Climatology of precipitation extremes in Estonia using the method of moving precipitation totals Tiina Tammets Estonian Meteorological and Hydrological Institute Jaak Jaagus * Department of Geography, Institute of Ecology and Earth Sciences, University of Tartu Abstract A method of moving precipitation totals is described and applied for the analysis of precipitation extremes in Estonia. Numbers of extremely wet and extremely dry days other indices of precipitation extremes were calculated using the daily precipitation data measured at 51 stations over Estonia during 1957–2009. Mean regularities of spatial and seasonal distribution were determined. Long-term changes were detected using Sen’s method and Mann-Kendall test. The highest risk of heavy precipitation is in the regions of higher mean precipitation on the uplands, and on the belt of higher precipitation in the western part of continental Estonia. Wet spells have their sharp maxima in July and August. The highest risk of droughts is observed in the coastal regions of West Estonia. In the coastal area, droughts appear mostly in the first half of summer while in the eastern Estonia, they are usually observed during the second half of summer. Statistically significant increasing trends were, first of all, found in the time series of the indices of extreme precipitation all in winter. In summer and autumn, trends existed in some indices but in spring there were no trends at all. Consequently, extreme precipitation events have become more frequent and intense. There were no trends in time series of dryness indices in Estonia in 1957– 2009. * - corresponding author, jaak.jaagus@ut.ee, tel. +3727375824, fax +3727375825 1 1. Introduction Precipitation is the climatic variable that has the highest variability in space and time. Therefore, precipitation extremes – continuing periods with little or without any precipitation leading to droughts, or heavy rainfall-induced flash floods – are related to the severest damages to human activities in nearly all regions of the world. That is why the analysis of extreme precipitation and drought events and their long-term trends has become an important topic in climatological research. An increase in the number of extreme precipitation events has been noticed, and proposed to be generally connected with possible climate warming in the future (Groisman et al. 2005). Analyses of changes in climatic extremes during the second half of the 20th century revealed a significant increase in the amount of precipitation derived from wet spells and numerous heavy rainfall events (Frich et al. 2002). At the same time, a significant decreasing trend was noticed in global series of the annual maximum of consecutive dry days (CDD). Similar trends have been detected by many local studies (Karl and Knight 1998; Stone et al. 2000; Osborn et al. 2000; Zhang et al. 2001; Frei and Schär 2001; Alpert et al. 2002; Kiktev et al. 2003; Sen Roy and Balling 2004; Schmidli and Frei 2005). An increasing frequency of droughts is also evident in different regions of the world (Dai et al. 2004; Zou et al. 2005; Groisman et al. 2007; Balling and Goodrich 2010). A detailed overview of studies in the precipitation extremes, droughts and wet spells in the Baltic Sea region has been presented by the BACC Author Team (2008). The CDD trends are mostly insignificant in this region (Frich et al. 2002; Haylock and Goodess 2004). A slightly decreasing insignificant trend of CDD was revealed in Norway and Sweden (Førland 2000; Alexandersson 2002), while a positive trend was detected in Germany for the summer season (Klein Tank and Können 2003). A comprehensive overview of trends in temperature and precipitation extremes were given within the EU project EMULATE (Chen et al. 2006). Different indices of heavy rainfall events indicate positive tendencies, but also no changes, in the Baltic Sea region during the second half of the 20th century. The number of days with precipitation ≥10 mm has increased at German mountain stations and in western Norway (Heino et al. 1999). The maximum number of consecutive wet days has increased at many stations over the Scandinavian Peninsula during 1961-2004 (Achberger and Chen 2006). The simple daily intensity index (SDII) increased significantly in central and western Europe in winter in 1921-1999 (Moberg and Jones 2005). However, none of these studies used data from the territory of Estonia. Previous results have been confirmed by recent new investigations. Based on changes in the NAO from the 1960s to 1990s, and using simulation groups from a general circulation model (HadCM3), large changes were found in the frequency of the 90th percentile precipitation events over Europe in winter (Scaife et al. 2008). This would lead to growing occurrence of heavy precipitation over northern Europe and lessening occurrence over southern Europe during high NAO periods. Zolina et al. (2009) obtained mostly positive trends of daily precipitation and precipitation totals at 116 stations in different regions of Europe, including the Baltic Sea region, during 1950–2000. The positive trends were characteristic for winter and spring while the negative trends occurred mostly in summer and, in some cases, in autumn. Wet periods with the daily precipitation R>1 mm have become longer over most of Europe by about 15-20% during 1950–2008 (Zolina et al. 2010). The lengthening of wet periods was not caused by an increase in the annual total number of wet days. Becoming longer, wet periods in Europe are now characterised by more abundant precipitation. Heavy precipitation events during the last two decades have much more frequently become associated with longer wet spells and intensified in comparison with the 1950s and 1960s (Zolina 2011). An increasing trend was detected in the 95th and 99th percentiles in winter, spring and autumn, and a negative trend in summer in northern Germany in 1950–2004 (Zolina et al. 2008). 