Czech Hydrometeorological Institute

Abstract Global climate change impacts can already be tracked in many physical and biological systems; in particular, terrestrial ecosystems provide a consistent picture of observed changes. One of the preferred indicators is phenology, the science of natural recurring events, as their recorded dates provide a high‐temporal resolution of ongoing changes. Thus, numerous analyses have demonstrated an earlier onset of spring events for mid and higher latitudes and a lengthening of the growing season. However, published single‐site or single‐species studies are particularly open to suspicion of being biased towards predominantly reporting climate change‐induced impacts. No comprehensive study or meta‐analysis has so far examined the possible lack of evidence for changes or shifts at sites where no temperature change is observed. We used an enormous systematic phenological network data set of more than 125 000 observational series of 542 plant and 19 animal species in 21 European countries (1971–2000). Our results showed that 78% of all leafing, flowering and fruiting records advanced (30% significantly) and only 3% were significantly delayed, whereas the signal of leaf colouring/fall is ambiguous. We conclude that previously published results of phenological changes were not biased by reporting or publication predisposition: the average advance of spring/summer was 2.5 days decade −1 in Europe. Our analysis of 254 mean national time series undoubtedly demonstrates that species' phenology is responsive to temperature of the preceding months (mean advance of spring/summer by 2.5 days°C −1 , delay of leaf colouring and fall by 1.0 day°C −1 ). The pattern of observed change in spring efficiently matches measured national warming across 19 European countries (correlation coefficient r =−0.69, P <0.001).

Abstract We present a dataset of daily resolution climatic time series that has been compiled for the European Climate Assessment (ECA). As of December 2001, this ECA dataset comprises 199 series of minimum, maximum and/or daily mean temperature and 195 series of daily precipitation amount observed at meteorological stations in Europe and the Middle East. Almost all series cover the standard normal period 1961–90, and about 50% extends back to at least 1925. Part of the dataset (90%) is made available for climate research on CDROM and through the Internet (at http://www.knmi.nl/samenw/eca ). A comparison of the ECA dataset with existing gridded datasets, having monthly resolution, shows that correlation coefficients between ECA stations and nearest land grid boxes between 1946 and 1999 are higher than 0.8 for 93% of the temperature series and for 51% of the precipitation series. The overall trends in the ECA dataset are of comparable magnitude to those in the gridded datasets. The potential of the ECA dataset for climate studies is demonstrated in two examples. In the first example, it is shown that the winter (October–March) warming in Europe in the 1976–99 period is accompanied by a positive trend in the number of warm‐spell days at most stations, but not by a negative trend in the number of cold‐spell days. Instead, the number of cold‐spell days increases over Europe. In the second example, it is shown for winter precipitation between 1946 and 1999 that positive trends in the mean amount per wet day prevail in areas that are getting drier and wetter. Because of its daily resolution, the ECA dataset enables a variety of empirical climate studies, including detailed analyses of changes in the occurrence of extremes in relation to changes in mean temperature and total precipitation. Copyright © 2002 Royal Meteorological Society.

Abstract This paper describes the HISTALP database, consisting of monthly homogenised records of temperature, pressure, precipitation, sunshine and cloudiness for the ‘Greater Alpine Region’ (GAR, 4–19°E, 43–49°N, 0–3500m asl). The longest temperature and air pressure series extend back to 1760, precipitation to 1800, cloudiness to the 1840s and sunshine to the 1880s. A systematic QC procedure has been applied to the series and a high number of inhomogeneities (more than 2500) and outliers (more than 5000) have been detected and removed. The 557 HISTALP series are kept in different data modes: original and homogenised, gap‐filled and outlier corrected station mode series, grid‐1 series (anomaly fields at 1° × 1°, lat × long) and Coarse Resolution Subregional (CRS) mean series according to an EOF‐based regionalisation. The leading climate variability features within the GAR are discussed through selected examples and a concluding linear trend analysis for 100, 50 and 25‐year subperiods for the four horizontal and two altitudinal CRSs. Among the key findings of the trend analysis is the parallel centennial decrease/increase of both temperature and air pressure in the 19th/20th century. The 20th century increase (+1.2 °C/+ 1.1 hPa for annual GAR‐means) evolved stepwise with a first peak near 1950 and the second increase (1.3 °C/0.6hPa per 25 years) starting in the 1970s. Centennial and decadal scale temperature trends were identical for all subregions. Air pressure, sunshine and cloudiness show significant differences between low versus high elevations. A long‐term increase of the high‐elevation series relative to the low‐elevation series is given for sunshine and air pressure. Of special interest is the exceptional high correlation near 0.9 between the series on mean temperature and air pressure difference (high‐minus low‐elevation). This, further developed via some atmospheric statics and thermodynamics, allows the creation of ‘barometric temperature series’ without use of the measures of temperature. They support the measured temperature trends in the region. Precipitation shows the most significant regional and seasonal differences with, e.g., remarkable opposite 20th century evolution for NW (9% increase) versus SE (9% decrease). Other long‐ and short‐term features are discussed and indicate the promising potential of the new database for further analyses and applications. Copyright © 2006 Royal Meteorological Society.

