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In the winter, orographic precipitation falls as snow in the mid- to high latitudes where it causes avalanches, affects local infrastructure, or leads to flooding during the spring thaw. We present a technique to validate operational numerical weather prediction model simulations in complex terrain. The presented verification technique uses a combined retrieval approach to obtain surface snowfall accumulation and vertical profiles of snow water at the Haukeliseter test site, Norway. Both surface observations and vertical profiles of snow are used to validate model simulations from the Norwegian Meteorological Institute's operational forecast system and two simulations with adjusted cloud microphysics. Retrieved surface snowfall is validated against measurements conducted with a double-fence automated reference gauge (DFAR). In comparison, the optimal estimation snowfall retrieval produces +10.9% more surface snowfall than the DFAR. The predicted surface snowfall from the operational forecast model and two additional simulations with microphysical adjustments (CTRL and ICE-T) are overestimated at the surface with +41.0%, +43.8%, and +59.2%, respectively. Simultaneously, the CTRL and ICE-T simulations underestimate the mean snow water path by −1071.4% and −523.7%, respectively. The study shows that we would reach false conclusions only using surface accumulation or vertical snow water content profiles. These results highlight the need to combine ground-based in situ and vertically profiling remote sensing instruments to identify biases in numerical weather prediction. [ABSTRACT FROM AUTHOR]

The prevailing snowfall regimes at two Scandinavian sites, Haukeliseter, Norway, and Kiruna, Sweden, are documented using ground-based in situ and remote sensing methods. Micro Rain Radar (MRR) profiles indicate three distinct snowfall regimes occur at both sites: shallow, deep, and intermittent snowfall. The shallow snowfall regime produces the lowest mean snowfall rates and radar echo tops are confined below 1.5 km above ground level (AGL). Shallow snowfall occurs under areas of large-scale subsidence with a moist boundary layer and dry air aloft. The atmospheric ridge coinciding with shallow snowfall is highly anomalous over Haukeliseter but is more common in Kiruna where shallow snowfall was frequently observed. The shallow snowfall particle size distributions (PSDs) are broad with lower particle concentrations than other regimes, especially small particles. Deep snowfall events exhibit MRR profiles that extend above 2 kmAGL and tend to be associated with weak low pressure and high relative humidity throughout the troposphere. The PSDs in deep events are narrower with high concentrations of small particles. Increasing MRR re- flectivity toward the surface suggests aggregation as a possible growth process during deep snowfall events. The heaviest mean snowfall rates are associated with intermittent events that are characterized by deep MRR profiles but have variations in intensity and height. The intermittent regime is associated with anomalous, deep low pressure along the coast of Norway and enhanced relative humidity at lower levels. The PSDs reveal high concentrations of small and large particles. The analysis reveals that there are unique characteristics of shallow, deep, and intermittent snowfall regimes that are common between the sites. [ABSTRACT FROM AUTHOR]

Snowfall is one of the primary drivers of the global cryosphere and is declining in many regions of the world with widespread hydrological and ecological consequences. Previous studies have shown that the probability of snowfall occurrence is well described by wet-bulb temperatures below 1°C (1.1°C) over land (ocean). Using this relationship, wet-bulb temperatures from three reanalysis products as well as multisatellite and reanalysis precipitation data are analyzed from 1979 to 2017 to study changes in potential snowfall areas, snowfall-to-rainfall transition latitude, snowfall amount, and snowfall-to-precipitation ratio (SPR). Results are presented at hemispheric scales, as well as for three Köppen–Geiger climate classes and four major mountainous regions including the Alps, the western United States, High Mountain Asia (HMA), and the Andes. In all reanalysis products, while changes in the wet-bulb temperature over the Southern Hemisphere are mostly insignificant, significant positive trends are observed over the Northern Hemisphere (NH). Significant reductions are observed in annual-mean potential snowfall areas over NH land (ocean) by 0.52 (0.34) million km2 decade−1 due to an increase of 0.34°C (0.35°C) decade−1 in wet-bulb temperature. The fastest retreat in NH transition latitudes is observed over Europe and central Asia at 0.7° and 0.45° decade−1. Among mountainous regions, the largest decline in potential snowfall areas is observed over the Alps at 3.64% decade−1 followed by the western United States at 2.81% and HMA at 1.85% decade−1. This maximum decrease over the Alps is associated with significant reductions in annual snowfall of 20 mm decade−1 and SPR of 2% decade−1. [ABSTRACT FROM AUTHOR]