2 Positive trends were detected for the number of wet spells and days with precipitation at five stations in Poland during the second half of the 20th century, while negative trends were revealed for mean precipitation during a given spell (Wibig 2009). Positive as well as negative trends in the indices of precipitation extremes were detected at 48 synoptic stations in Poland during 1951– 2006; however, the decreasing trends were prevailing (Łupikasza 2010). The highest number of decreasing trends was revealed in summer, while the number of increasing trends was more pronounced in spring and autumn. Increasing trends in time series of the number of days with heavy precipitation (above 10 mm) and of the percentage of heavy precipitation in the total annual amount were detected in Lithuania during 1961-2008 (Rimkus et al. 2011). This tendency is especially clear in summer months when an increase in precipitation extremity can be observed on the background of neutral or negative tendencies in the total summer precipitation. A significant increase in the number of days with heavy precipitation (≥10 mm) was also observed in Latvia during 1924-2008 (Avotniece et al 2010). Statistically significant trends in two drought indices were not detected for Vilnius, Lithuania, in 1891–2010; however, a small decrease in dryness was still observed (Valiukas 2011). Although Estonia is characterised by a humid climate where annual precipitation exceeds annual evaporation, dry periods occur rather often during the warm half-year. They cause severe damages to human activities – loss of agricultural production, water shortage, forest fires, expanding of different diseases, etc. Flash floods caused by heavy rainfall and continuing wet periods pose serious problems for agriculture, transport and everyday life of local residents. Estonia is located on the eastern coast of the Baltic Sea between 57.5 and 59.5 degrees north. Climatically, it lies in the transition zone between the area of rather maritime climate on the islands and the coast of the Baltic Sea in the west, and the area of much more continental climate in the south-east and east. Precipitation regime is similar to that of the neighbouring regions with maximum rainfall in summer and autumn and minimum in winter and spring. The annual mean amount of precipitation in Estonia varies between 550 mm in the coastal zone and up to 750 mm on the windward slopes of the uplands (Jaagus and Tarand 1988). Time series analysis has revealed an increasing trend of precipitation in winter and autumn (Jaagus 1996). Clear periodical fluctuations of 50-60, 25-30 and 6-7 years have been detected for annual precipitation in Estonia since 1866 (Jaagus 1992). Relationships between atmospheric circulation indices and precipitation totals are the strongest in winter and spring (Keevallik et al. 1999; Keevallik 2003; Jaagus 2006). The dependence of precipitation pattern on local landscape characteristics in the three Baltic countries is analysed using data mining tools (Jaagus et al. 2010; Remm et al. 2011). Extreme precipitation events in Estonia were analysed in relation to different types of largescale atmospheric circulation (Grosswetterlagen) (Merilain and Post 2006) and cyclones of different trajectories (Mätlik and Post 2008). It was found that more than 25% of all heavy rainfall events (at least 50 mm per 24 hours) in Estonia have been caused by the southern cyclones originated in the region of the Black and Mediterranean Seas. The frequency of the 95th and 99th percentile daily precipitation events at 40 stations in Estonia has increased significantly during 1961–2008 (Päädam and Post 2011). The main disadvantage of the studies on extreme precipitation and extreme dry spells is the calculation of characteristics of extremes in discrete time intervals (seasons, months, ten-day periods, days etc.). To estimate water content in the soil, it is not so important to know how much it has rained during the observed period but how much it had rained before that period. In other words, for many purposes it is essential not to show how much it rains in a day but to show how much it has rained till this day. Following this ideology, the first author of the current article has proposed a methodology for analysing precipitation extremes by using moving precipitation totals (Tammets 2007; 2010). Using this method, wet and dry days have been defined and their numbers are analysed for Estonia. A day is considered as extremely wet when the moving total of 3 precipitation is at least 100 mm on ten successive days leading up to this day and as extremely dry when there was no precipitation during the successive twenty days till the observed day. An increase in the number of extreme (wet and dry) days was distinguished during 1957-2006. To present the minimum and maximum of precipitation totals in long time series of a station in any time period, which has been measured by days, months or years, special relationships were built (Tammets 2010). This study is the continuation of a previous one. Its main objectives are to compare the usual characteristics of precipitation extremes with the number of extremely dry and wet days; to develop a method for characterising the climatology of extreme precipitation totals for any time periods (the number of following days, months or years using moving totals); to detect the most severe wet and dry spells in Estonia during the last 50 years; to analyse trends in the frequency of extremely wet and dry periods using different indices and to compare the results. 2. Data and method Daily precipitation data at 51 stations (25 meteorological stations and 26 precipitation stations) over Estonia (Figure 1) during 53 years (1957-2009) was used as the initial data in this study. The stations were selected according to the quality and continuity of time series. There were only very few gaps in the data series, which were filled by measured data from a neighbouring station located in the similar landscape conditions and having the highest correlation. There have been relocations of the measuring sites at the Tallinn, Tartu, Narva and Pärnu stations, which may cause inhomogeneities in the data series. It is difficult to detect the influence of data inhomogeneity on the estimates of precipitation extremes. Therefore, the time series used are considered reliable. Fig. 1. Location map of precipitation stations in Estonia used in this study 4 All the daily precipitation totals are registered at 06 p.m. GMT. The only exception was the period from 11 February 2005 until 30 April 2009 when the measurements in the meteorological stations were made in the morning of the next day at 06 a.m. GMT. The precipitation measurements were made manually using Tretyakov gauges. Since 1966, wetting corrections of 0.2 mm for liquid and 0.1 mm for solid precipitation were added if at least 0.1 mm had been measured. Some gaps in 2009 were filled with the data from an automatic weather station MILOS 520, which are less reliable. The VRG type automatic precipitation gauge was applied in Kuressaare since 1 January 2008 and in Heltermaa since 1 May 2009. Following the method of moving totals, a day is an extreme day if the amount of precipitation