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On the basis of data from the period 1966-2020 from about 50 stations in Poland the precipitation and potential evaporation indices will be estimated and their variability analyzed. Different formulas of potential evapotranspiration, based on air temperature, relative humidity, saturation deficit, wind speed and/or sunshine duration, will be compared. Climatic water balance defined as a difference of precipitation and potential evapotranspiration will be calculated and its intraannual variability analyzed. Long-term changes of all three parameters (precipitation, evapotranspiration and climatic water balance) in the context of contemporary warming will be examined. On this basis, the problem of water availability in various regions of the country will be discussed, especially in the context of changes in the spatial distribution and duration of the snow cover.

The Arctic region has undergone a substantial climatic changes in recent years with dramatic sea-ice loss, evident particularly in late summer and early autumn. According to many findings the near-surface temperature in high latitudes of the Northern Hemisphere has been rising at rates double that of the lower latitudes and was amplified by rapid loss of sea ice in the Arctic through the snow and ice albedo feedback. Open, free of ice water has a smaller albedo than ice, absorbs more energy incoming from the Sun, resulting in much faster increase in temperature than other areas. The abrupt acceleration in Arctic warming well seen since the middle 1990s coincides with changes in climate in the mid-latitudes. The aim of this paper is to analyse the changes in atmospheric circulation in the mid to high latitudes in the response to low extent of ice cover in Arctic in the preceding months.To analyze the variability of sea ice cover in Arctic region we used daily sea-ice fraction data which are defined as the fraction of a grid box which is covered by sea ice. These data were multiplied by field of each grid box. After summing up all recalculated grid points, a new parameter named – area ice cover (AIC) was defined. We analyzed the AIC variability in the whole Artcic and in three sectors Atlantic - from 75°W to 60°E, Asian - from 60.25°E to 189.75°W and American - from 170°W to 75.25°W.Then AIC index was correlated to selected northern hemisphere teleconnection: North Atlantic Oscillation (NAO), the Scandinavian pattern (SCAND), East Atlantic (EA), Polar/Eurasia index (POL). The Monthly NAO, SCAND, EA and POL values were provided by U.S. National Centers for Environmental Prediction (NCEP). The AIC preceded circulation indices by 1 to 12 months.In regions and periods of significant correlation, composite maps of temperature anomalies, sea level pressure and geopotential altitude of 100 and 200 hPa in the years with low icing were created and analysed.The analysis showed that the greatest changes in AIC are observed in Asian sector of Arctic and they have the strongest impact on the atmospheric circulation. AIC changes modulate circulation in both winter and summer period. However the nature of the impact is clearly different. A sensitive regions turns to be Barents and Kara Seas and Novaya Zemlya.

This study analyses long-term changes in drought indices (Standardised Precipitation Index—SPI, Standardised Precipitation–Evapotranspiration Index—SPEI) at 1 and 3?months scales at 182 stations in 11 central and eastern European countries during 1949–2018. For comparative purposes, the necessary atmospheric evaporative demand (AED) to obtain SPEI was calculated using two methods, Hargreaves-Samani (SPEIH) and Penman-Monteith (SPEIP). The results show some relevant changes and tendencies in the drought indices. Statistically significant increase in SPI and SPEI during the cold season (November–March), reflecting precipitation increase, was found in the northern part of the study region, in Estonia, Latvia, Lithuania, northern Belarus and northern Poland. In the rest of study domain, a weak and mostly insignificant decrease prevailed in winter. Summer season (June–August) is characterized by changes in the opposite sign. An increase was observed in the north, while a clear decrease in SPEI, reflecting a drying trend, was typical for the southern regions: the Czech Republic, Slovakia, Hungary, Romania, Moldova and southern Poland. A general drying tendency revealed also in April, which was statistically significant over a wide area in the Czech Republic and Poland. Increasing trends in SPI and SPEI for September and October were detected in Romania, Moldova and Hungary. The use of SPEI instead of SPI generally enhances drying trends.

The wind was analyzed on the basis of speed and direction data from three observation times at 41 meteorological stations in Poland from 1966–2018. The average annual and seasonal wind speeds and the incidence of atmospheric calms were calculated. Wind frequencies from different directions were determined using a 16-sector wind rose. Particular attention was paid to winds exceeding 8 ms−1. The seasonal and annual trends of strong winds were counted. The average wind speed was 3.6 ms−1 and ranged from 1.4 ms−1 in Zakopane to 12 ms−1 on Śniezka. On the daily scale, the highest wind speed appeared at noon. In the annual cycle, the average wind speed at noon was the highest in spring and in the other times in winter, the lowest was always in summer. Windless weather occurred most often in the morning (12.3% of observations), less often in the evening (8.3%) and the least often at noon (3.0%). The long-term average of the highest annual wind speed oscillated between 8.3 ms−1 in Tarnow and 42.5 ms−1 on Śniezka. Strong winds were most often recorded from the west, west southwest and west northwest directions. In southern Poland it was also the southern direction. The decreasing trends in annual and seasonal series of wind speed were observed in Poland as well as in other European countries. A number of authors attribute this trend to an increase in surface roughness.

