Search

South-western France has witnessed some of the most devastating extreme precipitation events that eventually lead to record-breaking severe flash flooding in the region and cause losses to human lives, urban transportation, agriculture, and infrastructure. In this study, two cases of deadly flash floods that occurred/reported in the Aude watershed in south-western France during 12-13 November 1999 and 14-15 October 2018 are studied using the WRF4.3.1 model simulations, with a particular emphasis on the model ability to capture these heavy precipitation events. We performed two simulations one with parameterized convection and one without the use of convection parameterizations for each case at gray-zone resolution (~9 km horizontal grid spacing) using the ERA5 reanalysis as the lateral boundary condition. In addition, attempts have been made to investigate the role of large-scale atmospheric circulation and atmospheric rivers in the production of these heavy precipitation events. The results from model simulations are compared quantitatively with available observations and reanalysis and found that the simulations at ~9 km gray-zone resolution capture the observed spatio-temporal distribution of precipitation characteristics during both extreme cases. The added value of gray-zone resolution simulations over driving coarse-scale ERA5 reanalysis datasets is observed in the representation of the precipitation characteristics. It has also been observed that the model simulation without the use of convection parameterization yields a reasonable and realistic representation of the precipitation characteristics during both extreme cases, and this suggests that at this “gray-zone” resolution the organized mesoscale convective systems/processes can be resolved explicitly by the model dynamics. The contribution of the large-scale atmospheric circulation and the atmospheric river (i.e., moisture transport) in the production of these flood events has also been observed.

The hydrological cycle over southern France represents an interplay between highly variable regional precipitation impacting the variability of soil moisture and continental runoff. The moisture sources for regional precipitation are associated with the moisture advection from the Mediterranean Sea (Gulf of Lion). Moreover, the strong coupling between the atmosphere and the land surface processes is affecting all branches of the regional hydrological cycle, being modulated by the complicated orography of the region. Potential intensification of the regional hydrological cycle under the ongoing and projected climate change may result in extreme hydroclimate events. Assessment of their nature and mechanisms is challenging for understanding climate variability over the Mediterranean. We perform the analysis of long-term daily precipitation time series over southern France (more than 300 rain gauge stations from METEO-FRANCE collection) along with the modern era Reanalyses (ERA5, JRA55) and satellite datasets (TRMM, GPCP, and PERSIANN CDR) over the period from 1979-2014. Diagnostics include estimates of linear trends and interannual variability in precipitation totals, number of wet days, precipitation intensity and different measures of precipitation extremes. Further we quantify the differences in precipitation variability patterns across different data products and demonstrate their strengths and weaknesses. We also examined regional responses of soil moisture to precipitation of different intensities using data from the GLEAM-GLDAS dataset and 14 SMOSANIA stations. Finally, we analyse the representation of the regional hydrological cycle in historical simulations with regional (Euro-CORDEX) and global (CMIP6) climate models. This allows for establishing a statistical links between precipitation, soil moisture, and continental runoff and for quantifying the role of regional atmospheric circulation and moisture advection in forming multidecadal changes in the regional hydrological cycle.

The Red Sea (RS) has a high evaporation rate, exceeding 2 m of water per year. The water vapor is transported from the shorelines by sea breezes as far as 200 km landward. Relative humidity in the vicinity of the RS exceeds 80% in summer. Nevertheless, precipitation is scarce in most of the Arabian RS coastal plain. In this work we use the Weather Research and Forecasting (WRF) regional model to assess how deliberate changes (geoengineering) in the surface albedo or conversion of bare land to wide-leaf forests over a vast coastal plain region affect precipitation over the Arabian RS coast. Our simulations show that geoengineering of land surface characteristics perturbs coastal circulation; alters temperature, moisture, and momentum exchange between the land surface and atmosphere; and changes the breeze intensity, cloud cover, and eventually the amount of precipitation. We find that extended afforestation and increased surface albedo are not effective in triggering rainfall over the RS coastal plains. Conversely, decreasing surface albedo to 0.2, assuming installation of solar panels over the coastal plains, increases surface air temperature by 1–2 K, strengthens horizontal surface temperature differences between sea and land, intensifies breezes, increases water vapor mixing ratio in the boundary layer above 3 km by about 0.5 g kg−1, enhances vertical mixing within the planetary boundary layer, and generates 1.5 Gt of extra rainwater, equivalent to the annual consumption of five million people. Thus, this form of regional land surface geoengineering, along with advanced methods of collection and underground storage of freshwater, provides a feasible solution to mitigation of the existing water crisis in the arid coastal regions.

Empirical study of the isotopic features of river runoff were carried out at three hydrological posts in 3 different river basins: the Zakza river in the center of East European Plane (southwest of the Moscow), the Dubna river (north of the Moscow) and the Sosna river in the south of central region. Samples of river water, groundwater and precipitation for the October 2019–October 2021 were collected at weekly intervals. A significant supply of melted snow during spring freshet is the key factor influencing water regimes for these three river basins; varying degrees of anthropogenic flow regulation are also present. During the observation period, there were fundamentally two completely different conditions in terms of runoff formation. First, October 2019–October 2020, there was abnormal low spring freshet instead there was high rain flood in summer. In October 2020–October 2021 there was a normal intra-annual flow pattern with high spring freshet. The new data of stable isotope signature of river runoff component can help to study the response of a river runoff to changing climate conditions.

The hydrological cycle over southern France is an interplay between highly variable precipitation impacting the variability of soil moisture and continental runoff. The regional hydrological cycle experiences strong impacts of the Mediterranean Sea (Gulf of Lion) through the moisture advection. Moreover, the strong coupling between the atmosphere and the land surface is affecting all branches of the regional hydrological cycle, especially due to the specific structure of the orography in the region. Under the climate change scenarios potential intensification of the regional hydrological cycle, resulting in extreme hydroclimate events is a critically important phenomenon for understanding mechanisms of climate variability over the Mediterranean.We perform the analysis of long-term daily precipitation time series over southern France (300 stations from METEO-FRANCE collection) along with modern era Reanalysis (ERA5, JRA55) and satellite datasets (TRMM, GPCP, and Persian) over the period from 1979 onwards. Diagnostics estimate linear trends and interannual variability in precipitation totals and precipitation extremes and allow for quantifying the differences in precipitation variability patterns across the different datasets, demonstrating their strengths and weaknesses. Then we associated changes in precipitation with tendencies in soil moisture derived from the GLEAM-GRACE datasets, thus quantifying regional responses of soil moisture to precipitation impacts. Finally, we analyze the representation of the observed regional hydrological cycle in historical simulations with regional (Euro-CORDEX) and global (CMIP6) climate models. We, thus, establish a statistical association between precipitation, soil moisture, and continental runoff and quantify the role of regional atmospheric circulation in forming multidecadal changes in the regional hydrological cycle.