up to this day has been too small or too large. The preliminary task, when looking at dry or wet spells, is to define a dry and a wet day. The simplest definition of a dry day is zero rainfall, and a wet day is the day with a precipitation threshold, depending on climate conditions of observed area. An extremely wet or a dry day is a day with too much or too few precipitation in a period, which lasts n days and ends on the observed day. Limits of the precipitation amount M and the number of subsequent days n in the observed period depend on the study object, which could be an agricultural field, a river basin, etc. In agro-meteorological studies, these limits depend on the plant species, the state of vegetation, soil conditions, air temperature, humidity, etc. Counting of the moving totals of precipitation continues throughout the whole observation period without separating time into months or years. To point out the meteorological, agro-meteorological, hydrological or socio-economical drought or flooding conditions, indicators should be calculated for the specified numbers of days n and thresholds M (Tammets 2007; 2010). A similar idea for describing drought conditions was applied to work out the concept of effective precipitation (Byun and Wilhite 1998; Kim and Byun 2009). Mathematically, the sequence of moving totals (averages){sj(n),1 £ j £ N-n+1} is derived from a sequence {ai, 1 £ i £ N} obtained by taking the totals (averages) of the subsequent n terms: i+n− 1 sj(n)= ∑ j=i aj (by moving averages sj(n)= 1 n i+n− 1 ∑ j=i aj), , where N is the total number of days in the precipitation time series and n the number of days through which the moving average is calculated. We find extremely dry and extremely wet days after calculating sj(n) with the time period n for each day i in precipitation time series choosing the days with values of sj(n) that are smaller or larger than the given threshold M (Fig. 2). In different studies and climate conditions, instead of the daily time step, a monthly or even an annual time step could be used for calculating the moving totals of precipitation. Soil moisture conditions respond to precipitation anomalies in a relatively short time. Groundwater, stream flow and reservoir storage reflect the longer-term precipitation anomalies. Extreme precipitation totals for a year and a longer period are one of the essential factors of the water balance in the region. To find the extreme precipitation totals for any number of days or months in the observed period, graphs were built with the y-axis showing the maximum and minimum of the moving total of precipitation in dependence on the number of days or months in the calculated period, shown on the x-axis (Fig. 3 and 4) (Tammets 2010). The highest curve on the daily graph (Fig. 3) shows values of the highest maximum, the second high curve – of the lowest maximum precipitation of all studied stations; the lowest curve shows values of the lowest minimum and the second curve above it – of the highest minimum of precipitation in dependence on n (number of days) in the observed period. These graphs were calculated over 51 Estonian meteorological stations during 1957–2009 and they can be used as one of the characteristics of the precipitation regime for Estonia. This method allows to analyse the precipitation conditions in any period. After calculating of moving totals through the time series, the period under observation should be separated and statistically analysed. 5 if there are no precipitation on a day and previous n days the day has been counted as a extremely dry day (EDD) days 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 days 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 17 21 21 22 23 if the total of precipitation on a day and previous n days is M mm and more the day has been counted as a extremely wet day (EWD) 18 19 20 22 23 Fig. 2. The scheme for the calculation of extremely dry days and extremely wet days using a moving precipitation total of the preceding days. In this example, n = 20 in case of dry days and n = 10 in case of wet days Some limits for defining extreme weather conditions in Estonia have been given in agrometeorological studies (Kivi 1998). Extremely wet conditions are observed when the mean daily precipitation amount is equal to or larger than 10 mm during 10 consecutive days (n = 10). We calculate the sequences si (10) giving the moving total for 10-day periods. If si >= 100 mm, the last day of the period is counted as an extremely wet day (EWD). Extremely dry conditions for field plants emerge if there are no precipitation in the duration of 20 consecutive days ( n = 20). If si (20) = 0, then the last day of the period is counted as an extremely dry day (EDD). Extremely wet days and also extremely dry days frequently follow each other. In such case, the number of EWDs or the number of EDDs show the intensity of a wet or a dry spell. To estimate differences between the number of extremely dry days with the amount of precipitation of 0 mm and 1 mm (maximum error measurements with Tretyakov gauge for 20 days) we also calculated the number of extremely dry days with the precipitation amount of only 1 mm (EDD1) in 20 consecutive days ( n = 20). Annual and seasonal numbers of extremely dry and wet days (EDD, EDD1, EWD) were calculated for each of the 51 stations during the period 1957-2009. To analyse the inter-annual variability of the average number of wet and dry days EDD and EWD together, the relative numbers of dry and wet days were calculated by dividing the number of EDDs and EWDs in a year by their average in 1957-2009. We also calculated the relative numbers of extremely wet and dry days for every season: spring (MAM), summer (JJA), autumn (SON) and winter (DJF). 