The book presents the results of climate research throughout Poland in the pre-instrumental period (using proxy data), instrumental period (using mainly statistical methods, based on data from weather stations and grid data) and projected changes (using regional climate models). A total of 1100 years are covered, i.e. the period from about 1000 to 2100. The majority of examined climate elements, meteorological phenomena and indices show statistically significant changes at least in certain areas of Poland and at certain seasons of the year. Moreover, many elements demonstrate significant year-to-year variability and temporal fluctuations. Changes of particular climate elements are interrelated. The primary causative factors are both anthropogenic changes (greenhouse gas emissions resulting in increased greenhouse effect and global warming, local sources of air pollution) and natural changes: (1) circulation factors: changes in the intensity and location of atmospheric activity centres, changes in the frequency of advection from a specific sector, and the frequency of cyclonic and anticyclonic systems over Poland and (2) radiation factors (changes in values of global solar radiation, sunshine duration and cloudiness). These changes, especially visible after the 1980s, affect the trends of most climatic elements, meteorological phenomena and indices. The effects of these changes, both positive and negative, are evident in people’s daily lives (e.g. decrease in bioclimatic cold stress, increase in bioclimatic heat stress, changes in conditions for recreation and sport) and economy (e.g. improvement of thermal agricultural conditions, changes in energy demand for heating buildings, air conditioning and refrigeration). A better understanding of the relationships of trends of the different climatic elements should be the aim of further research into the climate of Poland.

The study covers the entire area of Poland. Part II of the book presents the results of the study of climate change before instrumental measurements in Poland in the last millennium. More than 50 proxy series including documentary evidences, dendrochronological records and varved sediment records were analysed. On this basis, reconstructions of: summer and winter air temperature, winter severity, precipitation (liquid and solid) and droughts were performed. Part III analysed the long-term changes and variability of 36 climate characteristics for 14 climate elements, indices, meteorological phenomena and weather types using data from 79 weather stations. Most of the climate elements studied were analysed for the base period 1951–2018 (68 years). For some elements and indices shorter study periods were considered. In addition, climate variability was analysed for 10 long measuring series up to 239 years. Variability of circulation indices over a period of 147 years (1873–2019) was investigated as well. Uniform research methods common to all elements and indices were used: (1) coefficient of variability (%), (2) absolute trend (unit/10y); (3) relative trend (%/10y). Part IV of the book deals with projected changes in temperature, precipitation and thermal indices related to the agriculture and energy sectors. In this study, 8 regional climate models from the EURO-CORDEX experiment were used for 2 representative concentration pathways: (1) corresponding to radiative forcing value +4.5 W.m−2 in 2100 (RCP4.5) and (2) corresponding to radiative forcing value +8.5 W.m−2 in 2100 (RCP8.5) relative to pre-industrial values. Two future time horizons were carried out for each concentration pathway: (1) near future: 2021–2050 and (2) far future: 2071–2100 with reference to the period of 1971–2000. Future projections were created for: 8 characteristics of temperature, 10 characteristics of precipitation, 5 characteristics for agriculture indices and 4 for energy demands indices.

Analyses of homogeneity of climate series and correction of heterogeneous series were undertaken in order to avoid making incorrect conclusions about climate change in Poland. The climate series homogenisation procedure was typically carried out in several stages: (1) collecting information on the history of meteorological stations (metadata), especially on significant changes in their location and measurements methods; (2) selecting the test for homogeneity control, calculating values of the test, selecting series with test values exceeding the specified critical level; (3) correction of non-homogeneous series; (4) recheck of the revised series. The correction of the climatic series analysed for the purposes of this book was carried out using various methods, specific to the climate element analysed. The most common was the relative and absolute Alexandersson test (Standard Normal Homogeneity Test, SNHT). The most common reasons for breaking the homogeneity of the climatic series were a significant change of station location and a change of the measuring instrument. However, in many cases, the cause of the series’ heterogeneity is not clear.