Globally, thermodynamics explains an increase in atmospheric water vapor with warming of around 7%/°C near to the surface. In contrast, global precipitation and evaporation are constrained by the Earth's energy balance to increase at ∼2-3%/°C. However, this rate of increase is suppressed by rapid atmospheric adjustments in response to greenhouse gases and absorbing aerosols that directly alter the atmospheric energy budget. Rapid adjustments to forcings, cooling effects from scattering aerosol, and observational uncertainty can explain why observed global precipitation responses are currently difficult to detect but are expected to emerge and accelerate as warming increases and aerosol forcing diminishes. Precipitation increases with warming are expected to be smaller over land than ocean due to limitations on moisture convergence, exacerbated by feedbacks and affected by rapid adjustments. Thermodynamic increases in atmospheric moisture fluxes amplify wet and dry events, driving an intensification of precipitation extremes. The rate of intensification can deviate from a simple thermodynamic response due to in-storm and larger-scale feedback processes, while changes in large-scale dynamics and catchment characteristics further modulate the frequency of flooding in response to precipitation increases. Changes in atmospheric circulation in response to radiative forcing and evolving surface temperature patterns are capable of dominating water cycle changes in some regions. Moreover, the direct impact of human activities on the water cycle through water abstraction, irrigation, and land use change is already a significant component of regional water cycle change and is expected to further increase in importance as water demand grows with global population.

Over the first half of 2020, Siberia experienced the warmest period from January to June since records began and on the 20th of June the weather station at Verkhoyansk reported 38 °C, the highest daily maximum temperature recorded north of the Arctic Circle. We present a multi-model, multi-method analysis on how anthropogenic climate change affected the probability of these events occurring using both observational datasets and a large collection of climate models, including state-of-the-art higher-resolution simulations designed for attribution and many from the latest generation of coupled ocean-atmosphere models, CMIP6. Conscious that the impacts of heatwaves can span large differences in spatial and temporal scales, we focus on two measures of the extreme Siberian heat of 2020: January to June mean temperatures over a large Siberian region and maximum daily temperatures in the vicinity of the town of Verkhoyansk. We show that human-induced climate change has dramatically increased the probability of occurrence and magnitude of extremes in both of these (with lower confidence for the probability for Verkhoyansk) and that without human influence the temperatures widely experienced in Siberia in the first half of 2020 would have been practically impossible. Supplementary Information The online version contains supplementary material available at 10.1007/s10584-021-03052-w.

This paper evaluates several daily precipitation products over western and central Europe, identifies and documents their respective strengths and shortcomings, and relates these to uncertainties associated with each of the products. We analyze one gauge-based, three satellite-based, and two reanalysis-based products using high-density rain gauge observations as reference. First, we assess spatial patterns and frequency distributions using aggregated statistics. Then, we determine the skill of precipitation event detection from these products with a focus on extremes, using temporally and spatially matched pairs of precipitation estimates. The results show that the quality of the datasets largely depends on the region, season, and precipitation characteristic addressed. The satellite and the reanalysis precipitation products are found to have difficulties in accurately representing precipitation frequency with local overestimations of more than 40%, which occur mostly in dry regions (all products) as well as along coastlines and over cold/frozen surfaces (satellite-based products). The frequency distributions of wet-day intensities are generally well reproduced by all products. Concerning the frequency distributions of wet-spell durations, the satellite-based products are found to have clear deficiencies for maritime-influenced precipitation regimes. Moreover, the analysis of the detection of extreme precipitation events reveals that none of the non-station-based datasets shows skill at the shortest temporal and spatial scales (1 day, 0.25°), but at and above the 3-day and 1.25° scale the products start to exhibit skill over large parts of the domain. Added value compared to coarser-resolution global benchmark products is found both for reanalysis and satellite-based products.

Atmospheric moisture convergence on ice sheets provides an estimate of snow accumulation, which is critical to quantifying sea-level changes. In the case of East Antarctica, we computed moisture transport from 1980 to 2016 in five reanalyses and in radiosonde observations. Moisture convergence in reanalyses is more consistent than net precipitation but still ranges from 72 to 96 mm yr−1 in the four most recent reanalyses, ERA-Interim, NCEP CFSR, JRA 55 and MERRA 2. The representation of long-term variability in reanalyses is also inconsistent, which justified resorting to observations. Moisture fluxes are measured on a daily basis via radiosondes launched from a network of stations surrounding East Antarctica. Observations agree with reanalyses on the major role of extreme advection events and transient eddy fluxes. Although assimilated, the observations reveal processes that reanalyses cannot model, some due to a lack of horizontal and vertical resolution, especially the oldest, NCEP DOE R2. Additionally, the observational time series are not affected by new satellite data unlike the reanalyses. We formed pan-continental estimates of convergence by aggregating anomalies from all available stations. We found statistically significant trends neither in moisture convergence nor in precipitable water.

We present in this paper the results of the Russian Academy of Sciences North Atlantic Atmospheric Downscaling (RAS-NAAD) project, which provides a 40-yr 3D hindcast of the North Atlantic (10°–80°N) atmosphere at 14-km spatial resolution with 50 levels in the vertical direction (up to 50 hPa), performed with a regional setting of the WRF-ARW 3.8.1 model for the period 1979–2018 and forced by ERA-Interim as a lateral boundary condition. The dataset provides a variety of surface and free-atmosphere parameters at sigma model levels and meets many demands of meteorologists, climate scientists, and oceanographers working in both research and operational domains. Three-dimensional model output at 3-hourly time resolution is freely available to the users. Our evaluation demonstrates a realistic representation of most characteristics in both datasets and also identifies biases mostly in the ice-covered regions. High-resolution and nonhydrostatic model settings in NAAD resolve mesoscale dynamics first of all in the subpolar latitudes. NAAD also provides a new view of the North Atlantic extratropical cyclone activity with a much larger number of cyclones as compared with most reanalyses. It also effectively captures highly localized mechanisms of atmospheric moisture transports. Applications of NAAD to ocean circulation and wave modeling are demonstrated.