6 total precipitation, mm 1000 absolute maximum of stations 900 lowest maximum of stations 800 absolute minimum of stations 700 highest minimum of stations 600 500 400 300 200 100 0 0 20 40 60 80 100 120 140 160 180 200 220 number of days 4 years Fig. 3. Maximum and minimum totals of precipitation at Estonian meteorological stations in 19572009 in dependence on the number of days in the period 4000 absolute minimum of stations absolute maximum of stations 3000 2 years lowest maximum of stations 2500 2000 1 year precipiation total, mm 3 years highest minimum of stations 3500 1500 1000 500 0 0 3 6 9 12 15 18 21 24 27 30 33 36 39 42 45 48 51 number of months Fig. 4. Maximum and minimum totals of precipitation at Estonian meteorological stations in 19572009 in dependence on the number of months in the period 7 Annual, seasonal and monthly precipitation totals are used to characterise the general precipitation regime in Estonia. Numbers of extremely wet days (EWD) and extremely dry days (EDD) in 1957-2009 were compared with other more frequently used indices of precipitation extremes. In this study we used the following indices: Wet conditions SDII – simple daily intensity index: annual total precipitation divided by the number of days with R ≥ 1 mm (mm/wet day); CWD – maximum number of consecutive wet days R ≥ 1 mm (days); R10mm – number of heavy precipitation days R ≥ 10 mm (days); R20mm – number of heavy precipitation days R ≥ 20 mm (days); RX1day – the highest 1-day precipitation amount (mm); RX5day – the highest 5-day precipitation amount (mm); R95p – days with RR> 95th percentile daily amount (R ≥ 1 mm); R99p – days with RR> 99th percentile daily amount (R ≥ 1 mm). Dry conditions CDD – maximum number of consecutive dry days R < 1 mm (days); XCDD – maximum number of consecutive dry days R = 0 mm (days); PDD – probability of a dry day following a dry day (mean dry-day persistence); DSMEA1 - mean dry spell length with R < 1mm (days); DSMEA0 – mean dry spell length with R = 0 mm (days). These indices of precipitation extremes were calculated for every station by years and seasons, and their mean values for 1957-2009 were found. The absolute maximum and minimum values of the indices with the corresponding year and station name were determined. The maximum and minimum for years (mean of stations) and maximum and minimum for separate stations were calculated as well (Tables 1, 2, 3). Seasonal values of R99p and R95p were calculated using annual limits (Table 1). Maps for the indices of precipitation extremes were drawn to analyse the spatial and annual distribution of extreme wet and dry periods in Estonia. Precipitation totals, as well as many indices of precipitation extremes are not normally distributed. Therefore, Sen’s method and Mann-Kendall test (Salmi et al 2002) were used for trend analysis. Trends are expressed in millimetres per decade. Trend time series of the indices of precipitation extremes are presented in Tables 1, 2 and 3, where statistically significant trends are marked in bold. 3. Results 3.1. Mean precipitation regime in Estonia Before analysing the precipitation extremes we describe the mean precipitation regime based on the data of 51 stations in 1957–2009, used in this study (Table 1). The spatio-temporal variability of precipitation is very high on the territory of Estonia (Fig. 5a). Variability of the indices of precipitation extremes is even higher. This is clearly demonstrated in Table 1, where mean indices of the 51 stations over years 1957–2009, the annual maxima of the stations’ mean values, their changes by trend, and the absolute maxima at single stations with the years of the corresponding maxima are presented. 8 Table 1. Mean values of precipitation totals and mean indices of extremely wet conditions at 51 stations in Estonia during 1957–2009, their maximum values and the absolute maxima of the station data (mm), years of the corresponding maxima and trends (per decade) of the stations’ mean values. Statistically significant changes at p<0.05 level are typed in bold Year Winter Spring Summer Autumn Index PREC EWD SDII CWD R10mm R20mm RX1day RX5day R95p R99p PREC SDII CWD R10mm R20mm RX1day RX5day R95p R99p PREC SDII CWD R10mm R20mm RX1day RX5day R95p R99p PREC EWD SDII CWD R10mm R20mm RX1day RX5day R95p R99p PREC EWD SDII CWD Mean 649 0.83 5.42 7.53 14.8 2.85 34.2 59.3 6.24 1.23 128 4.03 5.53 1.55 0.07 12.7 26.9 0.34 0.01 108 4.72 4.26 1.97 0.26 16.0 29.1 0.68 0.09 215 0.72 7.33 5.07 6.47 1.87 31.6 54.2 3.44 0.91 196 0.11 5.54 6.25 Maximum Year of Year of of stations the Absolute the means maximum maximum maximum 858 2008 1120 1981 4.7 1978 20 1978 6.28 1978 7.72 1997 9.82 1984 19 5 years 21.5 2001 35 2003 5.49 2008 12 1991 52.5 1987 131 1988 85.6 1987 184 1997 10.6 2008 19 2009 2.55 2008 7 2009 206 1999 320 1990 5.12 1993 7.32 1993 8.20 2007 16 3 years 4.55 1999 11 2 years 0.41 1990 4 2003 18.1 1989 41.3 1978 38.9 2003 80.4 1990 1.27 1999 6 2003 0.08 2004 1 16 years 176 1995 226 1995 6.56 1969 12.2 2005 6.24 1990 12 1986 4.31 1969 9 1983 1.04 1995 4 2 years 29.8 1995 65.4 1998 40.1 1996 88 1999 1.94 1969 5 1977 0.57 1995 3 1983 360 1998 508 1978 3.86 1978 20 1978 8.69 1986 17.5 1997 7.57 2005 15 2 years 11.4 1998 18 1981 3.94 1998 10 1998 49.0 1987 131 1988 81.3 1978 184 1997 6.59 1998 12 2 years 2.12 2008 6 3 years 268 1978 380 1974 0.8 1978 10 1987 7.05 2001 9.41 2001 9.04 1978 19 5 years 9 Trend 25.8 0.13 0.11 0.15 0.93 0.32 0.89 1.53 0.58 0.11 5.8 0.07 0.28 0.15 0.02 0.54 1.6 0.05 0.03 1.1 0.03 0.04 0.02 0.00 0.14 0.54 0.02 0.00 10.2 0.11 0.19 0.12 0.47 0.15 0.86 1.71 0.28 0.06 5.1 0.01 0.12 0.02 R10mm R20mm RX1day RX5day R95p R99p 4.81 0.65 21.1 43.1 1.78 0.21 8.76 1.41 27.7 57.4 3.49 0.63 2001 2008 1997 1997 2001 2005 16 7 78.4 141 9 3 2 years 1991 2000 1985 2 years 6 years 0.02 0.09 0.75 1.88 0.14 0.03 Mean spatial distribution of annual precipitation PREC (Fig. 5a) varies between 535 mm (Kihnu Island) and 732 mm (Mauri, Haanja Upland in South-East Estonia). Lower annual precipitation is generally typical for coastal stations, and for the coast of Lakes Peipsi and Võrtsjärv. Higher precipitation can be seen in continental Estonia on the uplands and, especially, on the meridional zone parallel to the western coast at the distance of 20-100 km from the mean coastline. The rainiest seasons are summer and autumn while spring is the driest (Table 1). There are remarkable differences between the coastal regions and the hinterland in the annual curve of monthly precipitation totals (Fig. 6). Western Estonia is much drier in spring and the first half of summer than central and eastern Estonia. In the second half of summer and especially in autumn, western Estonia receives much more precipitation. This can be explained by different thermal regimes of sea and land surfaces. In spring, sea surface is cold causing a stable air stratification upon it, which prevents convective air uplift, the formation of clouds and precipitation. Sea surface warms up during summer, inducing unstable air stratification, convective ascending of the air, and the formation of clouds and precipitation. Sea surface acts as a warmer surface throughout autumn and winter. The highest mean seasonal precipitation of 246 mm was observed in summer in Mauri (Haanja Upland) while the lowest summer mean values are typical for the coastal stations (155 mm in Vilsandi). The corresponding mean precipitation amounts in autumn were recorded in Uue-Lõve (243 mm) and in Praaga (158 mm). a) b) Fig. 5. Mean (a) and maximum (b) annual precipitation in Estonia during 1957-2009 using the data of 51 meteorological stations 10 90 VÕRU VILSANDI 80 precipitation, mm 70 60 50 40 30 20 10 0 I II III IV V VI VII VIII IX X XI XII months Fig. 6. Monthly mean precipitation in Vilsandi and Võru during 1957–2009 3.2. Extremely wet conditions The highest annual precipitation PREC in Estonia varies between 1120 mm (Tahkuse, 1981) and 759 mm (Kihnu Island, 2001) (Fig. 5b). Absolute maximum of the 12-month moving precipitation total was 1146 mm in Tahkuse (December 1980 till November 1981) (Fig. 4). 