Special Issue: VALUE: Validating and Integrating Downscaling Methods for Climate Change Research., Credible information about the properties and changes of extreme events on the regional and local scales is of prime importance in the context of future climate change. Within the EU-COST Action VALUE a comprehensive validation framework for downscaling methods has been developed. Here we present validation results for extremes of temperature and precipitation from the perfect predictor experiment that uses reanalysis-based predictors to isolate downscaling skill. The raw reanalysis output reveals that there is mostly a large bias with respect to the extreme index values at the considered stations across Europe, clearly pointing to the necessity of downscaling. The performance of the downscaling methods is closely linked to their specific structure and setup. All methods using parametric distributions require non-standard distributions to correctly represent marginal aspects of extremes. Also, the performance is much improved by explicitly including a seasonal component, particularly in case of precipitation. With respect to the marginal aspects of extremes the best performance is found for model output statistics (MOS), weather generators (WGs) as well as perfect prognosis (PP) methods using analogues. Spell-length-related extremes of temperature are best assessed by MOS and WGs, spell-length-related extremes of precipitation by MOS and PP methods using analogues. The skill of PP methods with transfer functions varies strongly across the methods and depends on the extreme index, region and season considered., This work has been carried out as part of the EU‐COST Action VALUE (ES1102). The authors are particularly grateful for the instructive and very helpful comments of two anonymous reviewers. P.M.M.S. thanks the Portuguese Foundation for Science and Technology (FCT) for funding under Project SOLAR‐PTDC/GEOMET/7078/2014.

The aim of the paper was to present the temporal and spatial variability of precipitation in the Łodź province in the period 1961–2015. Daily precipitation totals from 20 precipitation stations in the province were used. It was shown that the mean yearly precipitation sum averaged across the province was 582 mm, and at particular stations ranged from 515 to 648. The totals were observed to be highest in the southern part of the province and they dropped to the north, where water deficits happened most often. Summer totals were more than double the winter ones. Spring and autumn precipitation totals were similar, with a slight advantage to spring totals. Daily sums most often fall in the range 1–10 mm. Daily totals exceeding 50 mm were extremely rare, and appeared once every few years, mainly in summer. At the moment, there are no statistically significant changes in the structure and level of precipitation, but there is a slight tendency towards increasing winter totals and decreasing summer totals.

The work presents a historical outline and main achievements of Lodz researchers in the field of urban climate. Although regular meteorological measurements in Lodz began at the beginning of the 20th century, the development of urban climatology, understood as the city's influence on the field of meteorological elements, dates back to the 1950s. The last decade of the 20th century and the beginning of the present century brought a special intensification of activities in the field of urban climatology in Lodz. Apart from studies on the classic problems of the urban heat island or solar relations, unique measurements of the heat balance, surface exchange of greenhouse gases (CO2 and CH4) or radon concentration in the city were carried out during this period; the innovative measurement methods (scintillometry) were used and original numerical models were developed.

This work gives an overview on how the projected changes in the extremes in Poland might impact human health and economy. For that purpose, statisti-cally corrected data from 7 regional climate models were used. A significant increase of extreme hot events (i.e. heat waves, tropical nights) is projected for Central and Southern Poland for the end of the 21st century which might seri-ously affect a society living in large urban areas. Less extreme cold events im-prove thermal comfort in winter. The negative impact of the warming will affect energy systems with higher demand for electricity in summer and agriculture: an earlier beginning of the growing season and flower blooming will enhance the risk of frost damages in spring, whereas excessive heat will reduce yields in summer. Polish tourism should benefit from higher thermal comfort (except for hot July and August in the far future and warming in the winter season bring-ing snow cover depletion in the near future)., Ovaj rad daje pregled kako projicirane promjene ekstrema u Poljskoj mogu utjecati na ljudsko zdravlje i njezino gospodarstvo. U tu svrhu korišteni su statistički korigirani podaci iz 7 regionalnih klimatskih modela. Za srednju i južnu Poljsku predviđa se za kraj 21. stoljeća značajan porast ekstremnih vrućih događaja (tj. toplinskih valova, tropskih noći), što bi moglo ozbiljno utjecati na društvo koje živi u velikim urbanim područjima. Manje ekstremni hladni događaji poboljšavaju toplinsku ugodnost zimi. Negativni utjecaj zagrijavanja utjecat će na energetske sustave s većom potražnjom električne energije ljeti, a u poljoprivredi: raniji početak vegetacijske sezone i cvjetanje cvijeća povećat će rizik od oštećenja od mraza u rano proljeće, dok će pretjerana vrućina smanjiti prinose ljeti. Poljskom turizmu trebala bi pogodovati veća toplinska ugodnost (osim vrućeg srpnja i kolovoza u dalekoj budućnosti te zagrijavanja tijekom zimske sezone u bliskoj budućnosti, što će dovesti do smanjenja snježnog pokrivača).