Presented are four winter seasons of data from an enhanced precipitation instrument suite based at the National Weather Service (NWS) Office in Marquette (MQT), Michigan (250–500 cm of annual snow accumulation). In 2014 the site was augmented with a Micro Rain Radar (MRR) and a Precipitation Imaging Package (PIP). MRR observations are utilized to partition large-scale synoptically driven (deep) and surface-forced (shallow) snow events. Coincident PIP and NWS MQT meteorological surface observations illustrate different characteristics with respect to snow event category. Shallow snow events are often extremely shallow, with MRR-indicated precipitation heights of less than 1500 m above ground level. Large vertical reflectivity gradients indicate efficient particle growth, and increased boundary layer turbulence inferred from observations of spectral width implies increased aggregation in shallow snow events. Shallow snow events occur 2 times as often as deep events; however, both categories contribute approximately equally to estimated annual accumulation. PIP measurements reveal distinct regime-dependent snow microphysical differences, with shallow snow events having broader particle size distributions and comparatively fewer small particles and deep snow events having narrower particle size distributions and comparatively more small particles. In addition, coincident surface meteorological measurements indicate that most shallow snow events are associated with surface winds originating from the northwest (over Lake Superior), cold temperatures, and relatively high surface pressures, which are characteristics that are consistent with cold-air outbreaks. Deep snow events have meteorologically distinct conditions that are accordant with midlatitude cyclones and frontal structures, with mostly southwest surface winds, warmer temperatures approaching freezing, and lower surface pressures. [ABSTRACT FROM AUTHOR]

Retrievals of falling snow from space-based observations represent key inputs for understanding and linking Earth's atmospheric, hydrological, and energy cycles. This work quantifies and investigates causes of differences among the first stable falling snow retrieval products from the Global Precipitation Measurement (GPM) Core Observatory satellite and CloudSat's Cloud Profiling Radar (CPR) falling snow product. An important part of this analysis details the challenges associated with comparing the various GPM and CloudSat snow estimates arising from different snow–rain classification methods, orbits, resolutions, sampling, instrument specifications, and algorithm assumptions. After equalizing snow–rain classification methodologies and limiting latitudinal extent, CPR observes nearly 10 (3) times the occurrence (accumulation) of falling snow as GPM's Dual-Frequency Precipitation Radar (DPR). The occurrence disparity is substantially reduced if CloudSat pixels are averaged to simulate DPR radar pixels and CPR observations are truncated below the 8-dBZ reflectivity threshold. However, even though the truncated CPR- and DPR-based data have similar falling snow occurrences, average snowfall rate from the truncated CPR record remains significantly higher (43%) than the DPR, indicating that retrieval assumptions (microphysics and snow scattering properties) are quite different. Diagnostic reflectivity (Z)–snow rate (S) relationships were therefore developed at Ku and W band using the same snow scattering properties and particle size distributions in a final effort to minimize algorithm differences. CPR–DPR snowfall amount differences were reduced to ~16% after adopting this diagnostic Z–S approach. [ABSTRACT FROM AUTHOR]

The ability of in situ snowflake microphysical observations to constrain estimates of surface snowfall accumulations derived from coincident, ground-based radar observations is explored. As part of the High-Latitude Measurement of Snowfall (HiLaMS) field campaign, a Micro Rain Radar (MRR), Precipitation Imaging Package (PIP), and Multi-Angle Snow Camera (MASC) were deployed to the Haukeliseter Test Site run by the Norwegian Meteorological Institute during winter 2016/17. This measurement site lies near an elevation of 1000 m in the mountains of southern Norway and houses a double-fence automated reference (DFAR) snow gauge and a comprehensive set of meteorological observations. MASC and PIP observations provided estimates of particle size distribution (PSD), fall speed, and habit. These properties were used as input for a snowfall retrieval algorithm using coincident MRR reflectivity measurements. Retrieved surface snowfall accumulations were evaluated against DFAR observations to quantify retrieval performance as a function of meteorological conditions for the Haukeliseter site. These analyses found differences of less than 10% between DFAR- and MRR-retrieved estimates over the field season when using either PIP or MASC observations for low wind "upslope" events. Larger biases of at least 50% were found for high wind "pulsed" events likely because of sampling limitations in the in situ observations used to constrain the retrieval. However, assumptions of MRR Doppler velocity for mean particle fall speed and a temperature-based PSD parameterization reduced this difference to +16% for the pulsed events. Although promising, these results ultimately depend upon selection of a snowflake particle model that is well matched to scene environmental conditions. [ABSTRACT FROM AUTHOR]