2008 has been the year with the maximum precipitation amount over Estonia with the average value of 858 mm. More than 800 mm was observed also in 1981, 1978 and 1990 when many maximum values of extreme precipitation indices were recorded (Table 1). The highest mean seasonal precipitation amount was registered in summer 1998 (360 mm). The absolute maximum totals of precipitation for the winter, spring, summer and autumn seasons were consequently 320.2 mm (Kuusiku, 1989/1990), 225.9 mm (Tudu, 1995), 507.9 mm (Praaga, 1978) and 380.3 mm (Uue-Lõve, 1974). The absolute maximum of the 90-days precipitation, calculated through the moving total over all days and stations during 53 years was 578.7 mm (Fig. 3) in Tõrva from 25 June till 22 September 1978. Precipitation extremes of Estonia are characterised using the number of extremely wet days (EWD). A day is estimated as extremely wet if it is the last day of a 10-day period with the precipitation total of 100 mm or more. In Estonia, such days have been observed only in summer and autumn during 1957–2009, i.e. from June to November. The highest number of such days was recorded in July and August (Fig. 7). The mean EWD per year was 0.83 (Table 1) while its annual maximum (20 days) was observed at Praaga in 1978, calculated in two wet periods – one in 12-22 July and the second one in 10-18 August. In autumn, the maximum number of EWD was twice lower – 10 in 1987. Spatial distribution of EWD shows that the greatest danger for extremely wet periods appears in south-eastern and northern Estonia (Fig. 8a). Annual mean EWD varies highly at different stations. Its maximum was found in northern Estonia (Vanaküla, 1.79) and minimum in western Estonia (Sõrve, 0.13). Consequently, the frequency of wet spells varies by many times over the Estonian territory. Annual maximum of EWDs is rather randomly distributed over the territory (Fig. 8b). Generally, the same pattern as in case of EWDs is also typical for other indices of precipitation extremes. 11 1.2% 90 EWD precipitation 80 70 0.8% 60 50 0.6% 40 0.4% 30 precipitation, mm relative number of days 1.0% 20 0.2% 10 0.0% 0 01 02 03 04 05 06 07 08 09 10 11 12 months Fig. 7. Monthly mean relative number of extremely wet days (EWDs divided by the total number of days in the months) and average monthly precipitation (PREC) at the Estonian meteorological stations in 1957–2009 a) b) Fig. 8. Annual mean (a) and maximum (b) number of extremely wet days (EWD) A simple daily intensity index (SDII) indicates the mean precipitation amount per one wet day (≥1 mm). Its annual mean value was 5.4 mm/wet day. Lower values were observed in winter and spring, slightly higher in autumn, and much higher in summer (mean value 7.3 mm/wet day). SDII is spatially variable with higher values in the western part of the continental Estonia. The absolute maximum of 17.5 mm per wet day was recorded in Räpina in 1997 during the summer season. The highest intensity of precipitation was calculated for 1978, when mean SDII of the stations reached to 6.3 mm/wet day. The maximum number of consecutive wet days (≥1 mm) (CWD) has the highest mean value not in summer but in autumn (especially in October), and the lowest in spring (Table 1). The annual maximum of CWDs at stations varies between 10 and 19 days while it is higher in the hinterland and lower at the sea coast. 12 The number of wet days is usually described by the number of heavy precipitation days R10mm (R≥10 mm) and R20mm (R≥20 mm). Their mean numbers were correspondingly 15 and 3 during the study period. Days with precipitation above 10 mm may occur in Estonia in every month. Very heavy precipitation days R20mm may also happen in every month, but more often in July and August. The spatial pattern of R10mm is in a good accord with the patterns of mean precipitation, as well as of other indices of precipitation extremes (Fig. 9). Fig. 9. Annual mean number of days with heavy precipitation (R10mm) The absolute maximum of daily precipitation (RX1day) during the study period (130.8 mm) was measured in Võru on 3 July 1988, and of 5-day precipitation (RX5day) – 183.5 mm – in Räpina on 15-19 July 1997. The corresponding mean values of the stations were 34.2 mm and 59.3 m (Table 1). The highest values of RX1day and RX5day were observed in summer and lowest ones in winter and spring. RX1day maximum in 1957-2009 varied between the stations from 53 mm to 131 mm. The highest values were measured in south-eastern Estonia. The RX5day varied between 85 and 184 mm and its spatial pattern is similar to the RX1day. The last year of the study period, 2009, was very specific for the 95th and 99th percentile daily precipitation (R95p, R99p). Record high values were documented at many stations. The mean R95p for all stations and all years was 6.24 days and the mean R99p – 1.23 days. The spatial pattern of these indices was in a good concordance with mean territorial distribution of precipitation in Estonia. Their maximum values were concentrated in the rainy zone of the western Estonia. 