The Dry Land Belt (DLB) of Northern Eurasia is the largest contiguous dryland on Earth. Changes in DLB during the last century have included resource extraction (e.g., coal, oil, gas, and mineral ores), rapid land cover and land use change (e.g., expansion of irrigated croplands and cities), institutional shifts (e.g., collapse of the Soviet Union, China’s entry to the World Trade Organization), global and regional climatic changes, and natural disturbances (e.g., wildfire, drought, dust). These factors intertwine, overlap, and sometimes mitigate but sometimes feedback to each other exacerbating their synergistic and cumulative effects. Thus, it is important to document properly the key external and internal factors and to characterize structural relationships among them to study better approaches to alleviating negative consequences of these regional environmental changes. This chapter addresses key climatic changes observed over the DLB in recent decades and it outlines possible linkages of these changes with other external and internal factors of the contemporary regional environmental change and human activities within the DLB.

A method for discriminating among different types of precipitation is presented. The method is based on surface observations of precipitation, present and past weather, and the morphological types of clouds. The climatology of showery, nonshowery, and drizzle precipitation in Northern Eurasia is studied using the data of 529 Russian weather stations for the period of 1966–2014. Showery precipitation dominates in Northern Eurasia. In general, showery precipitation has greater temporal (monthly and diurnal) and spatial variability than nonshowery precipitation. The majority of showers are registered in summer (the maximum is in July), whereas the high est total monthly nonshowery precipitation is observed in autumn (the maximum is in October). The daily intensity values of showery and nonshowery precipitation are generally close, the maximum intensity is recorded in July–August. For three-hour in tervals, the shower in tensity is by 1.1–1.5 times higher. The drawbacks of the presented methodology are discussed.

The major sources of atmospheric moisture over the Red Sea are analyzed using ERA-Interim for the 1979–2013 period. The vertical structure of moisture transports across the coastlines has been computed separately for the western and eastern coasts of the Red Sea. The vertical structure of the moisture transport from the Red Sea to the continents is dominated by a breeze-like circulation in the near-surface layer and the Arabian high above 850 hPa. The lower-layer, breeze-like circulation is acting to export the moisture to the northwest of Africa and to the Arabian Peninsula and contributes about 80% of the moisture exports from the Red Sea, dominating over the transport in the upper layer, where the moisture is advected to the Arabian Peninsula in the northern part of the sea and to the African continent in the southern part. Integrated moisture divergence over the Red Sea decreased from the early 1980s to 1997 and then increased until the 2010s. Associated changes in the moisture export were provided primarily by the increasing intensity of the breeze-associated transports. The transports above the boundary layer, while being strong across the western and the eastern coasts, have a smaller effect on the net moisture export. The interannual variability of the moisture export in the near-surface layer was found to be closely correlated with the variability in sea surface temperature, especially in summer. Implications of the observed changes in the moisture advection for the hydrological cycle of the Middle East are discussed.

Atmospheric moisture convergence on ice-sheets provides an estimate of snow accumulation which is critical to quantify sea level changes. In the case of East Antarctica, we computed moisture transport from 1980 to 2016 in five reanalyses and in radiosonde observations. Moisture convergence in reanalyses is more consistent than net preciptation but still ranges from 72 to 96 mm per year in the four most recent reanalyses, ERA Interim, NCEP CFSR, JRA 55 and MERRA 2. The representation of long term variability in reanalyses is also inconsistent which justified the resort to observations. Moisture fluxes are measured on a daily basis via radiosondes launched from a network of stations surrounding East Antarctica. Observations agree with reanalyses on the major role of extreme advection events and transient eddy fluxes. Although assimilated, the observations reveal processes some reanalyses cannot model due to a lack of horizontal and vertical resolution especially the oldest, NCEP DOE R2. Additionally, the observational time series are not affected by new satellite data unlike reanalyses. We formed pan-continental estimates of convergence by aggregating anomalies from all available stations. We found no statistically significant trend neither in moisture convergence nor in precipitable water.

International audience; The Warm Arctic-cold Siberia surface temperature pattern during recent boreal winter is suggested to be triggered by the ongoing decrease of Arctic autumn sea ice concentration and has been observed together with an increase in mid-latitude extreme events and a meridionalization of tropospheric circulation. However, the exact mechanism behind this dipole temperature pattern is still under debate, since model experiments with reduced sea ice show conflicting results. We use the early twentieth-century Arctic warming (ETCAW) as a case study to investigate the link between September sea ice in the Barents-Kara Sea (BKS) and the Siberian temperature evolution. Analyzing a variety of long-term climate reanalyses, we find that the overall winter temperature and heat flux trend occurs with the reduction of September BKS sea ice. Tropospheric conditions show a strengthened atmospheric blocking over the BKS, strengthening the advection of cold air from the Arctic to central Siberia on its eastern flank, together with a reduction of warm air advection by the westerlies. This setup is valid for both the ETCAW and the current Arctic warming period.

This study uses daily observations and modern reanalyses in order to evaluate reanalysis products over northern Eurasia regarding the spring snow albedo feedback (SAF) during the period from 2000 to 2013. We used the state-of-the-art reanalyses from ERA-Interim/Land and the Modern-Era Retrospective Analysis for Research and Applications version 2 (MERRA-2) as well as an experimental set-up of ERA-Interim/Land with prescribed short grass as land cover to enhance the comparability with the station data while underlining the caveats of comparing in situ observations with gridded data. Snow depth statistics derived from daily station data are well reproduced in all three reanalyses. However day-to-day albedo variability is notably higher at the stations than for any reanalysis product. The ERA-Interim grass set-up shows improved performance when representing albedo variability and generates comparable estimates for the snow albedo in spring. We find that modern reanalyses show a physically consistent representation of SAF, with realistic spatial patterns and area-averaged sensitivity estimates. However, station-based SAF values are significantly higher than in the reanalyses, which is mostly driven by the stronger contrast between snow and snow-free albedo. Switching to grass-only vegetation in ERA-Interim/Land increases the SAF values up to the level of station-based estimates. We found no significant trend in the examined 14-year time series of SAF, but interannual changes of about 0.5 % K−1 in both station-based and reanalysis estimates were derived. This interannual variability is primarily dominated by the variability in the snowmelt sensitivity, which is correctly captured in reanalysis products. Although modern reanalyses perform well for snow variables, efforts should be made to improve the representation of dynamic albedo changes.