Special Issue: VALUE: Validating and Integrating Downscaling Methods for Climate Change Research: et al., VALUE is an open European collaboration to intercompare downscaling approaches for climate change research, focusing on different validation aspects (marginal, temporal, extremes, spatial, process-based, etc.). Here we describe the participating methods and first results from the first experiment, using “perfect” reanalysis (and reanalysis-driven regional climate model (RCM)) predictors to assess the intrinsic performance of the methods for downscaling precipitation and temperatures over a set of 86 stations representative of the main climatic regions in Europe. This study constitutes the largest and most comprehensive to date intercomparison of statistical downscaling methods, covering the three common downscaling approaches (perfect prognosis, model output statistics—including bias correction—and weather generators) with a total of over 50 downscaling methods representative of the most common techniques. Overall, most of the downscaling methods greatly improve (reanalysis or RCM) raw model biases and no approach or technique seems to be superior in general, because there is a large method-to-method variability. The main factors most influencing the results are the seasonal calibration of the methods (e.g., using a moving window) and their stochastic nature. The particular predictors used also play an important role in cases where the comparison was possible, both for the validation results and for the strength of the predictor–predictand link, indicating the local variability explained. However, the present study cannot give a conclusive assessment of the skill of the methods to simulate regional future climates, and further experiments will be soon performed in the framework of the EURO-CORDEX initiative (where VALUE activities have merged and follow on). Finally, research transparency and reproducibility has been a major concern and substantive steps have been taken. In particular, the necessary data to run the experiments are provided at http://www.value-cost.eu/data and data and validation results are available from the VALUE validation portal for further investigation: http://www.value-cost.eu/validationportal., This work has been performed in the framework of the VALUE COST Action ES1102, under FP7 programme J.M.G. and S.H. acknowledge partial funding from MULTI-SDM project (MINECO/FEDER, CGL2015-66583-R). B.H. and D.R. acknowledge COMPLEX project (FP7-ENV-2012, No: 308601). M.T. was supported by HOPE project (MINECO, CGL2014-52571-R). Participation of M.D., P.S. and R.H. was funded by the Ministry of Education, Youth, and Sports of the Czech Republic contracts LD12029, LD14043 and LD12059, respectively.

Snow accumulation and associated synoptic conditions in the Russian Arctic are analysed based on snow depth data from 1950 to 2013 from the All-Russian Research Institute of Hydrometeorological Information—World Data Centre data set. The mean duration of snow coverage in the Russian Arctic is approximately eight to nine months. While the period of snowmelt is usually very short (one or two months), snow accumulates during most of the cold season (October–May). Snow accumulation is associated with negative anomalies of sea level pressure and positive anomalies of air temperature, which means increased cyclonic activity over the analysed region. The cyclones differ in intensity and localization, depending on the area of snowfall. In the western part of the Russian Arctic the cyclones and air masses that bring snowfall may originate from the North Atlantic, while in the eastern part they originate from the Bering Sea, Okhotsk Sea or the North Pacific. The cyclones that bring snowfall may also form locally along the zonal border between two different air masses: the very cold, polar, continental air originating from the Siberian High and the Arctic air from the north, which is often warmer and always more humid than the continental air. Keywords: Polar climate; snow cover; air circulation patterns. (Published: 30 March 2016) To access the supplementary material for this article, please see the supplementary files in the column to the right (under Article Tools). Citation: Polar Research 2016, 35 , 25916, http://dx.doi.org/10.3402/polar.v35.25916

This paper outlines the symptoms of contemporary global warming, reviews its possible driving factors and presents some projections for future. Key among the symptoms are those related to temperature, with the increase in average global temperature since 1880 now reaching a value of 0.85°C. While warming has encompassed almost the whole world, the high latitudes have warmed more than the low, and maximum temperature has increased more than average temperature. Warming has been causing sea level rise, thanks to both the thermal expansion of warming water and the melting of ice on land. The other consequence of warming is a change in precipitation pattern, manifesting itself in higher precipitation in certain parts of the world (generally at low and high latitudes), but also lower precipitation in other parts (mainly the Tropics); as well as in changes in the intraannual course characterising precipitation (with more falling in winter and less in summer), and in the frequency and intensity of rainfall (more intense heavy-precipitation events and higher variability where the frequency of precipitation is concerned). Among the possible driving factors, the most important are those related to the increase of CO2 and mixing ratios of other greenhouse gases in the troposphere. Land-use changes and emissions of aerosols to the atmosphere also exert a major impact on temperature. These are mainly anthropogenic factors. While natural drivers also modulate the climate markedly, they tend to warm and cool the globe alternately, stepping up warming when they are in a warming phase, but slowing down or even offsetting warming during a cooling phase. Projections for the future are entirely dependent on socio-economic scenarios of future development. All the (economically) realistic scenarios point to a continuation of the warming trend, with a further intense sea-level rise and precipitation changes, albeit with the rate of change varying in line with the rate of increase in concentrations of the greenhouse gases. The realistic range of values for average rise in global temperature is between 2 and even 6 degrees Celsius.