A warming climate is expected to have an impact on the magnitude and timing of river floods; however, no consistent large-scale climate change signal in observed flood magnitudes has been identified so far. We analyzed the timing of river floods in Europe over the past five decades, using a pan-European database from 4262 observational hydrometric stations, and found clear patterns of change in flood timing. Warmer temperatures have led to earlier spring snowmelt floods throughout northeastern Europe; delayed winter storms associated with polar warming have led to later winter floods around the North Sea and some sectors of the Mediterranean coast; and earlier soil moisture maxima have led to earlier winter floods in western Europe. Our results highlight the existence of a clear climate signal in flood observations at the continental scale.

We analyze century‐long daily temperature and precipitation records for stations in Europe west of 60°E. A set of climatic indices derived from the daily series, mainly focusing on extremes, is defined. Linear trends in these indices are assessed over the period 1901–2000. Average trends, for 75 stations mostly representing Europe west of 20°E, show a warming for all temperature indices. Winter has, on average, warmed more (∼1.0°C/100 yr) than summer (∼0.8°C), both for daily maximum (TX) and minimum (TN) temperatures. Overall, the warming of TX in winter was stronger in the warm tail than in the cold tail (1.6 and 1.5°C for 98th and 95th, but ∼1.0°C for 2nd, 5th and 10th percentiles). There are, however, large regional differences in temperature trend patterns. For summer, there is a tendency for stronger warming, both for TX and TN, in the warm than in the cold tail only in parts of central Europe. Winter precipitation totals, averaged over 121 European stations north of 40°N, have increased significantly by ∼12% per 100 years. Trends in 90th, 95th and 98th percentiles of daily winter precipitation have been similar. No overall long‐term trend occurred in summer precipitation totals, but there is an overall weak (statistically insignificant and regionally dependent) tendency for summer precipitation to have become slightly more intense but less common. Data inhomogeneities and relative sparseness of station density in many parts of Europe preclude more robust conclusions. It is of importance that new methods are developed for homogenizing daily data.

Abstract. The COST (European Cooperation in Science and Technology) Action ES0601: advances in homogenization methods of climate series: an integrated approach (HOME) has executed a blind intercomparison and validation study for monthly homogenization algorithms. Time series of monthly temperature and precipitation were evaluated because of their importance for climate studies and because they represent two important types of statistics (additive and multiplicative). The algorithms were validated against a realistic benchmark dataset. The benchmark contains real inhomogeneous data as well as simulated data with inserted inhomogeneities. Random independent break-type inhomogeneities with normally distributed breakpoint sizes were added to the simulated datasets. To approximate real world conditions, breaks were introduced that occur simultaneously in multiple station series within a simulated network of station data. The simulated time series also contained outliers, missing data periods and local station trends. Further, a stochastic nonlinear global (network-wide) trend was added. Participants provided 25 separate homogenized contributions as part of the blind study. After the deadline at which details of the imposed inhomogeneities were revealed, 22 additional solutions were submitted. These homogenized datasets were assessed by a number of performance metrics including (i) the centered root mean square error relative to the true homogeneous value at various averaging scales, (ii) the error in linear trend estimates and (iii) traditional contingency skill scores. The metrics were computed both using the individual station series as well as the network average regional series. The performance of the contributions depends significantly on the error metric considered. Contingency scores by themselves are not very informative. Although relative homogenization algorithms typically improve the homogeneity of temperature data, only the best ones improve precipitation data. Training the users on homogenization software was found to be very important. Moreover, state-of-the-art relative homogenization algorithms developed to work with an inhomogeneous reference are shown to perform best. The study showed that automatic algorithms can perform as well as manual ones.

Data assimilation (DA) methods for convective‐scale numerical weather prediction at operational centres are surveyed. The operational methods include variational methods (3D‐Var and 4D‐Var), ensemble methods (LETKF) and hybrids between variational and ensemble methods (3DEnVar and 4DEnVar). At several operational centres, other assimilation algorithms, like latent heat nudging, are additionally applied to improve the model initial state, with emphasis on convective scales. It is demonstrated that the quality of forecasts based on initial data from convective‐scale DA is significantly better than the quality of forecasts from simple downscaling of larger‐scale initial data. However, the duration of positive impact depends on the weather situation, the size of the computational domain and the data that are assimilated. Furthermore it is shown that more advanced methods applied at convective scales provide improvements over simpler methods. This motivates continued research and development in convective‐scale DA. Challenges in research and development for improvements of convective‐scale DA are also reviewed and discussed. The difficulty of handling the wide range of spatial and temporal scales makes development of multi‐scale assimilation methods and space–time covariance localization techniques important. Improved utilization of observations is also important. In order to extract more information from existing observing systems of convective‐scale phenomena (e.g. weather radar data and satellite image data), it is necessary to provide improved statistical descriptions of the observation errors associated with these observations.

ARPEGE/Aladin is a limited-area 3D primitive equation model, which belongs to the integrated NWP ARPEGE/IFS system. Like its global counterpart, the limited-area version has a spectral representation of variables in the horizontal but uses double-Fourier series instead of the classical spherical harmonies, in the manner introduced by Machenhauer and Haugen. Following the suggestion of Laprise, a nonhydrostatic version of ARPEGE/Aladin has been developed using hydrostatic pressure as an independent variable. The dynamics employ the fully elastic Euler equations of motion, orography being introduced via a terrain-following hybrid coordinate. A semi-implicit scheme has been formulated to control both acoustic and gravity waves. The discrete linear operators appear to have the same form as in the hydrostatic dynamics, except an additional one representing the vertical part of the Laplacian operator. To keep an elegant elimination, it was necessary to modify the approximation of logarithmic thicknesses of the model layers. It is noteworthy that the Helmholtz matrix has a tridiagonal form, confirming a local character to the nonhydrostatic dynamics. The representation of the horizontal pressure gradient term fulfills the rules of conservation of energy and angular momentum. Some instability problems were encountered and it was thus necessary to introduce an additional semi-implicit type of correction of the nonlinear part of the total 3D divergence, a solution that calls for iterations of the semi-implicit scheme. The results of a few idealized numerical simulations and of a real situation are presented.