3.3. Extremely dry conditions Spatial distribution of annual minimum precipitation in Estonia (Fig. 10) is quite variable but similar to the mean annual pattern (Fig. 5a). The annual minimum is generally lower in eastern Estonia than in its western part except the coastal zone. The absolute minimum value of annual precipitation – 335 mm – was registered in Kunda, North Estonia, in 1964 (Table 2). The absolute minimum of 12-month moving precipitation total in Estonia was 278 mm (Fig. 4), again in Kunda, from September 2005 till August 2006. The year 1964 was the driest in the period of 1957–2009 with the minimum of mean annual precipitation of 453 mm over the stations. Minimum values of precipitation for the winter, spring, summer and autumn seasons were 36 mm (Võru, 1963/1964), 16 mm (Sõrve, 1964), 44 mm (Rohuküla, 1992) and 54 mm (Piigaste, 1988), consequently. The absolute minimum precipitation found from the moving 90-day totals of precipitation during the 13 study period was only 10 mm (Fig. 3), in Väike-Maarja from 28 December 1971 to 26 March 1972. Fig. 10. Minimum of annual precipitation in Estonia in 1957-2009 Table 2. Mean values of precipitation and mean minimum precipitation (the stations mean), absolute minima of the station data (mm), years of the corresponding minima and trends of the minimum values of the station data (mm per decade) in Estonia during 1957–2009. Statistically significant changes at p<0.05 level are typed in bold Year Winter Spring Summer Autumn Mean 649 128 108 215 196 Minimum of stations means 453 66 50 118 116 Year of the minimum 1964 1964 1974 1983 1958 Absolute minimum 335 36 16 44 54 Year of the minimum 1964 1964 1964 1962 1988 Trend 18.1 5.50 0.06 5.83 3.80 Duration of an extremely dry period in Estonia has been characterised by the number of extremely dry days (EDD). A day is estimated as extremely dry if it is the last day of a 20-day period without precipitation. These days have occurred in Estonia in all months (Fig. 11). The maximum number of EDD days is observed in August. A clear minimum in July is highly remarkable (Fig. 11). Hereby, it should be taken into account that EDD describes a dry day that follows a 19-day dry period. Therefore, the minimum of EDDs in July implies on less frequent dry periods during midsummer. This is an interesting feature, which has not yet been detected in other regions. The annual mean number of EDDs during the study period was 1.2. The largest dryness risk, i.e. the highest average number of EDDs was obtained in the north-western part of Estonian mainland with the mean value above 2 days (maximum 3.2 days in Rohuküla). The least number of dry days was calculated for Jõhvi and Tartu (0.38, Fig. 12). The highest annual maximum of EDDs was registered in Mauri in 2002, showing 30 extremely dry days from August and September. The same year was extremely dry over the whole of Estonia – the mean number of EDDs over all stations was 12.6 days (Table 3). 14 0.9% 90 relative number of days 0.8% precipitation 80 0.7% 70 0.6% 60 0.5% 50 0.4% 40 0.3% 30 0.2% 20 0.1% 10 0.0% precipitation, mm EDD 0 01 02 03 04 05 06 07 08 09 10 11 12 Fig. 11. Monthly mean relative number of extremely dry days (EDD divided by the total number of days in the months) and average monthly precipitation totals at Estonian meteorological stations in 1957–2009 a) b) Fig. 12. Annual mean and maximum number of extremely dry days (EDD) The spatio-temporal distribution of EDD is characterised by the fact that its maximum values in spring and summer were observed in the coastal region of West Estonia but in autumn in eastern Estonia. The maximum number of EDD in summer (21 days) occurred twice – in 1997 (Valga) and in 2002 (Rohuküla). In spring and in autumn, the drought risk has been lower – the maximum number of EDD in spring was 18 (Kuusiku in 1972 and Räpina in 2009) and in autumn 13 days, calculated for different stations, mostly in East Estonia (Lüganuse, Tudu, Tudulinna, Mauri) in 2000 and 2002. If the daily precipitation limits of a dry day are higher – up to 1 mm, the number of extremely dry days EDD1 increases significantly (Table 3). In this case the mean annual number of EDD1 was 3.9 with the maximum in the spring season – 1.9. The maximum EDD1 of 45 days was measured at Sõrve in spring 1964. 15 Table 3. Mean indices of extremely dry conditions at 51 stations in Estonia during 1957–2009, their maximum values and absolute maxima of the station data (mm), years of the corresponding maxima and trends (per decade) of the stations mean values. The trends are insignificant Year Winter Spring Summer Autumn Index EDD EDD1 XCDD CDD PDD DSMEA0 DSMEA1 EDD EDD1 XCDD CDD PDD DSMEA0 DSMEA1 EDD EDD1 XCDD CDD PDD DSMEA0 DSMEA1 EDD EDD1 XCDD CDD PDD DSMEA0 DSMEA1 EDD EDD1 XCDD CDD PDD DSMEA0 DSMEA1 Mean 1.20 3.92 16.6 22.0 0.34 2.99 4.16 0.02 0.30 7.92 13.6 0.25 2.40 3.84 0.42 1.86 13.4 19.0 0.45 3.70 5.60 0.48 1.18 11.9 15.2 0.40 3.44 4.51 0.28 0.73 10.1 14.3 0.28 2.76 3.73 Maximum of stations means 12.6 22.3 29.9 39.6 0.46 4.19 5.40 0.20 1.04 14.3 24.9 0.37 3.36 6.11 3.31 13.3 20.5 39.5 0.65 6.93 13.1 10.1 13.9 23.47 24.4 0.61 6.45 7.82 3.31 7.24 20.5 34.6 0.51 4.18 6.17 Year of the Absolute maximum maximum 2002 30 2002 45 2002 49 1972 71 1959 0.56 1959 5.68 1959 6.75 1983, 1984 6 1986 11 1984 25 1972 44 1996 0.57 1993 3.36 1970 10.8 2006 18 1972 45 2006 46 1972 71 1974 0.79 1974 11.0 1974 31.7 21 2002 2002 28 2002 36 1992 57 1997 0.76 2002 11.2 1969 13.3 1961 13 2002 18 1997 49 2002 52 1993 0.67 2005 7 1961 8.5 Year of the maximum 2002 1964 2002 1964 1959 1959 1996 1993 1993 1993 1987 1993 1993 1993 1972 1964 1964 1964 1974 1974 1974 2002 1994 2002 2008 1997 1959 1994 2002 1961,2000 2002 2002 1993 1993,1998 1998 Trend 0.09 0.19 0.17 0.22 0.00 0.03 -0.06 0.00 0.00 0.12 0.11 0.00 0.02 -0.07 0.00 0.00 -0.21 -0.19 0.00 0.01 -0.10 0.00 0.00 0.07 -0.19 0.00 0.01 0.01 0.00 0.00 0.28 -0.11 0.00 0.03 0.01 The numbers of consecutive dry days XCDD (R=0 mm) and CDD (R<1 mm) also describe a deficit of precipitation. Their mean values were 16.6 and 23.0 days, correspondingly. Average XCDD varied between 13.5 (Jõhvi) and 19.2 (Rohuküla). Among seasons, their highest values are typical for spring and the lowest for winter. Due to their similar to EDD definition, the maximum number of consecutive dry days XCDD was also the highest in 2002 at the Mauri station – 49 days (Table 3). The maximum number of consecutive dry days CDD – 71 – was recorded at the coastal station of Heltermaa on Hiiumaa Island in February-April 1964. Average CDD was 22 days with 16 the highest values in spring. It varies from 20.5 (Mauri) to 26.1 days (Heltermaa). 2002 was also the year of the highest average of CDD over the stations. Annual variation (mean of stations) of CDD varies from 14.6 in 1977 to 39.6 in 1972. The 51 stations-mean of the dry-day persistence PDD was 0.34 during 1957–2009. This means that a dry day will occur after a dry day in one third of the cases. The probability of a dry day was significantly higher in spring and lower in autumn (Table 3). Spatial differences are not large in Estonia but PDD was higher mostly in the dry coastal zone. The absolute maximum of 0.79 was observed at Kasari in spring 1974. Mean dry spell durations DSMEA1 and DSMEA0 were 4.2 and 3.0 days, correspondingly. They were shorter than the average in autumn and winter, and longer in spring and summer. The absolute maximum DSMEA1 was registered at Heltermaa in 1976. The spatial pattern of mean dry spell duration on Fig. 13 is similar to the patterns of other dryness indices in Estonia. The driest regions are located in the coastal zone (Vihterpalu, Kunda, Virtsu, Rohuküla, Sõrve, Kihnu). Fig. 13. Annual mean duration of the dry period (DSMEA0). 