This study is focused on early fall season in southern Siberia (50–60 N) and is purposed as a contribution to the discussion on the climatic relevance of October Eurasian snow cover. Analysis is based on the daily snow depth data from 43 stations from years 1980–2012, available in the database of All-Russian Research Institute of Hydrometeorological Information—World Data Centre. The snow cover season in southern Siberia starts in early autumn and the number of days with snowfall varies from less than 5 days in the southernmost zone along the parallel 50 N to more than 25 days in the northeastern part of the analyzed area. October snowfall in southern Siberia is associated with occurrence of negative anomalies of sea level pressure (SLP), usually spreading right over the place of recorded intense snowfall or extending eastward from it. Negative anomalies of air temperature at the 850 hPa geopotential level (T850) occurring with increased cyclonic activity are also observed. Negative T850 anomalies are located west or northwest of the SLP depressions. Counterclockwise circulation around low-pressure systems transports cold Arctic air from the north or even colder Siberian polar air from the east, to the west, and northwest parts of cyclones, and induces negative anomalies of temperature. The pattern of T850 anomalies during heavy snowfalls in the eastern part of the southern Siberia is shifted counterclockwise in regard to SLP anomalies: the strongest negative T850 anomalies are located west or northwest of the SLP depressions.

The aim of the paper was to assess the robustness of four bias correction techniques: simple bias correction, distribution based bias correction, delta change and distribution based delta change. Data from nine RCM simulations of CORDEX project and 41 Polish weather stations were used. The methods were calibrated in the period 1971–1985 and evaluated in 1991–2005. The improvement in mean, 10th and 90th percentiles was shown, without significant differences among methods. For 1st and 99th percentiles the improvement was generally weaker and simple methods seem to be more robust than the distribution based ones. Strong differences between individual models were found, so the use of model ensemble is recommended.

The mean duration of snow coverage in the West Siberian Plain is approximately eight months in the north to about five months in the south. While the period of intense snow melting is short (one or two months between March and May), snow accumulation persists for most of the cold season. Snow accumulation is associated with negative anomalies of sea level pressure, which means increased cyclonal activity and weaker than normal Siberian High. Much lower anomalies of sea level pressure occur during snow ablation. This suggests smaller influence of air circulation on snow cover reduction in spring.

Various indices of hot weather frequency and intensity were analysed in the area of Poland in the period between 1951 and 2006. An increase of majority of them was shown in the whole year and all summer months but September, when significant decrease in all indices was apparent. The correlation of selected hot weather indices and precipitation totals in a month of hot weather event and the preceding months were also calculated to check if prolonged dry weather can constitute a forcing factor for hot event creation. Because significant correlations appear mainly in the cases when precipitation is for the same month as the hot weather index, it seems that in Poland the presence of high pressure systems is a more important factor of hot event creation than dry weather.

This is an open access article under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs License.-- et al., VALUE is an open European network to validate and compare downscaling methods for climate change research. VALUE aims to foster collaboration and knowledge exchange between climatologists, impact modellers, statisticians, and stakeholders to establish an interdisciplinary downscaling community. A key deliverable of VALUE is the development of a systematic validation framework to enable the assessment and comparison of both dynamical and statistical downscaling methods. In this paper, we present the key ingredients of this framework. VALUE's main approach to validation is user- focused: starting from a specific user problem, a validation tree guides the selection of relevant validation indices and performance measures. Several experiments have been designed to isolate specific points in the downscaling procedure where problems may occur: what is the isolated downscaling skill? How do statistical and dynamical methods compare? How do methods perform at different spatial scales? Do methods fail in representing regional climate change? How is the overall representation of regional climate, including errors inherited from global climate models? The framework will be the basis for a comprehensive community-open downscaling intercomparison study, but is intended also to provide general guidance for other validation studies., VALUE is funded via the EU FP7 programme as EU COST Action ES1102.

Gridded data sets of observations are frequently used for the evaluation of climate extremes in climate models. However, it is necessary to understand how the behaviour of extremes is affected in such data sets. The aim of the paper is to analyse how the smoothing effect is related to the number of stations used for calculating the values in each gridcell. A very dense network of stations with highly correlated records of temperature and precipitation was used. The set of grid points was established and for each grid point all data from stations located in the circles with a radius of 75 km in the case of temperature and of 50 km in the case of precipitation were used. Two distributions were compared. The first one was built of the data averaged from all selected stations. The second one was built of all the data from the same stations. Apart from high correlation of averaged records, the effect of reducing extremes is strong. In the case of temperature in winter, the areal averaging causes the overestimation of temperature in the left tail of distribution for the percentiles below the tenth. In summer, the strongest effect is in the right tail and the temperature in underestimated. In the case of precipitation, the higher the number of stations averaged the lower the number of dry days. At the same time, the highest daily totals are underestimated in the highest 5 percentiles (equal and higher than the 95th percentile). All the effects increase with the increasing number of averaged records.