It is generally accepted that drought is one of the most costly weather-related natural hazards. In 2015, a long-lasting drought hit Europe, particularly affecting central and eastern Europe. In some regions it was the driest (North Slovakia) and in others (Czech Republic and Poland) it was the second driest summer of the last 50 years (following 2003). Key questions are: (i) how extreme are these events, not only in terms of hydro-meteorological characteristics but also impacts? and (ii) how are these impacts managed? Droughts often are viewed from a climatic perspective (e.g. Herring et al., 2015; Heim, 2015), with their severity defined by the strength of the anomaly in meteorological conditions (e.g. sea surface temperature, geopotential height, precipitation or temperature). Normalized anomalies in climatic variables, such as the Standardized Precipitation Index (SPI, McKee et al., 1993; WMO, 2006) and the more recently developed Standardized Precipitation–Evapotranspiration Index (SPEI, Vicente-Serrano et al., 2010), have become standard tools to characterize drought. Although the SPI and SPEI have proved their applicability across a wide range of hydro-climatological regimes, there is a pressing need to monitor the impacts of climate and weather events in a more systematic way (Stahl et al., 2016). Many drought-related impacts (e.g. crop yields, water-borne transport, aquatic ecosystems, water supply, energy production) are associated with hydrology rather than solely with weather. Hydrologically oriented drought studies have shown that drought in groundwater or streamflow (hydrological drought) deviates from meteorological drought (precipitation anomalies) (Changnon, 1987; Peters et al., 2003; Vidal et al., 2010; Hannaford et al., 2011; Van Loon and Van Lanen, 2012; Van Dijk et al., 2013). Hydrological drought is a complex phenomenon that integrates many river basin characteristics, such as (but not limited to) land cover, topography, geology and river network structure (Van Lanen et al., 2013; Stoelzle et al., 2014). Minor meteorological droughts may not show up as a hydrological drought, whereas a series of meteorological droughts can merge to form a long-lasting hydrological drought, which usually has a later onset and recovery. Hydrological drought has in most cases a smaller intensity than meteorological drought. The areas that are covered by the different drought types are also varying (Peters et al., 2006; Tallaksen et al., 2009). Additionally, water managers take actions in response to the (forecasted) impacts (e.g. water storage, abstractions, water transfers) in which hydrology plays a key role. This commentary discusses how drought, from its origin as a meteorological anomaly, manifests itself as a deficiency in soil moisture and subsequently as a hydrological drought. Furthermore, the commentary emphasizes that better understanding and management of drought requires understanding this propagation of water deficits through the hydrological cycle, with consideration of the nature of the resultant impacts on socioeconomic and natural systems also of critical importance. Drought characterization from such a perspective requires concerted multi-disciplinary action from both the climatic and hydrological communities. Although some initiatives (Harding et al., 2011; Schellnhuber et al., 2013) are promising, more widespread and comprehensive action is necessary. We use the 2015 European drought as an example. The summer (June – August) of 2015 was characterized by daily maximum temperatures 2 °C higher than the seasonal mean over most of western Europe, and more than 3 °C higher in central Europe (Figure 1a). Large parts of Europe also experienced a severe lack of rainfall and higher evapotranspiration than normal, with negative values of the three-month standardized precipitation–evaporation anomaly (SPEI3) from June onwards across a widespread area. Summer SPEI3 values dropped to as low as −4 in central and eastern Europe (Figure 1b). Similar to the extreme 2003 summer drought, upper level atmospheric circulation over continental Europe was characterized by a large, positive 500-hPa geopotential height anomaly (Z500; Figure 1c). Positive anomalies first occurred in March, and persisted throughout the summer. This high pressure blocking pattern over Europe prevented the flow of moisture and precipitation across much of Europe. During summer, the positive European anomaly was bordered by a large negative Z500 over the central North Atlantic Ocean, extending to northern Scandinavia. Summer sea surface temperature (SST) was characterized by large negative anomalies in the central North Atlantic Ocean (with the peak difference approximately co-located with the peak Z500 difference), and large positive anomalies in the Mediterranean basin (Figure 1d). The 2015 negative Atlantic SST (JJA) anomaly was within the top 10 coldest summers in this region in the ERSST v4 record extending back to 1854. Vegetation stress in summer 2015 (anomaly of absorbed photosynthetically active radiation; Figure 2) displayed similarities to the SPEI pattern (Figure 1b), but also with obvious differences. At the end of June, only some scattered areas with vegetation stress occurred, mainly in eastern Europe (Ukraine, Romania, Balkan Adriatic coast). In August, these areas combined into a west-east zone stretching from central France into Ukraine and Belarus. In October, the west-east zone divided into three core regions: southern Germany, Poland and Ukraine, and some new areas (Latvia, northern Europe) in response to a precipitation deficit that developed in early autumn (not shown). In all cases, the area affected by vegetation stress was substantially smaller than the area experiencing moderate meteorological drought (SPEI < −1, Figure 1b), although they occupied similar regions. Low flow and drought characteristics were computed from about 800 daily streamflow time series across Europe (Laaha et al., 2016). The return period of the 7-day minimum flow in 2015 was determined for each month (Figure 3). In June, most gauging stations showed streamflow with return periods <2 years (Figure 3a), with a few exceptions (mostly <5 years). Although SPEI3 ≤ −1 in June occurred in a wide west-east band from the Benelux into Belarus and Ukraine (not shown), low flows remained in the normal range. In August, low flows became more extreme (Figure 3b) in a southwest-northeast zone north of the Alps. Particularly in central Europe (Czech Republic, Poland, southern Germany, northern Austria) and also France, the return period of the 7-day minimum flow increased to more than 50 years. In the Czech Republic and Poland (e.g. Vistula) many rivers recorded the lowest flow on record. Some recovery was seen in the autumn, but low flows were still extreme (return period >20 years) in southern Germany, southwestern Poland and the Czech Republic (Figure 3c). Return periods for drought duration (the period that streamflow is below flow equaled or exceeded 80% of the time over the period 1976–2010) are presented in Figure 3d. Drought characteristics could not be fully established for 2015, because for many gauging stations flow by the end of the autumn was still below the drought threshold. A typical feature of the 2015 drought was its long duration. For instance, one of the major rivers in Europe, the Rhine at the Dutch–German boundary, faced the longest running low flow period since the 1976 benchmark drought. Return periods in drought duration of more than 20 years were mainly seen in central Europe. The flow analysis showed that the drought followed the SPEI3 JJA pattern, but that the hydrological response was delayed through drought propagation and that local differences occurred because of catchment storage processes and antecedent conditions. The impacts of the 2015 drought were manifold across Europe, as derived from various text sources (e.g. reports, websites). The wide range of impacts is not uncommon as illustrated for previous events by the European Drought Impact Inventory, EDII (Stahl et al., 2016). In some central and eastern European regions the impacts continued even into 2016. No drought impacts were reported in Scandinavia and the UK, which matches the drought pattern in Figures 1–3. The vegetation stress (Figure 2) induced by excessive heat and soil water drought led to lower crop yields. For example, crop losses of up to 50% were reported in the Czech Republic, Germany, Poland and Slovakia for sugar beet and potatoes, while maize was unable to build cobs in some regions. The drought also had a significant impact on livestock farming, with a 50% lower hay harvest (Czech Republic), failing grass cuts (Germany, Slovakia) and substantially lower milk production (Slovakia and Romania). Czech authorities have estimated that the impact of the 2015 drought on agriculture amounts to € 50–100 million. The drought also led to worst summer for Czech firefighters in at least the last ten years, with almost twice as many fires as in 2014. In Austria the drought caused an exceptionally long wildfire season, lasting until the end of 2015. The hydrological component of the 2015 drought (Figure 3) had an impact on a wide range of sectors, including water supply, energy production, waterborne transportation, freshwater aquaculture and fisheries, water quality, fresh water ecology, tourism and recreation. A summary of these impacts follows. Across central Europe and parts of eastern Europe (e.g. Romania) hundreds of towns and villages faced drinking water supply deficiencies. In southern Germany, boreholes dried up in crystalline rocks leading to water supply shortages for cattle. In eastern Romania record-low groundwater levels were registered and because of groundwater overexploitation water quality deteriorated. Low flows and associated high water temperatures caused reduced energy production along