Effective disaster response requires well-coordinated efforts among individuals and agencies. Although collaborative disaster response increases in popularity, little has been accomplished within the hierarchical, centralized command and control context of China. This study examined collaborative disaster response in China based on the case of extraordinary serious cryogenic freezing rain and snow disaster. In addition, public managers were surveyed to investigate network establishment, with preliminary analysis on whole network using centrality measures. Subsequently, the blockmodel was employed to discuss the whole network structure followed by analysis on structural holes and intermediaries. Lastly, issues such as obstacles to effective collaboration and propositions proposed for further research were discussed. [ABSTRACT FROM AUTHOR]

The character of a winter can be defined by many of its features, including temperature averages and extremes, snowfall totals, snow depth, and the duration between onset and cessation of winter-weather conditions. The accumulated winter season severity index incorporates these elements into one site-specific value that defines the severity of a particular winter, especially when examined in the context of climatological values for that site. Thresholds of temperature, snowfall, and snow depth are assigned points that accumulate through the defined winter season; a parallel index uses temperature and precipitation to provide a snow proxy where snow data are unavailable or unreliable. The results can be analyzed like any other meteorological parameter to examine relationships to teleconnection patterns, determine trends, and create sector-specific applications, as well as to analyze an ongoing winter or any individual winter season to place its severity in context. [ABSTRACT FROM AUTHOR]

Winter precipitation can be very disruptive to travel by aircraft and by motor vehicles. Vehicle fatalities due to winter precipitation are considered 'indirect' and are not counted in Storm Data, the publication commonly used to evaluate losses from meteorological hazards. The goal of this study is to examine the spatial and temporal characteristics of these indirect transportation fatalities that involve winter precipitation for the period 1975-2011. Motor vehicle fatalities were gathered from the National Highway Traffic Safety Administration's (NHTSA) Fatality Analysis Reporting System (FARS) database, while aviation fatalities were collected from the National Transportation Safety Board's (NTSB) Aviation Accident database. Statistical analysis and geographic information systems (GIS) were used to assess the spatial and temporal characteristics of these deaths. Most winter-precipitation-related motor vehicle fatalities occur during the daylight hours. Fatal motor vehicle accident rates are higher than expected in the Northeast and Great Lakes regions, while winter-precipitation-related aviation fatalities are most common in the western United States. Vehicle fatality counts due to winter weather are compared to fatality counts for various hazards from Storm Data to highlight the differences between the datasets. Because of the exclusion of vehicle fatalities, Storm Data underestimates by an order of magnitude the number of fatalities that involve winter weather each year. It is hoped that a better understanding of winter precipitation mortality can be applied in order to reduce fatalities in the future. [ABSTRACT FROM AUTHOR]