The collapse in recruitment of the European eel (Anguilla anguilla) since the early 1980s has been ascribed to possible overfishing, poisoning, parasitism, habitat loss and changes in ocean circulation. It is unclear which mechanism is most important, and firm data are lacking to make an assessment of the factors that apply over the full continental range. On the other hand, the recruitment of the American eel (A. rostrata) has declined along the western Atlantic at about the same time. This suggests a candidate mechanism that can affect both species together. A change in ocean climate may be a likely explanation, which is supported by a possible link between the North Atlantic Oscillation and one important recruitment index. However, it is unsafe to discard the other possible mechanisms because of lack of evidence. Habitat loss, in particular, may be important. We review over a century of evidence to suggest how the eel may have declined through progressive habitat loss that accelerated in the early 1980s as the result of economic development linked with hydrological changes. Although no single line of evidence can definitely prove one hypothesis for the eel decline, the total body of information may indicate a pronounced susceptibility in the southwest corner of the continental range closest to the Sargasso Sea that has been particularly affected by drought and dam construction. The sexual dimorphism of the species together with the energy requirements of the spawning migration may provide insight to explain the population collapse.

The daily minimum and maximum temperatures at nine stations in Poland were used in an analysis of the occurrence of coldwaves, where these are defined as days with temperatures exceeding selected thresholds (tmin ≤ -20°C, tmin ≤ -15°C and tmax ≤ -10°C) in the period 1951-2006. Cold nights occurred more often than very cold days and nights but the mean lengths of waves of cold nights were similar – lasting a little more than 2 days on average. The frequencies of extremely cold days revealed a slight, but statistically non-signifi cant downward trend. The occurrence of coldwaves was associated with high-pressure systems over Central Europe and with blocking episodes, but it was always linked with a thick layer of cold air.

The analysis concerned the variability of daily precipitation totals observed during the second half of the 20th century at five stations in Poland. The elements examined were number of days with precipitation exceeding given thresholds, lengths of wet and dry spells and precipitation amounts in a single spell. There is an upward trend for the number of spells and days with precipitation, and a downward trend for mean precipitation during a given spell. Changes in peak precipitation are not uniform, there being negative trends at some stations and positive ones at others. However the lack of a trend for precipitation totals combines with the climatic warming now to be observed to imply risk of a water deficit.

In Łodź, the total cloud cover has significantly decreased in the second half of the 20th century. This change is opposite to that of the low-level cloud cover in the warm part of the year. But the most important are variations in frequencies of different cloud types. The stratiform clouds, St and Ns, became less frequent, whereas the convective ones, Cu and Cb, became more frequent. The rise in Ac and high-level clouds and the fall in As clouds were also observed. These changes are consistent with other observations in central and eastern Europe. Because different cloud types are created in different conditions, their variability is a valuable tool for assessment of changes in processes taking place in the troposphere. Such reorganization in cloud types distribution indicates significant changes in the vertical structure of the troposphere: fall of the frequency of temperature inversions in the lower troposphere and stronger lapse rate. Copyright © 2007 Royal Meteorological Society

Primary goal of presented study is to classify the most frequent patterns of the upper tropospheric jet stream over Europe. Wind fields were grouped into separated classes with the help of the correlation-based Lund’s technique. The treatment of vector fields with Lund’s method was achieved by replacement linear Pearson coefficient with vector correlation coefficient. The outstanding features of the upper-level circulation and ground-based weather associated with each jet type were analysed. Finally, basic statistics of jet stream patterns (frequency, duration time, day-to-day changes of jet structure) as well as their trends were estimated. The analysis was conducted on the basis of mean daily wind components at 200 hPa level, air temperature at 850 hPa, sea-level pressure, vertical velocity and geopotential at 500 hPa level. Data set was extracted from the NCEP/NCAR Reanalysis. The warm half-year in the period 1950–2001 was taken into consideration. The first 15 most frequent jet types, including 60.8% of the sample, were selected. Three jet stream types (C, E and I) are associated with distinct temperature changes in western Europe. Another three types (B, F and O) cause significant thermal advection in eastern and central Europe. Seasonal differences in frequency and duration time of jet stream patterns are also observed. Meridional types (A, C and D) dominate in spring, while in summer, patterns with intensified zonal flow prevail (B, E and J). At last, it is worth noticing that the majority of selected jet types pronounce an increase in day-to-day changes of wind field, which may indicate slight enhancement of circulation dynamics in the upper troposphere.

Data from two automatic stations in Łodź (one urban and one rural) for the period 1997–2002 are analyzed to reveal urban–rural contrasts of such parameters as air temperature, relative humidity, water vapour pressure and wind speed. Under favourable weather conditions the highest temperature differences between the urban and rural station exceeds 8 °C. Relative humidity is lower in the town, sometimes by more than 40%. Water vapour pressure differences can be either positive (up to 5 hPa) or negative (up to −4 hPa). Wind speed at the urban station is on average lower by about 34% in night and 39% during daytime. Regression analysis shows that for rural winds lower than 1.13 m s−1 urban winds can be stronger than rural speeds. Attention has also been paid to singularities in the course of the analyzed parameters over 24 hour periods. It is shown that the typical course of the urban heat island intensity under favourable conditions is similar in all season. Four stages of this course have been distinguished. Wind speed differences also seem to change in a typical way. Case studies show that humidity contrasts, unlike temperature, can evolve in different ways under fine weather conditions. Types of relative humidity evolution are proposed.