rivers in southern Germany, Czech Republic, Poland and European Russia. Some hydropower stations had to be shut down: in the northeast Czech Republic the majority of small hydropower plants were out of service for four months. In August, 1600 of the biggest companies in Poland suffered from power restrictions. French and Czech hydropower production was 30–50% lower than normal in some summer and autumn months. Similar reductions were reported for one of the main hydropower stations in the downstream part of the Don River (Russia). The 2015 drought significantly impacted water-borne transportation, notably in France, Germany and European Russia. In Germany, load losses on the Rhine, Danube, Elbe, Oder and Weser Rivers and in Russia on the Don River were up to 50%. The drought and associated heat triggered oxygen deficits and high temperatures in surface water bodies in Germany, Slovakia and European Russia, which influenced freshwater aquaculture and fisheries (lower fish yields), while causing other water quality issues (blue–green algae blooms and botulism). Dried-up fish breeding grounds and dying fish were reported in several central and eastern European countries. Fresh water ecosystems in the Czech Republic were also impacted by hydropower plants; 25% of the small plants could not comply with the ecological minimum flow standard. Violation of environmental flow requirements in upstream headwaters also happened in Germany. Tourism and recreation were impacted in several countries because low reservoir and river levels restricted leisure activities in these water bodies. Access to forests was also restricted because of the high fire risk. The impacts of the 2015 drought were also felt beyond the core region in central and eastern Europe. For example, in Belgium and the Netherlands a 1-in-20 year meteorological drought occurred from April to August. Some early crops (such as potatoes) had yield losses of up to 30%. Low flow in the major Dutch rivers caused salt water intrusion in the river mouths over tens of kilometres, affecting fresh and brackish water ecosystems. Water-borne transport in the Netherlands was strongly impacted. The shallow water depth affected transport until late November (with up to 50% cargo losses in the autumn), mainly because of little river inflow from upstream (Switzerland and southern Germany). Shrinkage of old peat dikes caused cracks leading to increased flood risk in the Netherlands. In the autumn many houseboats were sitting askew on dry stream bottoms because of the unprecedented low water levels. Surface water and reservoirs are particularly important means to manage a drought. For instance, in the Czech Republic, reservoirs were 90% full at the start of the 2015 summer. During the drought event, reservoirs were emptied to provide direct water and to increase low flows downstream. Reservoir storage remained above 30% with a few exceptions, but most reservoirs were still in decline at the end of October 2015, which had not happened since 2003. In the eastern part of Romania, the volume of some large inter-annual regulation reservoirs was also very low (remaining storage: about 30%) at the end of 2015. In northeastern France, reservoirs used for sustaining low flows had their available volume below the 1-in-10 year level in early September. In Germany, record water transfers from the Danube to Main basins were implemented for low flow augmentation. In the Dutch lowlands, surface water levels were raised to conserve water. Some canals or sections in northeastern France were closed to water-borne transportation for several months, not re-opening until the end of 2015. Transport in Romania also faced restrictions. In the Netherlands, boats had to cope with more costly lock operations. Special measures were implemented in the main river network until the end of the autumn as a response to the low inflow from upstream. In many European regions crops were irrigated when possible. Record irrigation of corn and tobacco was reported in the Upper Rhine Valley in Germany. In contrast, water abstraction restrictions were in place in 70 French departments in early August, which enforced a complete water abstraction ban for all non-priority uses, including irrigation. In early November some crisis orders were still active in Burgundy. In the Netherlands there were bans on abstraction of surface water for irrigation to avoid deterioration of water quality until mid-August when rain caused relief. Locally, in the Czech Republic, Poland, Slovakia and southern Germany tank trucks were ordered to fill reservoirs in municipalities with water supply deficiencies because of low inflow from local springs. Many municipal councils banned water use for watering gardens, swimming pools or washing cars. Additional flushing of the regional surface water system in the Netherlands using water from the main rivers occurred to avoid further salinization. Emergency pumps were installed to reroute surface water and in other places surface water was blocked from flowing into certain streams to avoid further deterioration of water quality. Various water inlets were closed to avoid spreading of blue–green algae. Natural swimming baths were closed (Germany, the Netherlands) due to the deteriorated water quality (blue–green algae bloom and botulism). Resettlement of fish was reported in Germany, Czech Republic and Slovakia. Aquaculture had increased costs for extra oxygenation. Various measures were also taken for human health and public safety reasons. In German, Dutch Slovak and Romanian cities, additional water was required for watering parks to avoid further development of the urban heat island and to maintain aesthetic value. In Bratislava and Bucharest, water tanks were used to supply tourists and city inhabitants at selected points. The Dutch Water Boards had to frequently inspect 3500 km of drought sensitive peat dikes and to irrigate in case of drought cracks. As shown for the 2015 European event, drought impacts are largely connected to soil water drought (crop yield, wildfires) or to hydrological drought (water supply, energy, transportation, recreation, water quality) rather than directly to the meteorological drought. This implies that knowledge of hydrology, i.e. the propagation of meteorological drought into a hydrological drought, including the role of antecedent water storage, is needed to understand drought impacts. It is also illustrated that stakeholders and water managers respond to impacts by taking measures (e.g. irrigation, water abstractions, use of reservoir storage, rerouting, transfers, conservation) to mitigate impacts, but which can also enhance impacts elsewhere (Van Dijk et al., 2013; Van Loon et al., 2016). Enhancement of impacts typically involves ecological minimum flows that cannot be sustained because of upstream water use. During droughts there is a high pressure on groundwater resources and in several regions more groundwater is abstracted than recharged (e.g. Castle et al., 2014; Panda and Wahr, 2015), leading to undesirable impacts (e.g. reduced groundwater flow to riparian areas and rivers). However, reports on declining groundwater tables are not everywhere available, or no separation is made between impacts due to the drought itself as compared to abstractions due to increased groundwater exploitation, as advised by Van Loon and Van Lanen (2013). The need for an enhanced hydrological perspective in terms of understanding and managing drought impacts requires urgent action. First, the European water sector should make near-real time hydrological data as readily available as meteorological data (Haylock et al., 2008; Hannah et al., 2011). Currently, large-scale observed flow data become available not earlier than a year after measurement (Global Runoff Data Centre, www.bafg.de/GRDC/EN), which forces experts to resort to simulated flow for pan-European studies (e.g. Gudmundsson and Seneviratne, 2015). Furthermore, drought impacts and response measures (including their success rate) should be archived, for example using the European Drought Impact Inventory (Stahl et al., 2016). Second, multi-monthly and seasonal drought forecasting should be improved beyond the currently available 10 or 14-day forecasted atmospheric indices and soil water anomalies. Some encouraging initiatives at the national scale are ongoing, as reported by the Hydrological Ensemble Prediction Experiment (HEPEX) community. For example, Prudhomme (2015) presented the first operational forecast system for Great Britain that delivers an outlook of 1 to 3 months for river flow and groundwater levels. Promising results on the forecasted 7-day minimum flow for major German waterways were also shown by Meißner et al. (2015), which are based upon the seasonal correlation between global oceanic and climatic data, soil moisture and low river flow (Ionita et al., 2008; 2015). Third, drought monitoring and forecasting should be embedded in drought policy. Wilhite (2014) provides a template for action, which in Europe could improve the drought chapter in the River Basin Management Plans. Managing drought in a pro-active way requires a concerted action of the hydrological and climatic communities. Such action should include pan-European monitoring of hydro-meteorological variables and multi-monthly and seasonal forecasting of both climatic and hydrological variables. Furthermore, impact assessments and exploration of potential promising measures to reduce impacts (considering context specific conditions at the river basin scale) represent a critical research direction for drought impact mitigation. This commentary is by a of European drought experts from the Low and Drought which to near-real time hydrological data and impact reports across Europe, which have Data by national hydro-meteorological the European Drought and and from the Access for most of the Access and was