On 8-9 February 2013, the northeastern United States experienced a historic winter weather event ranking among the top five worst blizzards in the region. Heavy snowfall and blizzard conditions occurred from northern New Jersey, inland to New York, and northward through Maine. Storm-total snow accumulations of 30-61 cm were common, with maximum accumulations up to 102 cm and snowfall rates exceeding 15 cm h−1. Dual-polarization radar measurements collected for this winter event provide valuable insights into storm microphysical processes. In this study, polarimetric data from the Weather Surveillance Radar-1988 Doppler (WSR-88D) in Upton, New York (KOKX), are investigated alongside thermodynamic analyses from the 13-km Rapid Refresh model and surface precipitation type observations from both Meteorological Phenomena Identification Near the Ground (mPING) and the National Weather Service (NWS) Forecast Office in Upton, New York, for interpretation of polarimetric signatures. The storm exhibited unique polarimetric signatures, some of which have never before been documented for a winter system. Reflectivity values were unusually large, reaching magnitudes >50 dB Z in shallow regions of heavy wet snow near the surface. The 0°C transition line was exceptionally distinct in the polarimetric imagery, providing detail that was often unmatched by the numerical model output. Other features include differential attenuation of magnitudes typical of melting hail, depolarization streaks that provide evidence of electrification, nonuniform beamfilling, a 'snow flare' signature, and localized downward excursions of the melting-layer bright band collocated with observed transitions in surface precipitation types. In agreement with previous studies, widespread elevated depositional growth layers, located at temperatures near the model-predicted −15°C isotherm, appear to be correlated with increased snowfall and large reflectivity factors Z H near the surface. [ABSTRACT FROM AUTHOR]

The eastern Great Lakes (Erie and Ontario) are often affected by intense lake-effect snowfalls. Lake-effect storms that form parallel to the major axes of these lakes can strongly impact communities by depositing more than 100 cm of snowfall in less than 24 h. Long-lake-axis-parallel (LLAP) storms are significantly different in structure and dynamics compared to the much more studied wind-parallel roll storms that typically form over the western Great Lakes. A Doppler on Wheels (DOW) mobile radar sampled several of these storms at fine spatial and temporal resolutions (and close to the surface) during the winter of 2010-11 over and downwind of Lake Ontario to document and improve understanding of how these storms develop. Over 1100 observations of vortices were catalogued within the 16 December 2010 and 4-5 January 2011 events. The majority of these vortices were less than 1 km in diameter with a statistical modal difference in Doppler velocity (delta-V) value across the vortex of 11 m s−1. Vortices developed along boundaries, which formed within the bands, suggesting horizontal shear instability was the main cause. Other features noted in the DOW observations included bounded weak echo regions, anvils, and horizontal vortices, typically on the south side of west-east-oriented LLAP bands. The reflectivity and velocity structure of LLAP bands were found to be much more complex than previously thought, which may impact localized precipitation amounts and errors in forecast location/intensity. [ABSTRACT FROM AUTHOR]

Winter storms, namely snowstorms and ice storms, are a major hazard and forecasting challenge across central North Carolina. This study employed a trajectory approach to analyze the ingredients (i.e., temperature, moisture, and lift) associated with heavy snowstorms and ice storms that occurred within the Raleigh, North Carolina, National Weather Service forecast region from 2000 to 2010. The Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) tool was used to calculate 72-h backward (i.e., upstream) air parcel trajectories from three critical vertical pressure levels at the time and location of heaviest precipitation for each storm. Analysis of composite trajectories revealed the source regions and meteorological properties of air parcels associated with heavy winter storms. Adiabatic and diabatic contributions to air parcel temperature and moisture content were also estimated along each trajectory to assess the physical processes connected with heavy winter precipitation in the region. Results indicate that diabatic warming and cooling contribute significantly to the vertical temperature profile during heavy winter storms and therefore dictate the resulting precipitation type. The main source of diabatic warming is fluxes of sensible and latent heat within the marine atmospheric boundary layer over the Gulf Stream. These fluxes contribute to a warming and moistening of air parcels associated with heavy ice storms. In contrast, heavy snowstorms are characterized by diabatic cooling in the lower troposphere above the marine atmospheric boundary layer. The most significant moisture source for heavy snowfall is the Caribbean Sea, while heavy ice storms entrain moisture from the Gulf of Mexico and Gulf Stream region near the Carolina coast. [ABSTRACT FROM AUTHOR]

Soil moisture changes are generally due to external factors (precipitation, evaporation, etc.) and internal forces (gravitational force, capillarity, transpiration, etc.). When soil temperatures remain below 0 °C for a long time (hours or even entire consecutive days), part of the liquid water content of the soil can freeze, thus freezing/thawing effects must be taken into account in those conditions. The present work is devoted to the numerical modeling of the water phase change in the soil. The model used in this study for the land surface processes is UTOPIA (University of TOrino land Process Interaction in Atmosphere) model, which is the updated version of LSPM (Land Surface Process Model). Scientific literature proposes some formulations to account for freezing/thawing processes. Three different parameterizations have been compared using a synthetic dataset in order to assess which one performs best from a physical point of view. Parameterizing freezing/thawing processes creates numerical instability and water overproduction in the UTOPIA model. These problems have been solved and described in the paper by means of synthetic data created to test the new parameterizations. The results show that UTOPIA is able to capture the freezing/thawing physical processes. [ABSTRACT FROM AUTHOR]