Dynamical processes in the atmosphere and ocean are central in determining the large-scale drivers of regional climate change, yet predictive understanding of climate dynamics is poor. In this perspective article, we identify three frontline problems in climate dynamics, in which progress is possible, and which will lead to improved predictions and projections and more robust information for climate change adaptation. These problems involve (i) the response to external forcing of storms, blocks and jet streams, (ii) ocean-basin to ocean-basin and tropical-extratropical teleconnections and (iii) the development of predictive theories of climate dynamics involving non-linear interactions between the dynamics and physics of the atmosphere and ocean. We highlight opportunities and techniques for making immediate progress in climate dynamics, namely the development of high-resolution coupled model simulations, partial coupling or pacemaker experiments and the development and use of dynamical metrics and dynamical hierarchies of models. Our particular focus is on the role of the ocean in providing a slowly-evolving boundary condition, and its coupling and interaction with the atmosphere, that plays a central role in driving internal climate variability and in modulating forced climate change.

During the past several decades, the Earth system has changed significantly, especially across NorthernEurasia. Changes in the socio-economic conditions of the larger countries in the region have also resulted ina variety of regional environmental changes that can have global consequences. The Northern Eurasia FutureInitiative (NEFI) has been designed as an essential continuation of the Northern Eurasia Earth SciencePartnership Initiative (NEESPI), which was launched in 2004. NEESPI sought to elucidate all aspects of ongoingenvironmental change, to inform societies and, thus, to better prepare societies for future developments. Akey principle of NEFI is that these developments must now be secured through science-based strategies codesignedwith regional decision-makers to lead their societies to prosperity in the face of environmental andinstitutional challenges. NEESPI scientific research, data, and models have created a solid knowledge base tosupport the NEFI program. This paper presents the NEFI research vision consensus based on that knowledge.It provides the reader with samples of recent accomplishments in regional studies and formulates new NEFIscience questions. To address these questions, nine research foci are identified and their selections are brieflyjustified. These foci include warming of the Arctic; changing frequency, pattern, and intensity of extreme andinclement environmental conditions; retreat of the cryosphere; changes in terrestrial water cycles; changes inthe biosphere; pressures on land use; changes in infrastructure; societal actions in response to environmentalchange; and quantification of Northern Eurasia’s role in the global Earth system. Powerful feedbacks betweenthe Earth and human systems in Northern Eurasia (e.g., mega-fires, droughts, depletion of the cryosphereessential for water supply, retreat of sea ice) result from past and current human activities (e.g., large-scalewater withdrawals, land use, and governance change) and potentially restrict or provide new opportunities forfuture human activities. Therefore, we propose that integrated assessment models are needed as the finalstage of global change assessment. The overarching goal of this NEFI modeling effort will enable evaluationof economic decisions in response to changing environmental conditions and justification of mitigation andadaptation efforts.

The STAMMEX (Spatial and Temporal Scales and Mechanisms of Extreme Precipitation Events over Central Europe) project has developed a high-resolution gridded long-term precipitation dataset based on the daily-observing precipitation network of the German Weather Service DWD, which runs one of the world's densest rain gauge networks, comprising more than 7,500 stations. Several quality-controlled daily gridded products with homogenized sampling were developed covering the periods 1931–onward (with 0.5° resolution), 1951–onward (0.5° and 0.25°), and 1971–2000 (0.5°, 0.25°, and 0.1°). Different methods were tested to select the best gridding methodology that minimizes errors of integral grid estimates over hilly terrain. Besides daily precipitation values with uncertainty estimates, the STAMMEX datasets include a variety of statistics that characterize temporal and spatial dynamics of the precipitation distribution (quantiles, extremes, wet/ dry spells, etc.). Comparisons with existing continental-scale daily...

Climate change is expected to change precipitation characteristics and particularly the frequency and magnitude of precipitation extremes. Satellite observations form an important part of the observing system necessary to monitor both temporal and spatial patterns of precipitation variability and extremes. As satellite-based precipitation estimates are generally only indirect, however, their reliability has to be verified. This study evaluates the ability of the satellite-based Global Precipitation Climatology Project One-Degree Daily (GPCP1DD) dataset to reliably reproduce precipitation variability and extremes over Europe compared to the European Daily High-resolution Observational Gridded Dataset (E-OBS). The results show that the two datasets agree reasonably well not only when looking at climatological statistics such as climatological mean, number of wet days (rain rates 1 mm), and mean intensity (i.e., mean over all wet days) but also with respect to their distributions. The results also reveal a pronounced seasonal cycle in the performance of GPCP1DD that is worse in winter and spring. Both deterministic and fuzzy verification methods are used to assess the ability of the GPCP1DD dataset to capture extremes. Fuzzy methods prove to be the better suited evaluation approach for such a highly variable parameter as precipitation because it compensates for slight spatial and temporal displacements. Whereas the deterministic diagnostics confirm previous findings on the deficiencies of satellite products, the “fuzzy” results show that at larger spatiotemporal scales (e.g., 3°/5 days) GPCP1DD has useful skill and is able to reliably represent the spatial and temporal variability of extremes.

Daily rain gauge data over Europe for the period from 1950 to 2009 were used to analyze changes in the duration of wet and dry spells. The duration of wet spells exhibits a statistically significant growth over northern Europe and central European Russia, which is especially pronounced in winter when the mean duration of wet periods increased by 15%–20%. In summer wet spells become shorter over Scandinavia and northern Russia. The duration of dry spells decreases over Scandinavia and southern Europe in both winter and summer. For the discrimination between the roles of a changing number of wet days and of a regrouping of wet and dry days for the duration of the period, the authors suggest a fractional truncated geometric distribution. The changing numbers of wet days cannot explain the long-term variability in the duration of wet and dry periods. The observed changes are mainly due to the regrouping of wet and dry days. The tendencies in duration of wet and dry spells have been analyzed for a number of European areas. Over the Netherlands both wet and dry periods are extended in length during the cold and the warm season. A simultaneous shortening of wet and dry periods is found in southern Scandinavia in summer. Over France and central southern Europe during both winter and summer and over the Scandinavian Atlantic coast in summer, opposite tendencies in the duration of wet and dry spells were identified. Potential mechanisms that might be responsible for the changing durations of wet and dry periods and further perspectives are discussed.