Abstract. The ALADIN System is a numerical weather prediction (NWP) system developed by the international ALADIN consortium for operational weather forecasting and research purposes. It is based on a code that is shared with the global model IFS of the ECMWF and the ARPEGE model of Météo-France. Today, this system can be used to provide a multitude of high-resolution limited-area model (LAM) configurations. A few configurations are thoroughly validated and prepared to be used for the operational weather forecasting in the 16 partner institutes of this consortium. These configurations are called the ALADIN canonical model configurations (CMCs). There are currently three CMCs: the ALADIN baseline CMC, the AROME CMC and the ALARO CMC. Other configurations are possible for research, such as process studies and climate simulations. The purpose of this paper is (i) to define the ALADIN System in relation to the global counterparts IFS and ARPEGE, (ii) to explain the notion of the CMCs, (iii) to document their most recent versions, and (iv) to illustrate the process of the validation and the porting of these configurations to the operational forecast suites of the partner institutes of the ALADIN consortium. This paper is restricted to the forecast model only; data assimilation techniques and postprocessing techniques are part of the ALADIN System but they are not discussed here.

The Carpathians are the longest mountain range in Europe and a geographic barrier between Central Europe, Eastern Europe, and the Balkans. To investigate the climate of the area, the CARPATCLIM project members collected, quality-checked, homogenized, harmonized, and interpolated daily data for 16 meteorological variables and many derived indicators related to the period 1961–2010. The principal outcome of the project is the Climate Atlas of the Carpathian Region, hosted on a dedicated website (www.carpatclim-eu.org) and made of high-resolution daily grids (0.1° × 0.1°) of all variables and indicators at different time steps. In this article, we analyze the spatial and temporal variability of 10 variables: minimum, mean, and maximum temperature, daily temperature range, precipitation, cloud cover, relative sunshine duration, relative humidity, surface air pressure, and wind speed at 2 m. For each variable, we present the gridded climatologies for the period 1961–2010 and discuss the linear trends both on an annual and seasonal basis. Temperature was found to increase in every season, in particular in the last three decades, confirming the trends occurring in Europe; wind speed decreased in every season; cloud cover and relative humidity decreased in spring, summer, and winter, and increased in autumn, while relative sunshine duration behaved in the opposite way; precipitation and surface air pressure showed no significant trend, though they increased slightly on an annual basis. We also discuss the correlation between the variables and we highlight that in the Carpathian Region positive and negative sunshine duration anomalies are highly correlated to the corresponding temperature anomalies during the global dimming (1960s and 1970s) and brightening (1990s and 2000s) periods.