The regional climate model RegCM3 is used to investigate potential future changes of temperature indices in Poland for the period 2011–2030. The model is forced by ECHAM5/MPI-OM GCM data from World Data Centre (WDCC) database for the 1971–1990 reference period and 2011–2030 projection period under SRES B1, A1B and A2 emission scenarios. Model output statistics methods are used to transform simulated minimum and maximum temperature data into realistic data. Selected indices of temperature extremes and their differences between the scenario simulations and the reference were calculated, for all scenarios, for the entire period and for each season. Results show a mean yearly increase in the number of summer and hot days and a decrease in the number of frost and ice days. Highest decline in the number of frost and ice days in autumn and an increase in spring is noticed. An highest increase in the number of summer and hot days is seen in summer. Future projections of these indices are relevant for studies on climate change impact in agriculture, tourism, health, transportation, road and building infrastructure. This work was carried out as part of the project KLIMAT "Impact of climate change on the society, the environment and the economy (changes, effects and ways of limiting them, conclusions for science and engineering practice and economic planning)". No POIG.01.03.01- 14-011/08 in frames of the Operational Programme Innovative Economy, co-financed by the European Regional Development Fund. Authors thank anonymous reviewers for valuable comments.

General (global) circulation models (GCMs) are a useful tool for studying how climate may change in the future. Although GCMs have high temporal resolution , their spatial resolution is low. To simulate the future climate of the Baltic Sea region, it is necessary to downscale GCM data. This chapter describes the two conceptually different ways of downscaling : regional climate models (RCMs) nested in GCMs and using empirical and/or statistical relations between large-scale variables from GCMs and small-scale variables. There are many uncertainties in climate models, including uncertainty related to future land use and atmospheric greenhouse gas concentrations, limits on the amount of input data and their accuracy, and the chaotic nature of weather. The skill of methods for describing regional climate futures is also limited by natural climate variability. For the Baltic Sea area, the lack of an oceanic component in RCMs and poor representation of forcing by aerosols and changes in land use are major limitations.

This chapter examines past and present studies of variability and changes in atmospheric variables within the North Sea region over the instrumental period; roughly the past 200 years. The variables addressed are large-scale circulation, pressure and wind, surface air temperature, precipitation and radiative properties (clouds, solar radiation, and sunshine duration). Temperature has increased everywhere in the North Sea region, especially in spring and in the north. Precipitation has increased in the north and decreased in the south. There has been a north-eastward shift in storm tracks, which agrees with climate model projections. Due to large internal variability, it is not clear which aspects of the observed changes are due to anthropogenic activities and which are internally forced, and long-term trends are difficult to deduce. The number of deep cyclones seems to have increased (but not the total number of cyclones). The persistence of circulation types seems to have increased over the past century, with ‘more extreme’ extreme events. Changes in extreme weather events, however, are difficult to assess due to changes in instrumentation, station relocations, and problems with digitisation. Without thorough quality control digitised datasets may be useless or even counterproductive. Reanalyses are useful as long as biases introduced by inhomogeneities are properly addressed. It is unclear to what extent circulation over the North Sea region is controlled by distant factors, especially changes in Arctic sea ice.

2 Institute of Meteorology and Water Management, Branch Wroc8aw, Parkowa 30, 51-616 Wroc8aw, Poland ABSTRACT: The variability of minimum and maximum temperature and the daily temperature range (DTR) in Poland was analyzed on the basis of the data from 9 stations with different periods of data (the longest was 98 yr). The long-term changes of seasonal means as well as for all Julian days were determined. The increase in the minimum temperature was accompanied by a slighter increase in the maximum temperature and a decrease in the DTR. It was found that the DTR changes correlate well with cloudiness, and the extreme temperature changes are related to the NAO (North Atlantic Oscillation) intensity, especially during winter and spring. The analysis of intra-annual changes has shown that the strongest increase in the minimum and maximum temperatures occurs in mid- and late winter, but there are also periods with decreasing tendencies, i.e. late autumn, the beginning of winter and the beginning of summer. All temperature indices indicate the cooling in autumn.