The atmospheric water cycle of the Arctic is evaluated via seven global reanalyses and in radiosonde observations covering the 1979–2013 period. In the regional moisture budget, evaporation and precipitation are the least consistent terms among different datasets. Despite the assimilation of radiosoundings, the reanalyses present a tendency to overestimate the moisture transport. Aside from this overestimation, the reanalyses exhibit a remarkable agreement with the radiosondes in terms of spatial and temporal patterns. The northern North Atlantic, subpolar North Pacific, and Labrador Sea stand out as the main gateways for moisture to the Arctic in all reanalyses. Because these regions correspond to the end of the storm tracks, the link between moisture transports and extratropical cyclones is further investigated by decomposing the moisture fluxes in the mean flow and transient eddy parts. In all reanalyses, the former term tends to cancel out when averaged over a latitude circle, leaving the latter to provide the bulk of the midlatitude moisture imports (89%–94% at 70°N). Although the Arctic warms faster than the rest of the world, the impact of these changes on its water cycle remains ambiguous. In most datasets, evaporation, precipitation, and precipitable water increase in line with what is expected from a warming signal. At the same time, the moisture transports have decreased in all the reanalyses but not in the radiosonde observations, though none of these trends is statistically significant. The fluxes do not scale with the Clausius–Clapeyron relation because the increasing humidity is not correlated with the meridional wind, particularly near the surface.

The long-term variability in heavy precipitation characteristics over Europe for the period 1950–2000 is analyzed using high-quality daily records of rain gauge measurements from the European Climate Assessment (ECA) dataset. To improve the accuracy of heavy precipitation estimates, the authors suggest estimating the fractional contribution of very wet days to total precipitation from the probability distribution of daily precipitation than from the raw data, as it is adopted for the widely used R95tot precipitation index. This is feasible under the assumption that daily precipitation follows an analytical distribution like the gamma probability density function (PDF). The extended index R95tt based on the gamma PDF is compared to the classical R95tot index. The authors find that R95tt is more stable, especially when precipitation extremes are estimated from the limited number of wet days of seasonal and monthly time series. When annual daily time series are analyzed, linear trends in R95tt and R95tot are qualitatively consistent; both hint at a growing occurrence of extreme precipitation of up to 3% decade−1 in central western Europe and in south European Russia, with a somewhat more evident trend pattern for the R95tt index. Linear trends estimated for individual seasons, however, exhibit pronounced differences when derived from both indices. In particular, in winter, R95tt clearly reveals an increasing occurrence of extreme precipitation in western European Russia (up to 4% decade−1), while during summer, a downward tendency in the fractional contribution of very wet days is found in central western Europe. The new index also allows for a better association of European extreme precipitation with the North Atlantic Oscillation (NAO) index by showing a more consistent spatial correlation pattern and higher correlation levels compared to R95tot.

Cyclone activity and life cycle are analysed in the coupled GCMs ECHAM5/OM and ECHAM4/OPYC3. First, the results for the present climate (1978–1999) are compared with ERA-40 and NCEP/NCAR reanalyses, showing a drastic improvement in the representation of cyclone activity in ECHAM5/OM compared to ECHAM4/OPYC3. The total number of cyclones, cyclone intensity, propagation velocity and deepening rates are found to be much more realistic in ECHAM5/OM relative to ECHAM4/OPYC3. Then, changes in extra tropical cyclone characteristics are compared between present day climate and future climate under the emission-scenario A1B using ECHAM5/OM. This comparison is performed using the 20-year time slices 1978–1999, 2070–2090 and 2170–2190, which were considered to be representative for the various climate conditions. The total number of cyclones does not undergo significant changes in a warmer climate. However, regional changes in cyclone numbers and frequencies are evident. One example is the Mediterranean region where the number of cyclones in summer increases almost by factor 2. Some noticeable changes are also found in cyclone life cycle characteristics (deepening rate and propagation velocity). Cyclones in the future climate scenario tend to move slower and their deepening rate becomes stronger, while cyclone intensity does not undergo significant change in a warmer climate. Generally, our results do not support the hypothesis of enhanced storminess under future climate conditions.

Statistical characteristics of daily precipitation in the reanalyses of the National Centers of Environmental Prediction (NCEP1 and NCEP2) and the European Center for Medium Range Weather Forecasts (ECMWF; ERA15 and ERA40) are intercompared with each other and with the in situ data assembled from different collections of station observations. Intercomparison is performed over the European continent. Precipitation statistics analyzed were the precipitation intensity, the parameters of Gamma distribution and the 99% percentiles of daily precipitation. NCEP1 and NCEP2 reanalyses show the higher occurrence of heavy precipitation than ECMWF products. Station data in comparison to the reanalyses show significantly higher estimates of heavy and extreme precipitation. Among the four reanalyses, NCEP2 demonstrates the closest to the station data estimates of extreme precipitation. The analysis of linear trends of statistical characteristics of heavy precipitation in ERA40 and NCEP1 for a 43-year period shows similarity of the trend patterns in winter and identifies strong local disagreements, resulting in the trends of opposite signs during summer. Interannual variability of statistical characteristics in different reanalyses is quite consistent over the Northern and Eastern Europe than in the mountain regions of the Southern Europe. Correlation between statistical characteristics of precipitation in different reanalyses and between the reanalyses and station data is 20–30% higher during the winter season. D 2004 Elsevier B.V. All rights reserved.

Recent observational studies have shown that the centers of action of interannual variability of the North Atlantic Oscillation (NAO) were located farther eastward during winters of the period 1978–97 compared to previous decades of the twentieth century. In this study, which focuses on the winter season (December–March), new diagnostics characterizing this shift are presented. Further, the importance of this shift for NAO-related interannual climate variability in the North Atlantic region is discussed. It is shown that an NAO-related eastward shift in variability can be found for a wide range of different parameters like the number of deep cyclones, near-surface air temperature, and turbulent surface heat flux throughout the North Atlantic region. By using a near-surface air temperature dataset that is homogenous with respect to the kind of observations used, it is shown that the eastward shift is not an artifact of changes in observational practices that took place around the late 1970s. Finally, an EOF-based Monte Carlo test is developed to quantify the probability of changes in the spatial structure of interannual NAO variability for a relatively short (20 yr) time series given multivariate “white noise.” It is estimated that the likelihood for differences in the spatial structure of the NAO between two independent 20-yr periods, which are similar (as measured by the angle and pattern correlation between two NAO patterns) to the observed differences, to occur just by chance is about 18%. From the above results it is argued that care has to be taken when conclusions about long-term properties of NAO-related climate variability are being drawn from relatively short recent observational data (e.g., 1978–97).