3.4. Changes in precipitation extremes Annual precipitation amount PREC has significantly increased at the majority of the Estonian stations during 1957–2009. The mean trend value averaged by the 51 stations was 25.8 mm per decade (Table 1). This increase is statistically significant also in the winter season (Fig. 14). In other seasons, significant trends were observed only at some stations. The most remarkable increase in precipitation was found in north-eastern Estonia. The trend was weaker at the coastal stations. 17 1000 900 summer winter annual Linear (annual ) Linear (summer ) Linear (winter ) precipitation, mm 800 700 600 500 400 300 200 100 2008 2005 2002 1999 1996 1993 1990 1987 1984 1981 1978 1975 1972 1969 1966 1963 1960 1957 0 Fig. 14. Time series of annual and seasonal precipitation (mean of the stations) in Estonia during 1957–2009 and their trend lines The relative number of extreme days, i.e. the sum of relative EWD and EDD has significantly increased (MK statistic 5.9) in Estonia during 1957–2009 (Fig.15). As a rule, extremely wet days prevailed in some years and extremely dry days in other years. Extremely dry days dominated in 1957-1977 while the number of extremely wet days was on the highest in 1978-1991. The wettest years were 1978 and 1987 when the mean relative number of wet days was by 4-6 times higher than the average of the study period. The number of dry days has substantially risen from the end of 20th century. The severest drought in 2002 with mean relative number of EDD (10 days) was observed in August and September. 12 EDD EWD 10 extreme days /EWD+EDD) relative number of days Linear (extreme days /EWD+EDD) ) 8 6 4 2 2008 2005 2002 1999 1996 1993 1990 1987 1984 1981 1978 1975 1972 1969 1966 1963 1960 1957 0 Fig. 15. Relative annual number of extreme dry, extreme wet and extreme days in Estonia during 1957-2009, and the trend line of the number of extreme days. 18 Relative values of EDD and EWD separately for each season show the most extreme dry or wet conditions in Estonia in 1957-2009 (Fig. 16). The highest number of extreme days was observed in spring and summer of 2002 when extra dry conditions predominated. The presence of extremely wet and extremely dry conditions together in 1997, 1999, 2006 and 2008 shows great spatial differences in the precipitation regime over Estonia. Generally, the indices of wet conditions (excl. CWD) had an increasing trend during 1957– 2009. It means that heavy rainfalls have become more frequent and more intense. The indices of extreme precipitation had statistically significant increasing trends in all cases in winter, but not at all in spring. In summer and autumn, some indices had trends, and some had no trends. In both cases (summer and autumn), trends were seen for SDII and R95p, and no trends for PREC and CWD (Table 1). At the same time there were no significant trends in the indices of extreme dry conditions (Table 3). EDD winter EDD autumn 35 EDD spring EWD summer EDD summer EWD autumn 30 25 20 15 10 5 2008 2005 2002 1999 1996 1993 1990 1987 1984 1981 1978 1975 1972 1969 1966 1963 1960 1957 0 Fig.16. Relative values of extremely dry and wet days in different seasons in 1957-2009. 4. Discussion and conclusions Using daily precipitation data measured at 51 stations in Estonia during 1957–2009, the numbers of extremely dry and extremely wet days were calculated together with other indices of precipitation extremes. Their spatial and seasonal distribution and long-term trends were analysed. The main results of these analyses are presented and discussed here. 4.1. Data quality Generally, the quality of precipitation data is not high. Data inhomogeneity has been discussed in many works, for example by Heino (1994). Precipitation inhomogeneities in the former USSR were thoroughly discussed by Groisman et al. (1991). During the time period involved in this article, the main change in the methodology of precipitation measurement occurred in 1966; since that time, a wetting correction was added to every measured amount of precipitation. It caused an upward shift in the time series of precipitation totals. We can assume that the increasing trend in annual and seasonal precipitation could partly be induced by the wetting correction. The role of the wetting correction in the extreme precipitation totals is negligible. Therefore, we consider these time series as homogeneous and not affected by wetting corrections. At the same 19 time, these corrections may significantly increase minimum precipitation and influence the indices of dryness. As these time series have no trends, we consider the effect of the wetting correction on the parameters of dry conditions also as not substantial. Relocations of some stations could have disturbed the homogeneity of data series. We assume that due to the relocation of the stations, an upward shift in precipitation was possible in Tallinn, Tartu and Pärnu, and a downward shift in Narva. But we have not noticed any influence on the time series of precipitation extremes. The possible inhomogeneities have no influence on the results of data analysis. 4.2. Mean precipitation The mean pattern of annual precipitation in Estonia for the period 1957–2009 coincides with the results of the previous studies, for example Jaagus et al. (2010). The western part of continental Estonia receives significantly more precipitation than the other regions. It could be explained with the coastal effect. It means that a large part of air moisture moving from the sea to the land area falls down in a moderate distance from the coast (20–80 km), forming a zone of higher precipitation parallel to the coastline. The influence of the Baltic Sea appears also in the annual curve of precipitation. Its clear maximum in continental Estonia is observed in July and August but on the western coast in October and November. This is caused by the thermal inertia of sea surface. 