet, and the flow pattern in and around a BBF has important consequences for the localized energy conversion between the electromagnetic and plasma mechanical energy forms. The plasma flow signature in and around BBFs is often rather complicated. Return flows and plasma vortices are expected to exist at the flanks of the main flow channel, especially near the inner plasma sheet boundary, but also further down-tail. A dipolarization front (DF) is often observed at the leading edge of a BBF, and a flux pileup region (FPR) behind the DF. Here we present Cluster data of three FPRs associated with vortex flows observed in the mid-tail plasma sheet on August 15, 2001. According to the principles of Fu et al . [2011] and Fu et al . [2012c], two of the FPRs are considered to be in an early stage of evolution (growing FPRs). The third FPR is in a later stage of evolution (decaying FPR). For the first time, the detailed energy conversion properties during various stages of the FPR evolution have been measured. We show that the later stage FPR has a more complex vortex pattern than the two earlier stage FPRs. The two early stage FPR correspond to generators, E  ⋅  J  〈 0, while the later stage FPR only shows weak generator characteristics and is instead dominated by load signatures at the DF, E  ⋅  J  〉 0. Moreover, to our knowledge this is one of the first times BBF related plasma vortices have been observed to propagate over the spacecraft in the mid-tail plasma sheet at geocentric distances of about 18 R E . Our observations are compared to recent simulation results [ Wiltberger et al ., 2000; Birn et al ., 2011] and previous observations [ Fu et al ., 2011, 2012c; Pang et al ., 2012].

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 %.