Synoptic-scale variability in the air‐sea turbulent fluxes in the areas of midlatitudinal western boundary currents is analyzed. In the Gulf Stream area, ocean‐atmosphere fluxes on synoptic time- and space scales are clearly coordinated with the propagating synoptic-scale atmospheric transients. The statistical analysis of 6-hourly resolution sea level pressure and surface turbulent fluxes from the NCEP‐NCAR reanalysis for the period from 1948 to 2000 in the area of strong sea surface temperature gradients in the Gulf Stream gives strong proof for the association between the propagating cyclones and synoptic patterns of surface turbulent fluxes. It is shown that sea‐air interaction in this area is controlled by the sharpness of surface temperature gradients in the ocean and by the intensity of the advection of the air masses in different parts of cyclones during the cold-air and warm-air outbreaks. A simple parameter based on the joint consideration of the characteristics of sea surface temperature and sea level pressure fields is used to characterize the synoptic variability of air‐sea turbulent fluxes. The effectiveness of the relationship between surface temperature and surface pressure on one side and air‐sea flux anomalies on the other vary from year to year in phase with variability in the frequencies of deep atmospheric cyclones in the Gulf Stream area. The limits of applicability of the approach, its sensitivity to higher-resolution sea surface temperature data, and the possibility of its further applications are discussed.

The uncertainties associated with the mapping of cyclone numbers and frequencies are analyzed using the 42-yr winter climatology of cyclone tracks derived from 6-hourly NCEP–NCAR reanalysis. Tracking is performed using an automated procedure, based on computer animation of the sea level pressure fields. Uncertainties in the mapping result from an incomplete catchment of cyclones by the grid cells: the coarse temporal resolution of data causes fast-moving storms to skip grid boxes. This introduces error into estimates of cyclone frequency, with cyclone counts systematically underestimated. To minimize these biases, it is possible to simulate higher temporal resolution of the storm tracks by linear interpolation applied to the original tracks. This simple procedure reduces bias in estimates of both cyclone frequencies and numbers and enables quantitative estimation of errors. Errors in cyclone counts are estimated over the Northern Hemisphere for different time resolutions and different grids. Stan...

The winter climatology of Northern Hemisphere cyclone activity was derived from 6-hourly NCEP/NCAR reanalysis data for the period from 1958 to 1999, using software which provides improved accuracy in cyclone identification in comparison to numerical tracking schemes. Cyclone characteristics over the Kuroshio and Gulfstream are very different to those over continental North America and the Arctic. Analysis of Northern Hemisphere cyclones shows secular and decadal-scale changes in cyclone frequency, intensity, lifetime and deepening rates. The western Pacific and Atlantic are characterized by an increase in cyclone intensity and deepening during the 42-year period, although the eastern Pacific and continental North America demonstrate opposite tendencies in most cyclone characteristics. There is an increase of the number of cyclones in the Arctic and in the western Pacific and a downward tendency over the Gulf Stream and subpolar Pacific. Decadal scale variability in cyclone activity over the Atlantic and Pacific exhibits south-north dipole-like patterns. Atlantic and Pacific cyclone activity associated with the NAO and PNA is analyzed. Atlantic cyclone frequency demonstrates a high correlation with NAO and reflects the NAO shift in the mid 1970s, associated with considerable changes in European storm tracks. The PNA is largely linked to the eastern Pacific cyclone frequencies, and controls cyclone activity over the Gulf region and the North American coast during the last two decades. Assessment of the accuracy of the results and comparison with those derived using numerical algorithms, shows that biases inherent in numerical procedures are not negligible.

Surface meteorological records from Ocean Weather Stations are used to characterise interannualchanges in the intensity of synoptic and subsynoptic processes over the North Atlanticmid-latitudes during the period from early the 1950s to 1990s. Individual 3-hourly data werefiltered to separate ultra-high frequency variability (0–2 days) and synoptic scale variability(2–6 days). Joint consideration of the intensities at these 2 scales in mid-latitudes makes itpossible to construct parameters which are highly correlated with the North Atlantic Oscillationindex. In high latitudes, intensities of synoptic and ultra-high-frequency processes are stronglynegatively correlated on interannual and decadal time scales. This phenomenon is discussed inthe context of interaction between high and mid-latitudinal synoptic variability. DOI: 10.1034/j.1600-0870.2000.d01-6.x

[1] Analysis of the duration of wet spells (consequent days with significant precipitation) in Europe and associated precipitation is performed over the period 1950–2008 using daily rain gauge data. During the last 60 years wet periods have become longer over most of Europe by about 15– 20%. The lengthening of wet periods was not caused by an increase of the total number of wet days. Becoming longer, wet periods in Europe are now characterized by more abundant precipitation. Heavy precipitation events during the last two decades have become much more frequently associated with longer wet spells and intensified in com‐ parison with 1950s and 1960s. The changes in the distri‐ bution of temporal characteristics of precipitation towards longer events and higher intensities should have a significant impact on the terrestrial hydrologic cycle including sub‐ surface hydrodynamics, surface runoff and European flooding. Citation: Zolina, O., C. Simmer, S. K. Gulev, and S. Kollet (2010), Changing structure of European precipitation: Longer wet periods leading to more abundant rainfalls, Geophys. Res. Lett., 37, L06704, doi:10.1029/2010GL042468.