The relationships of circulation patterns defined as rotated principal components of the 500 hPa geopotential heights in the Euro-Atlantic sector to precipitation distribution in Europe during the winter months (from December to March) were investigated. In each month, four to five patterns are distinguished: the North Atlantic Oscillation (NAO), Scandinavian, Central European, East European and East Atlantic. The first four are present each month, while the East Atlantic pattern is present only in December and March. Rotated principal components are used as indicators of circulation type intensity. The precipitation data consist of 321 series of monthly totals for the years 1951–1990. Maps of linear correlation coefficients between precipitation series and rotated principal components show the areas where precipitation is influenced by particular circulation types. The NAO has the greatest influence on precipitation in Western Europe; it enhances precipitation from France across the British Isles to Scandinavia and is a reason for low totals in Spain. The anticyclone over Scandinavia results in wet conditions in the Central Mediterranean and dry weather in northern Europe. The blocking phase of the Central European pattern combines with very low precipitation in Central Europe and higher-than-usual totals in the north. The East European pattern brings above-normal precipitation in north-eastern and south-western Europe, and below-normal precipitation in north-western and south-eastern areas in the phase when the geopotential level is low over the east Atlantic. Copyright © 1999 Royal Meteorological Society

Mean monthly air temperatures in Cracow for the period 1826–1990 were used. The temperatures of the coldest (Tlow) the warmest (Thigh) periods of the year, and mean annual temperatures (Ta), were calculated. The persistence of temperatures was examined using the correlation coefficients between monthly temperatures in succeeding months. A significant temperature persistence was found for a cold season (December-March) and also for the summer (July-August). Long-term fluctuations were analysed by means of 20-year and 30-year running mean values and the positive and negative squared deviations of moving 20-year subperiods. Two warm stages were found. One at the beginning of the present century, the second at the end of the period analysed. The first warm phase was connected with a decrease in the degree of continentality. Warm phases were also linked with a decrease in the temperature variability. The spectrum and the moving spectrum in 40-year subperiods of monthly and annual temperature records were calculated. An 8-year period was found in Tlow, Ta, and in the monthly series of the cold season, and 4- and 5-year periods were found in Thigh and in the monthly series of the warm season. By comparing the temperature in Cracow with that at the neighbouring stations of Wieliczka, Bochnia, and Tarnow, it was found that the difference in annual means increased from −0.3°C at the end of the previous century to 1.0°C in the last two decades. About 0.5°C of this increase occurred in the last 40-year period of abrupt expansion and industrialization of the city of Cracow.

Cold waves commonly occur in higher latitudes under prevailing high pressure systems especially during winter season which cause serious economical loss and cold related death. Accurate prediction of such severe weather events is important for decision making by administrators and for mitigation planning. An Advanced high resolution Weather Research and Forecasting mesoscale model is used to simulate a severe cold wave event occurred during January 2006 over Europe. The model is integrated for 31 days starting from 00UTC of 1 January 2006 with 30 km horizontal resolution. Comparison of the model derived area averaged daily mean temperatures at 2m height from different zones over the central Europe with observations indicates that the model is able to simulate the occurrence of the cold wave with the observed time lag of 1 to 3days but with lesser intensity. The temperature, winds, surface pressure and the geopential heights at 500 hPa reveal that the cold wave development associates with the southward progression of a high pressure system and cold air advection. The results have good agreement with the analysis fields indicates that the model has the ability to reproduce the time evolution of the cold wave event.

The correlation coefficients between air temperature, precipitation, and geopotential heights of the 500 hPa level for six stations in Europe, located along the 20°E meridian from Scandinavia to Greece, for the period of 1951–1985 are calculated. The fields of correlation coefficients for each station and season are described. It is observed that the correlation coefficients of temperatures are, as a rule, significantly greater than for precipitation. How meridional and zonal circulation determine temperatures and precipitation has been examined.

We investigate the winter temperature and precipitation evolution over Poland over the last half millennium in comparison with the European average in reconstructions/instrumental data and in the ECHO-G and HadCM3 models and discuss the physical processes behind those variations. Results indicate very good agreement between European land and Polish winter temperatures in reconstructions and in the models. Colder winter conditions were found within the ‘Little Ice Age’ and temperatures at the turn of the twenty first century are very likely the warmest in the context of the past. The strong agreement between Polish winter temperature and European mean conditions is of major interest since some of the longest European proxy information stem from Poland and therefore can improve European temperature reconstructions significantly. Precipitation results indicate that reconstructions over Poland agree well with those of the rest of Europe, though the agreement is poorer between the reconstruction and the models. The role of the large-scale atmospheric circulation dynamics/forcing connected with the observed Polish winter temperature/precipitation changes is investigated in the reconstructions and in the model world. The most important atmospheric circulation pattern for winter temperature variability is the North Atlantic Oscillation (NAO). The Scandinavian (SCAND) and to a lesser degree also the NAO and East Atlantic/Western Russia (EA/WRUS) are of relevance for winter precipitation variations in Poland. Canonical Correlation Analysis (CCA) results show that the leading circulation modes responsible for dry/wet and warm/cold Polish winter conditions are in good agreement in the reconstructions and models. These results suggest that the models are able to reproduce the links in instrumental and proxy data and also that the large- to regional-scale relationships are robust during the last centuries. The stability of the large- to regional-scale links is relevant for downscaling approaches and also for palaeoclimate reconstructions.