Abstract The environments of severe and nonsevere thunderstorms were analyzed using 16 421 proximity soundings from December 2007 to December 2013 taken at 32 central European stations. The soundings were assigned severity categories for the following hazards: hail, wind, tornado, and rain. For each of the soundings, parameters were calculated representing the instability, vertical wind profile, and moisture of the environment. The probability of the various hazards as a function of CAPE and 0–6-km bulk shear (DLS) is quite different for each of the hazards. Large hail is most likely for high CAPE and high DLS, a regime that also supports severe wind events. A second severe wind regime exists for low CAPE and very high DLS. These events are mostly cold season events. Storms with significant tornadoes occur with much higher DLS than storms with weak or no tornadoes, but with similar CAPE. The 0–1-km bulk shear (LLS) does not discriminate better than DLS between weak and significant tornadoes. Heavy rain events occur across a wide range of DLS, but with CAPE above the median for nonsevere thunderstorms and are most likely when both absolute humidity in the boundary layer and relative humidity in the low- to midtroposphere are high. LCL height does not discriminate well between the intensity categories of tornadoes, but higher LCL heights were associated with a higher probability of severe hail. Storm relative helicity shows similar results to DLS, but with more overlap among intensity categories.

Abstract The paper assesses the difficulties of running an operational NWP model in the resolution range of 3–8 km. In this case, deep convection cells are neither much smaller than the grid box as assumed by most parameterization schemes, nor completely resolved as would be required for them to be treated explicitly. A specific approach is proposed, with an integrated sequential treatment of resolved condensation, deep convection, and microphysics together with the use of prognostic variables. It currently allows for the production of consistent and realistic results at resolutions ranging from a few tens of kilometers down to less than 4 km. Model skill scores and an example of an operational forecast at different resolutions are presented.