[1] The newly updated collection of daily precipitation measurements over Western Germany (more than 2000 stations in total) is used to analyze linear trends in extreme and heavy precipitation for different seasons over the period 1950–2004. Heavy and extreme precipitation has been quantified using the 95% and 99% percentiles with respect to the Gamma distribution fitted to daily precipitation data. The significance of linear trends was quantified using several statistical tests including estimates of field significance. Positive linear tendencies in heavy precipitation for the winter, spring and autumn seasons were found for the whole domain with the largest increase of 13% per decade in Central and Southern Germany. For the summer season, however, heavy precipitation exhibits mostly negative trends of up to 8% per decade e.g., for the Central and Southwestern parts of Germany. Trends derived from the estimates of heavy precipitation without seasonal breakdown, however, do not show any clear spatial pattern. Estimates of field significance show that the conclusions concerning the seasonal diversity in trend sign hold for most of Western Germany. The results are insensitive to changes of the beginning and the end of the records by several years; thus the seasonal linear trend patterns are not influenced by interdecadal variability. Seasonality is also identified in the linear trends of mean precipitation characteristics. Analysis performed for different classes of precipitation intensity shows that during winter the linear increase of heavy and extreme precipitation is associated with downward linear tendencies for weak precipitation. In summer statistically significant negative linear trends were identified for all classes of precipitation intensities. Our results also imply that the amplitude of the annual cycle of heavy and extreme precipitation underwent a considerable decrease during the last 55 years between 30% to 60% per decade.

Climate projections point toward more frequent and intense weather and climate extremes such as heat waves, droughts, and floods, in a warmer climate. These projections, together with recent extreme climate events, including flooding in Pakistan and the heat wave and wildfires in Russia, highlight the need for improved risk assessments to help decision makers and the public. But accurate analysis and prediction of risk of extreme climate events require new methodologies and information from diverse disciplines. A recent workshop sponsored by the World Climate Research Programme (WCRP) and hosted at United Nations Educational, Scientific and Cultural Organization (UNESCO) headquarters in France brought together, for the first time, a unique mix of climatologists, statisticians, meteorologists, oceanographers, social scientists, and risk managers (such as those from insurance companies) who sought ways to improve scientists' ability to characterize and predict climate extremes in a changing climate.

The need for research and development of new cost-efficient methods for tracking and analyzing atmospheric cyclones is apparent. Currently storm tracking is performed either manually or using numerical codes. The manual approach is more accurate but it requires considerable time and labor. Numerical schemes [e.g., Murray and Simmonds, 1991] track the cyclones from digital sea level pressure (SLP) data, linking the sequential positions of cyclone centers using different assumptions based on atmospheric dynamics. This approach is very effective computationally but it creates a number of uncertainties and biases, especially in the Northern Hemisphere.

Snow is a critical component of the Arctic climate system. Over Northern Eurasia and North America the duration of snow cover is 7 to 10 months per year and a maximum snow extension is over 40% of the Northern Hemisphere land each year. Snow affects a variety of high latitude climate processes and feedbacks. High reflectivity of snow and low thermal conductivity have a cooling effect and modulates the snow-albedo feedback. A contribution from terrestrial snow to the Earth’s radiation budget at the top of the atmosphere is close to that from the sea ice. Snow also prevents large energy losses from the underlying soil and notably the ice growth and the development of seasonal permafrost. Being a natural water storage, snow plays a critical role in high latitude hydrological cycle, including evaporation and run-off. Snow is also one of the most variable components of climate system. With the Arctic warming twice as fast as the globe, the present and future variability of snow characteristics are crucially important for better understanding of the processes and changes undergoing with climate. However, our capacity to observe the terrestrial Arctic is limited compared to the mid-latitudes and climate models play very important role in our ability to understand the snow-related processes especially in the context of a warming cryosphere. In this respect representation of snow-associated feedbacks in climate models, especially during the shoulder seasons (when Arctic snow cover exhibits the strongest variability) is of a special interest.The focus of this study is on the representation of the Arctic terrestrial snow in global circulation models from Coupled Model Intercomparison Project (CMIP5) ensemble during the melting (March-April) and the onset (October-November) season for the period from 1979 to 2005. Snow characteristics from the general circulation models have been validated against in situ snow measurements, different satellite-based products and reanalyses.We found that snow characteristics in models have stronger bias in spring than in autumn. The annual cycle of snow cover is well captured by models in comparison with observations, however, the annual cycles of snow water equivalent and snow depth are largely overestimated by models, especially in North America. There is better agreement between models and observations in the snow margin position in spring rather than in autumn. Magnitudes of interannual variability for all snow characteristics are significantly underestimated in most CMIP5 models compared to observations. For both seasons, trends of snow characteristics in models are primarily negative but weaker and less significant than those from observations. The patterns of snow cover trends are relatively well reproduced in models, however, the spatial distribution of trends for snow water equivalent and snow depth display strong regional heterogeneities.Finally, we have concluded CMIP5 general circulation models provides valuable information about the snow characteristics in the terrestrial Arctic, however, they have still limitations. There is a lack of agreement among the ensemble of models in the spatial distribution of snow compared to the observations and reanalysis. And these discrepancies are accentuated in regions where variability of snow is higher in areas with complex terrain such as Canada and Alaska and during the melting and the onset season. Our goal in this study was to identify where and when these models are or are not reproducing the real snow characteristics in the Arctic, thus we hope that our results should be considered when using these snow-related variables from CMIP5 historical output in future climate studies., La neige est une composante essentielle du système climatique arctique. Au nord de l'Eurasie et de l'Amérique du Nord, la couverture neigeuse est présente de 7 à 10 mois par an et son extension saisonnière maximale représente plus de 40% de la surface terrestre de l'hémisphère nord. La neige affecte une variété de processus climatiques et de rétroactions aux hautes latitudes. Sa forte réflectivité et sa faible conductivité thermique ont un effet de refroidissement et modulent la rétroaction neige-albédo. Sa contribution au bilan radiatif de la Terre est comparable à celle de la banquise. De plus, en empêchant d'importantes pertes d'énergie du sol sous-jacent, la neige limite la progression de la glace et le développement du pergélisol saisonnier. Réserve d'eau naturelle, la neige joue un rôle essentiel dans le cycle hydrologique aux hautes latitudes, notamment en ce qui concerne l'évaporation et le ruissellement. La neige est l'une des composantes du système climatique présentant la plus forte variabilité. Le réchauffement de l'Arctique étant deux fois plus rapide que celui du reste du globe, la variabilité présente et future des caractéristiques de la neige est cruciale pour une meilleure compréhension des processus et des changements climatiques.Cependant, notre capacité à observer l'Arctique terrestre étant limitée, les modèles climatiques jouent un rôle clé dans notre aptitude à comprendre les processus liés à la neige. À cet égard, la représentation des rétroactions associées à la neige dans les modèles climatiques, en particulier pendant les saisons intermédiaires (lorsque la couverture neigeuse de l'Arctique présente la plus forte variabilité), est primordiale.Notre étude porte principalement sur la représentation de la neige terrestre arctique dans les modèles de circulation générale issus du projet CMIP5 (Coupled Model Intercomparison Project) au cours du printemps (mars-avril) et de l’automne (octobre-novembre) de 1979 à 2005. Les caractéristiques de la neige des modèles de circulation générale ont été validées par rapport aux mesures de neige in situ, ainsi qu’à des produits satellitaires et à des réanalyses.Nous avons constaté que les caractéristiques de la neige dans les modèles ont un biais plus marqué au printemps qu'en automne. Le cycle annuel de la couverture neigeuse est bien reproduit par les modèles. Cependant, les cycles annuels d'équivalent en eau de la neige et de sa profondeur sont largement surestimés par les modèles, notamment en Amérique du Nord. Il y a un meilleur accord entre les modèles et les observations dans la position de la marge de neige au printemps plutôt qu'en automne. Les amplitudes de variabilité interannuelle pour toutes les variables de la neige sont nettement sous-estimées par la plupart des modèles CMIP5. Pour les deux saisons, les tendances des variables de la neige dans les modèles sont principalement négatives, mais plus faibles et moins significatives que celles observées. Les distributions spatiales des tendances de la couverture neigeuse sont relativement bien reproduites par les modèles, toutefois, la distribution spatiale des tendances en équivalent-eau et en profondeur de la neige présente de fortes hétérogénéités régionales.Enfin, nous concluons que les modèles CMIP5 fournissent des informations précieuses sur les caractéristiques de la neige en Arctique terrestre, mais qu’ils présentent encore des limites. Il y a un manque d’accord entre l’ensemble des modèles sur la distribution spatiale de la neige par rapport aux observations et aux réanalyses. Ces écarts sont particulièrement marqués dans les régions où la variabilité de la neige est la plus forte. Notre objectif dans cette étude était d'identifier les circonstances dans lesquelles ces modèles reproduisent ou non les caractéristiques observées de la neige en Arctique. Nous attirons l’attention de la communauté scientifique sur la nécessité de prendre compte nos résultats pour les futures études climatiques.