The phase of precipitation formed within the atmosphere is highly dependent on the vertical temperature profile through which it falls. In particular, several precipitation types can form in an environment with a melting layer aloft and a refreezing layer below. These precipitation types include freezing rain, ice pellets, wet snow, and slush. To examine the formation of such precipitation, a bulk microphysics scheme was used to compare the characteristics of the hydrometeors produced by the model and observed by a research aircraft flight during the 1998 ice storm near Montreal, Canada. The model reproduced several of the observed key precipitation characteristics. Sensitivity tests on the precipitation types formed during the ice storm were also performed. These tests utilized temperature profiles produced by the North American Regional Reanalysis. The results show that small variations (±0.5°C) in the temperature profiles as well as in the precipitation rate can have major impacts on the types of precipitation formed at the surface. These results impose strong requirements on the accuracy needed by prediction models. [ABSTRACT FROM AUTHOR]

Global climate models predict that terrestrial northern high-latitude snow conditions will change substantially over the twenty-first century. Results from a Community Climate System Model simulation of twentieth and twenty-first (SRES A1B scenario) century climate show increased winter snowfall (+10–40%), altered maximum snow depth (−5 ± 6 cm), and a shortened snow-season (−14 ± 7 days in spring, +20 ± 9 days in autumn). By conducting a series of prescribed snow experiments with the Community Land Model, we isolate how trends in snowfall, snow depth, and snow-season length affect soil temperature trends. Increasing snowfall, by countering the snowpack-shallowing influence of warmer winters and shorter snow seasons, is effectively a soil warming agent, accounting for 10–30% of total soil warming at 1 m depth and ~16% of the simulated twenty-first century decline in near-surface permafrost extent. A shortening snow season enhances soil warming due to increased solar absorption whereas a shallowing snowpack mitigates soil warming due to weaker winter insulation from cold atmospheric air. Snowpack deepening has comparatively less impact due to saturation of snow insulative capacity at deeper snow depths. Snow depth and snow-season length trends tend to be positively related, but their effects on soil temperature are opposing. Consequently, on the century timescale the net change in snow state can either amplify or mitigate soil warming. Snow state changes explain less than 25% of total soil temperature change by 2100. However, for the latter half of twentieth century, snow state variations account for as much as 50–100% of total soil temperature variations. [ABSTRACT FROM AUTHOR]

Anomalous warming occurred over the Tibetan Plateau (TP) before and during the disastrous freezing rain and heavy snow hitting central and southern China in January 2008. The relationship between the TP warming and this extreme event is investigated with an atmospheric general circulation model. Two perpetual runs were performed. One is forced by the climatological mean sea surface temperatures in January as a control run; and the other has the same model setting as the control run except with an anomalous warming over the TP that mimics the observed temperature anomaly. The numerical results demonstrate that the TP warming induces favorable circulation conditions for the occurrence of this extreme event, which include the deepened lower-level South Asian trough, the enhanced lower-level southwesterly moisture transport in central-southern China, the lower-level cyclonic shear in the southerly flow over southeastern China, and the intensified Middle East jet stream in the middle and upper troposphere. Moreover, the anomalous TP warming results in a remarkable cold anomaly near the surface and a warm anomaly aloft over central China, forming a stable stratified inversion layer that favors the formation of the persistent freezing rain. The possible physical linkages between the TP warming and the relevant resultant circulation anomalies are proposed. The potential reason of the anomalous TP warming during the 2007–08 winter is also discussed. [ABSTRACT FROM AUTHOR]