A common problem with nonlinear advection schemes is the false accumulation of energy at the smallest resolvable scales. To keep this process under control, following Arakawa (1966), a number of energy and enstrophy conserving schemes for staggered and semi-staggered grids have been designed. In this paper, it is demonstrated that, in contrast to the staggered grid, the conservation of energy and enstrophy on the semi-staggered gods does not guarantee that the erroneous transport of energy from large to small scales will be effectively restricted. Using a new approach to the application of the Arakawa Jacobian, a scheme for a semi-staggered grid which exactly reflects the Arakawa theory for nondivergent flow is obtained for the first time. This is achieved by conservation of energy and enstrophy as defined on the staggered grid. These two quantities are of higher accuracy and cannot be calculated directly from the dependent variables on the semi-staggered grid. It is further demonstrated that the amount of energy which can be transported toward smaller scales is more restricted than for any other scheme of this type on both staggered and semi-staggered grid. Experiments performed with the proposed scheme and a scheme which conserves energy and enstrophy as defined on the semi-staggered grid reveal visible differences in long-term integrations which are in agreement with the theory and demonstrate the advantages of the new scheme.

The Arctic polar vortex exhibited widespread regions of low temperatures during the winter of 2005, resulting in significant ozone depletion by chlorine and bromine species. We show that chemical loss of column ozone (ΔO 3 ) and the volume of Arctic vortex air cold enough to support the existence of polar stratospheric clouds (V PSC ) both exceed levels found for any other Arctic winter during the past 40 years. Cold conditions and ozone loss in the lowermost Arctic stratosphere (e.g., between potential temperatures of 360 to 400 K) were particularly unusual compared to previous years. Measurements indicate ΔO 3 = 121 ± 20 DU and that ΔO 3 versus V PSC lies along an extension of the compact, near linear relation observed for previous Arctic winters. The maximum value of V PSC during five to ten year intervals exhibits a steady, monotonic increase over the past four decades, indicating that the coldest Arctic winters have become significantly colder, and hence are more conducive to ozone depletion by anthropogenic halogens.

We present validation studies of MLS version 2.2 upper tropospheric and stratospheric ozone profiles using ozonesonde and lidar data as well as climatological data. Ozone measurements from over 60 ozonesonde stations worldwide and three lidar stations are compared with coincident MLS data. The MLS ozone stratospheric data between 150 and 3 hPa agree well with ozonesonde measurements, within 8% for the global average. MLS values at 215 hPa are biased high compared to ozonesondes by ∼20% at middle to high latitude, although there is a lot of variability in this altitude region. Comparisons between MLS and ground‐based lidar measurements from Mauna Loa, Hawaii, from the Table Mountain Facility, California, and from the Observatoire de Haute‐Provence, France, give very good agreement, within ∼5%, for the stratospheric values. The comparisons between MLS and the Table Mountain Facility tropospheric ozone lidar show that MLS data are biased high by ∼30% at 215 hPa, consistent with that indicated by the ozonesonde data. We obtain better global average agreement between MLS and ozonesonde partial column values down to 215 hPa, although the average MLS values at low to middle latitudes are higher than the ozonesonde values by up to a few percent. MLS v2.2 ozone data agree better than the MLS v1.5 data with ozonesonde and lidar measurements. MLS tropical data show the wave one longitudinal pattern in the upper troposphere, with similarities to the average distribution from ozonesondes. High upper tropospheric ozone values are also observed by MLS in the tropical Pacific from June to November.

Abstract We analysed monthly precipitation trends on the eastern Mediterranean fringe of the Iberian Peninsula (IP) by means of a new monthly precipitation database [Monthly Precipitation Dataset, Mediterranean Spain (MOPREDA MES )]. This database was created following an exhaustive quality control of the archives from the Meteorological Agency of Spain (Instituto Nacional de Meteorología, INM), and comprises 1113 complete and homogeneous monthly precipitation series (1951–2000), covering 180 000 km 2 (one‐third of IP area). The new data set currently offers the highest spatial density of stations on the IP and around the Mediterranean Sea (1 station/150–200 km 2 ), and includes available information at 1500 m asl. The analyses of monthly precipitation trends indicate high spatial and temporal variability. No global trend in the study area was found, except for March, when significant negative trends affect the whole study area. Trends for winter months (December–January–February) are dominated by an East–West gradient with a latitudinal temporal shift. Positive trends are mainly located in coastland areas and negatives ones predominate inland. April shows a North‐positive South‐negative gradient, and the reverse is true for June and September. Negative trends are dominant in October over most of the study area, except for the Pyrenees area. July and August show the most complex spatial distribution pattern, with well‐delimited areas of positive trends to the south and northwest. We generally detected no significant trends in May and November. In order to analyse the nature of rainfall variability in the study area, we also studied the synchronous influence of different low‐variability modes [the North Atlantic Oscillation (NAO), the Mediterranean Oscillation (MO) and the Western Mediterranean Oscillation (WeMO)]. On the Mediterranean fringe of the IP precipitation is mainly related to negative phases of the three low‐frequency variability patterns analysed, and the MO and the WeMO emerge as predominant teleconnection patterns. These results suggest that monthly variations in rainfall may result from the simultaneous effects of different atmospheric modes of low variability, especially those linked to the Mediterranean region. Studies making use of high‐density precipitation databases, as is the case in this paper, are useful for a better understanding of precipitation behaviour in a complex area like the Mediterranean fringe of the IP. The results also provide valuable information for downscaling and hydrological management. Copyright © 2008 Royal Meteorological Society