The water vapour converging to the polar regions condenses into heat-trappingclouds and eventually precipitates, freshening the polar oceans and thickening the ice-sheets. Modulo circulation changes, the moisture transport is expected to increase in a warmer climate. While the extra precipitation could dampen the ice sheets’ contribution to sea level rise, the surplus of moisture could also feed back on the surface warming. However, the present variability of the polar moisture budgets must be known precisely before they can be projected with confidence into the future.This study examines the atmospheric water cycle of both the Arctic and the Antarctic in seven global reanalyses and in radiosonde observations covering the 1979-2013 period. The impacts of known model and assimilation flaws and of the various numerical approximations were evaluated and proven to be limited, at least for the moisture flux variable and the more recent reanalyses.In the Arctic, aside from a slight overestimation, the northward fluxes in reanalyses exhibit a remarkable agreement with the radiosoundings in terms of spatial and temporal patterns. In all reanalyses, transient eddies provide the bulk of the mid-latitude moisture imports – 89-94% at 70◦ N. In most datasets, evaporation, precipitation and precipitable water increase in line with what is expected from a warming signal. However fluxes do not scale with the Clausius-Clapeyron relation because the increasing humidity is not correlatedwith the meridional wind, particularly near the surface.The representations of the Antarctic atmospheric water cycle in reanalyses suffer from the scarcity of observations : the moisture convergence estimations vary from 117 to 156 mm per year. On the coast, the mean moisture flux results from the interplay between transient eddies and katabatic winds, which are particularly sensitive to the orography. The coastalradiosonde sites report significant increases of the southward moisture fluxes but otherwise there are practically no trends in reanalyses on a continental scale.Finally, the share of transient eddies in moisture advection is qualified using alternate methods. In particular, extratropical cyclones leave a characteristic imprint on the transport field, which can be detected and quantified., La vapeur d’eau convergeant vers les régions polaires se condense en nuages quiretiennent la chaleur terrestre. Ces nuages donnent lieu à des précipitations, qui adoucissent les océans polaires et épaississent les calottes de glace. Sans changement des vents, le transport de vapeur d’eau est appelé à augmenter dans un climat plus chaud et donc les chutes de neige sur les calottes aussi. Le surplus d’humidité risque cependant de rétroagir sur le réchauffement de surface.Afin de contraindre les projections futures, cette thèse se propose d’évaluer la variabilité actuelle du cycle de l’eau dans les hautes latitudes. Elle s’appuie sur sept réanalyses globales et des observations par radiosondages allant de 1979 à 2013. Leurs biais intrinsèques et les approximations de calcul n’entament pas les conclusions principales de cette étude.En Arctique, mise à part une légère surestimation, le transport d’humidité dans les réanalyses est remarquablement proche des observations, aussi bien dans le temps que dans l’espace. Dans toutes les réanalyses, les vents dominants n’advectent qu’une fraction de la vapeur d’eau, de 6 à 11%, au profit des perturbations. D’après la plupart des sources, évaporation, précipitation et humidité atmosphérique augmentent en accord avec l’élévation des températures. Toutefois, les flux de vapeur d’eau ne suivent pas la loi de Clausius-Clapeyron car humidité et vents sont moins corrélés, notamment près de la surface.En Antarctique, le manque d’observations se fait sentir : la convergence de vapeur d’eau sur la calotte varie de 117 à 156 mm par an selon les réanalyses. Le transport côtier, très variable dans l’espace, résulte de l’alternance entre vents catabatiques et passage de perturbations. Sur la côte, les radiosondages signalent une augmentation significative des flux d’humidité vers le Sud. À l’échelle du continent en revanche, les réanalyses ne font étatde quasiment aucune tendance.Enfin, le rôle des phénomènes météorologiques d’échelle courte est évalué de nouveau, selon plusieurs méthodes. En particulier, les cyclones extratropicaux laissent dans les flux de vapeur d’eau une empreinte caractéristique qui peut être détectée et quantifiée.