This paper investigated the interactions between the synoptic patterns, quasi-stationary fronts, eastward-propagating cloud clusters from the Tibetan Plateau, surface conditions, and atmospheric stratification processes associated with a 20-day event of freezing precipitation over southern China from January to February 2008. It was found that the long duration of the freezing precipitation process was primarily caused by stationary and anomalous synoptic weather patterns such as a blocking high pressure system in the northern branch and a trough in the south branch of the westerlies, which resulted in the convergence of cold air from northern China and warm, moist air from the south. The cloud clusters over the Tibetan Plateau propagated eastward and showed noticeable impacts in the local areas when they moved over southern China during several similar cloud propagation processes from January to February 2008. An east–west-oriented quasi-stationary front system in southern China, which is rare during the Asian winter monsoon season, is responsible for producing freezing precipitation and snowstorms. A stronger horizontal gradient of the isolines of the pseudo-equivalent potential temperature and higher temperatures at the inversion layer in the western part of the front than that in its eastern part can be found. At the same time, low-level moisture convergence ahead of the front enhanced the formation, development, and persistence of freezing precipitation in the west part of the front. The thickness of the warm layer and the temperature inversion layer also modulated the intensity and duration of freezing rain and ice pellets. Temperature from about −1° to −3°C and weak winds were found to be favorable meteorological factors at the surface level for freezing precipitation. These analysis results are synthesized into a conceptual model that coherently describes the physics processes associated with the synoptic features and quasi-stationary front system as well as the atmospheric stratification process during the freezing precipitation event. [ABSTRACT FROM AUTHOR]

In January 2008, central and southern China experienced persistent low temperatures, freezing rain, and snow. The large-scale conditions associated with the occurrence and development of these snowstorms are examined in order to identify the key synoptic controls leading to this event. Three main factors are identified: 1) the persistent blocking high over Siberia, which remained quasi-stationary around 65°E for 3 weeks, led to advection of dry and cold Siberian air down to central and southern China; 2) a strong persistent southwesterly flow associated with the western Pacific subtropical high led to enhanced moisture advection from the Bay of Bengal into central and southern China; and 3) the deep inversion layer in the lower troposphere associated with the extended snow cover over most of central and southern China. The combination of these three factors is likely responsible for the unusual severity of the event, and hence a long return period. [ABSTRACT FROM AUTHOR]

The National Operational Hydrologic Remote Sensing Center (NOHRSC) Snow Model (NSM) is an energy- and mass-balance model used by the National Oceanic and Atmospheric Administration’s National Weather Service for moderate-resolution spatially distributed snow analysis and data assimilation over the United States. The NSM was evaluated in a one-dimensional mode using meteorological and snowpit observations from five sites in Colorado collected during 2002–03. Four parameters estimated by the NSM [snow water equivalent (SWE), snow depth, average snowpack temperature, and snow surface temperature] were compared with snowpit observations and with estimates from another snow energy and mass-balance model, SNTHERM. Root-mean-squared differences (RMSDs) between snowpit SWE observations (January–June) at all sites and estimates from the NSM were about 11% (RMSD = 0.073 m) of the average maximum observed SWE from all sites of 0.694 m. SNTHERM exhibited only a slightly better agreement (RMSD = 0.066 m). During the winter and early spring period before snowpacks became isothermal at 273.15 K, both NSM and SNTHERM simulated significantly cooler average snowpack temperatures than observed (RMSD = 3 and 2 K, respectively). During this snow accumulation period estimates of SWE by both models were very similar. Differences in modeled SWE were traced to short periods (5–21 days) during isothermal conditions in early spring when the two models diverged. These events caused SWE differences that persisted throughout the ablation period and resulted in a range in melt-out times of 0.2–7.2 days between depth observations and modeled estimates. The divergence in SWE resulted from differences in snowmelt fluxes estimated by the two models, which are suggested to result from 1) liquid water fractions within a snowpack being estimated by the NSM using an internal energy method and by SNTHERM using a semiempirical temperature-based approach, and 2) SNTHERM, but not the NSM, accounting for the small liquid water fraction that coexists in equilibrium with snow when the snowpack surface is dry (<273.15 K). [ABSTRACT FROM AUTHOR]