Between 2007-2011, the European COST Action ES0601 called HOME project was devoted to evaluate the performance of homogenization methods used in climatology and produce a software that would be a synthesis of the best aspects of some of the most efficient methods. HOMER (HOMogenizaton softwarE in R) is a software for homogenizing essential climate variables at monthly and annual time scales. HOMER has been constructed exploiting the best characteristics of some other state-of-the-art homogenization methods, i.e., PRODIGE, ACMANT, CLIMATOL, and the recently developed joint-segmentation method (cghseg). HOMER is based on the methodology of optimal segmentation with dynamic programing, the application of a network-wide two-factor model both for detection and correction, and some new techniques in the coordination of detection processes from multiannual to monthly scales. HOMER also includes a tool to assess trend biases in urban temperature series (UBRIS). HOMER's approach to the final homogenization results is iterative. HOMER is an interactive method, that takes advantage of metadata. A practical application of HOMER is presented on temperature series of Wien, Austria and its surroundings.

Abstract. The EU CANDIDOZ project investigated the chemical and dynamical influences on decadal ozone trends focusing on the Northern Hemisphere. High quality long-term ozone data sets, satellite-based as well as ground-based, and the long-term meteorological reanalyses from ECMWF and NCEP are used together with advanced multiple regression models and atmospheric models to assess the relative roles of chemistry and transport in stratospheric ozone changes. This overall synthesis of the individual analyses in CANDIDOZ shows clearly one common feature in the NH mid latitudes and in the Arctic: an almost monotonic negative trend from the late 1970s to the mid 1990s followed by an increase. In most trend studies, the Equivalent Effective Stratospheric Chlorine (EESC) which peaked in 1997 as a consequence of the Montreal Protocol was observed to describe ozone loss better than a simple linear trend. Furthermore, all individual analyses point to changes in dynamical drivers, such as the residual circulation (responsible for the meridional transport of ozone into middle and high latitudes) playing a key role in the observed turnaround. The changes in ozone transport are associated with variations in polar chemical ozone loss via heterogeneous ozone chemistry on PSCs (polar stratospheric clouds). Synoptic scale processes as represented by the new equivalent latitude proxy, by conventional tropopause altitude or by 250 hPa geopotential height have also been successfully linked to the recent ozone increases in the lowermost stratosphere. These show significant regional variation with a large impact over Europe and seem to be linked to changes in tropospheric climate patterns such as the North Atlantic Oscillation. Some influence in recent ozone increases was also attributed to the rise in solar cycle number 23. Changes from the late 1970s to the mid 1990s were found in a number of characteristics of the Arctic vortex. However, only one trend was found when more recent years are also considered, namely the tendency for cold winters to become colder.

Abstract. Biogenic volatile organic compounds (BVOCs) emitted from the terrestrial vegetation into the Earth's atmosphere play an important role in atmospheric chemical processes. Gridded information of their temporal and spatial distribution is therefore needed for proper representation of the atmospheric composition by the air quality models. Here we present three newly developed high-resolution global emission inventories of the main BVOC species including isoprene, monoterpenes, sesquiterpenes, methanol, acetone and ethene. Monthly mean and monthly averaged daily profile emissions were calculated by the Model of Emission of Gases and Aerosols from Nature (MEGANv2.1) driven by meteorological reanalyses of the European Centre for Medium-Range Weather Forecasts for the period of 2000–2019. The dataset CAMS-GLOB-BIOv1.2 is based on ERA-Interim meteorology (0.5∘ × 0.5∘ horizontal spatial resolution); the datasets CAMS-GLOB-BIOv3.0 and v3.1 were calculated with ERA5 (both 0.25∘ × 0.25∘ horizontal spatial resolution). Furthermore, European isoprene emission potential data were updated using high-resolution land cover maps and detailed information of tree species composition and emission factors from the EMEP MSC-W model system. Updated isoprene emissions are included in the CAMS-GLOB-BIOv3.1 dataset. The effect of annually changing land cover on BVOC emissions is captured by the CAMS-GLOB-BIOv3.0 as it was calculated with land cover data provided by the Climate Change Initiative of the European Space Agency (ESA-CCI). The global total annual BVOC emissions averaged over the simulated period vary between the datasets from 424 to 591 Tg (C) yr−1, with isoprene emissions from 299.1 to 440.5 Tg (isoprene) yr−1. Differences between the datasets and variation in their emission estimates provide the emission uncertainty range and the main sources of uncertainty, i.e. meteorological inputs, emission potential data and land cover description. The CAMS-GLOB-BIO time series of isoprene and monoterpenes were compared to other available data. There is a general agreement in an interannual variability in the emission estimates, and the values fall within the uncertainty range. The CAMS-GLOB-BIO datasets (CAMS-GLOB-BIOv1.2, https://doi.org/10.24380/t53a-qw03, Sindelarova et al., 2021a; CAMS-GLOB-BIOv3.0, https://doi.org/10.24380/xs64-gj42, Sindelarova et al., 2021b; CAMS-GLOB-BIOv3.1, https://doi.org/10.24380/cv4p-5f79, Sindelarova et al., 2021c) are distributed from the Emissions of atmospheric Compounds and Compilation of Ancillary Data (ECCAD) system (https://eccad.aeris-data.fr/, last access: June 2021).