Abstract: An extraction method for the quantitative analysis of polybrominated diphenyl ethers (PBDEs) in aqueous samples has been evaluated. The analytical methodology includes the sample filtration through glass fiber filter and subsequent extraction of dissolved phase compounds by C18 solid-phase disk extraction. Dependence of extraction efficiency on factors such as pollutant concentrations, sample volume, and stability during storage has been investigated. Mean extraction efficiencies of 97% for total PBDEs (13 tri- to heptabrominated congeners at spiking levels in the range of 15–90pg) with a RSD between 9 and 20% were achieved. Higher recoveries were observed for the more volatile PBDEs (112%) in relation to more brominated congeners (88%). The developed methodology was successfully applied to the analysis of these compounds in atmospheric deposition and snow samples from remote sites in Europe with method detection and quantitation limits in the range of 2.1–10pgL−1 for almost all congeners, which allow the determination of PBDEs in remote areas with levels in the range of low to medium pgL−1 for ΣPBDEs. [Copyright &y& Elsevier]

This study provides an assessment of changes in the occurrence frequency of four types of adverse-weather (freezing precipitation, blowing snow, fog, and low ceilings) and no-weather (i.e., no precipitation or visibility obscuration) events as observed at 15 Canadian Arctic stations of good hourly weather observations for 1953–2004. The frequency time series were subjected to a homogenization procedure prior to a logistic regression–based trend analysis. The results show that the frequency of freezing precipitation has increased almost everywhere across the Canadian Arctic since 1953. Rising air temperature in the region has probably resulted in more times that the temperature is suitable for freezing precipitation. On the contrary, the frequency of blowing snow occurrence has decreased significantly in the Canadian Arctic. The decline is most significant in spring. Changes in fog and low ceiling (LC) occurrences have similar patterns and are most (least) significant in summer (autumn). Decreases were identified for both types of events in the eastern region in all seasons. In the southwest, however, the fog frequency has increased significantly in all seasons, while the LC frequency has decreased significantly in spring and summer. The regional mean rate of change in the frequency of the four types of adverse weather was estimated to be 7%–13% per decade. The frequency of no-weather events has also decreased significantly at most of the 15 sites. The decrease is most significant and extensive in autumn. Comparison with the adverse-weather trends above indicates that the decline in no-weather occurrence (i.e., increase in weather occurrence) is not the result of an increase in blowing snow or fog occurrence; it is largely the result of the increasing frequency of freezing precipitation and, most likely, other types of precipitation as well. This is consistent with the reported increases in precipitation amount and more frequent cyclone activity in the lower Canadian Arctic. [ABSTRACT FROM AUTHOR]

Ice in the troposphere affects a variety of processes, including the formation of precipitation, and cloud lifetime, albedo, dynamics, and electrification. A lack of understanding of the ways in which ice is created and multiplied hampers progress in understanding all of these processes. We survey the state of knowledge, starting with homogeneous nucleation, where current formulations for freezing from both pure water and solutions have considerable predictive power. However, debate still exists on the underlying mechanisms of nucleation. Using the concepts and framework that homogeneous nucleation provides, heterogeneous nucleation, where neither a commonly agreed upon theory nor even standard measurement technique exists, is considered. Investigators have established the ice-nucleating characteristics of broad classes of substances, such as mineral dust and soot, which are important ice nuclei in the atmosphere, but a coherent theory of why these substances act as they do has yet to emerge. All ice in clouds is the result of a nucleation event, but its concentration can be enhanced by secondary processes that multiply and magnify the original nucleation events. Riming particles splinter in certain conditions, and this process explains many, but not all, instances of ice concentrations that are greater than those created via primary nucleation. It seems that important secondary processes have not been identified, especially in cases with no liquid water. [ABSTRACT FROM AUTHOR]

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The article reports on the extraordinary snowfall in Buenos Aires, Argentina in July 2007. It is stated that the snow covered the ground of the city during the early period of the month. According to the author, the city's last recorded snowfall was in 1973 while the first time that snow had blanketed the city occurred in 1918. Héctor Ciappesoni, vice president of the Argentina Meteorology Center, denotes that the incident coincided with the unusual frigid arctic conditions in South America having low temperatures, freezing rain and other wintry weather. He adds that such event caused more than 100 deaths, disrupted transportation and strained energy supplies across the continent.