4.3. Method of moving totals The proposed method of moving totals of precipitation was successfully applied for the study of precipitation extremes. Weather conditions in the previous days are taken into account in defining extremely wet and extremely dry days. There might be different criteria for the extremes. We used a ten-day precipitation total of 100 mm for describing extremely wet days and a 20-day period without precipitation for describing extremely dry days. The extremely wet days were observed only in summer and autumn. August and July are the months most affected by the risk of wet spells in Estonia. This lies in a good concordance with the maximum of precipitation in these months. Surprisingly, the annual curve of the numbers of extremely dry days in Estonia has two clearly expressed maxima and a period with less frequent droughts between them around the midsummer day, i.e. the second half of June and the first half of July. The two-peaked distribution of dry periods has not been mentioned in literature. We explain it with two main factors that cause droughts in Estonia. The late spring and early summer droughts are probably related to the coastal regions. Comparatively cool sea surface prevents the formation of convective rainfall and induces long dry periods. The droughts in the second half of summer are usually related to the extensive areas of high pressure over the East European Plain that have influence, first of all, on the eastern parts of Estonia. 4.4. Comparing results Comparing the indices of extreme precipitation in Estonia with the same variables in other regions we can conclude that the intensity of rainfall is slightly lower in Estonia. The mean number of days with precipitation above 10 mm was about 15 in Estonia, but in Latvia (Avotniece et al. 2010) and Lithuania it has been higher, between 12 and 22 (Rimkus et al. 2011). Comparing the mean indices of precipitation extremes in Estonia and Europe (Klein Tank and Können 2003), we can see that the latter are remarkably higher. The corresponding values are the following: R10mm 15 in Estonia (21 in Europe), R20mm 3 (7), R1day 34 (44), R5day 59 (77). We suggest that higher 20 latitude, lower temperature and a shorter warm period are the main reasons for lower precipitation extremes in Estonia. 4.5. Trends Time series of the indices of extreme precipitation have statistically significant increasing trends in Estonia during 1957–2009. The frequency and intensity of heavy precipitation has increased; this is assumed to be a consequence of global climate warming. This change has been determined also in many other regions of the world (Karl and Knight 1998; Frich et al. 2002; Groisman et al. 2005) and in Europe (Klein Tank and Können 2003; Haylock and Goodess 2004; Moberg and Jones 2005; Zolina et al. 2009; 2010; Zolina 2011). An increase in total precipitation and extreme precipitation was detected in Fennoscandia in 1951–2002 and in the western part of the former USSR in 1936–1997 (Groisman et al. 2005). This increase in extremely wet conditions is seasonally different. All indices of extreme precipitation have increased annually (except CWD) and, first of all in the winter season. Some indices have significant trends also in summer and autumn but no trends at all in spring. We assume that the most remarkable change in winter is directly related to temperature change that has also been the highest in winter. The intensification of cyclone activity over northern Europe in winter during the last decades has caused the temperature increase, higher storminess, precipitation and precipitation extremes. All these changes are governed by the North Atlantic Oscillation (Scaife et al. 2008). No trends were detected for the indices of dry extremes. It means that the frequency of droughts has not changed during the study period, although two extremely dry years were observed in the end of the time series (2002, 2006). This result lies in a good concordance with analogous studies from the neighbouring regions (Haylock and Goodess 2004). Acknowledgements This study has been supported by the grant No. 7510 of the Estonian Science Foundation and by the target financed project SF0180049s09 of the Ministry of Education and Science of the Republic of Estonia. The authors thank their colleagues Raivo Aunap and Andres Luhamaa for the assistance in preparing cartographic images. References Achberger C, Chen D (2006) Trend of extreme precipitation in Sweden and Norway during 19642004. Research Report C72, Earth Science centre, Gothenburg University Alexandersson H (2002) Temperature and precipitation in Sweden 1860-2001. SMHI Meteorologi, 104 pp Alpert P, Ben-Gai T, Baharad A, Benjamini Y, Yekutieli D, Colacino M, Diodato L, Ramis C, Homar V, Romero R, Michaelides S, Manes A (2002) The paradoxical increase of Mediterranean extreme daily rainfall in spite of decrease in total values. Geophys Res Lett 29. doi:10.1029/2001GL013554 Avotniece Z, Rodinov V, Lizuma L, Briede A, Kļaviņš M (2010) Trends in the frequency of extreme climate events in Latvia. Baltica 23:135–148 BACC Author Team (2008) Assessment of climate change for the Baltic Sea basin. Springer-Verlag, Berlin, Heidelberg, 473 pp Balling RC, Goodrich GB (2010) Increasing drought in the American Southwest? A continental perspective using a spatial analytical evaluation of recent trends. Phys Geogr 31:293–306 21 Dai A, Trenberth KE, Qian T (2004) A global data set of Palmer Drought Severity Index for 1870– 2002: Relationship with soil moisture and effects of surface warming. J Hydrometeorol 5:1117– 1130 Førland EJ (2000) Trends in precipitation intensity in Norway and the Nordic region during the 20th century. ECAC2000: 3rd European Conference on Applied Climatology, 16-20 October 2000, Pisa, Italy. ISBN 88-900502-0-9 Frei C, Schär C (2001) Detection probability of trends in rare events: theory and application to heavy precipitation in the Alpine region. J Climate 14:1568–1584 Frich P, Alexander LV, Della-Marta P, Gleason B, Haylock M, Klein Tank AMG, Peterson T (2002) Observed coherent changes in climatic extremes during the second half of the twentieth century. Clim Res 19:193–212 Groisman PY, Knight RW, Easterling DR, Karl TR, Hegerl GC, Razuvaev VN (2005) Trends in intense precipitation in the climate record. J Climate 18:1326–1350 Groisman PY, Sherstyukov BG, Razuvaev VN, Knight RW, Enloe JG, Stroumentova NS, Whitfield PH, Førland E, Hannsen-Bauer I, Tuomenvirta H, Alexandersson H, Mescherskaya AV, Karl TR (2007) Potential forest fire danger over Northern Eurasia: Changes during the 20th century. Global Planet Change 56:371–386 Haylock MR, Goodess CM (2004) Interannual variability of European extreme winter rainfall and links with large-scale circulation. Int J Climatol 24:759–776 Heino R, Brázdil R, Førland EJ, Tuomenvirta H, Alexandersson H, Beninston M, Pfister C, Rebetez M, Rosenhagen G, Rösner S, Wibig J (1999) Progress in study of climatic extremes in Northern and Central Europe. Clim Change 42:151–181 Jaagus J (1992) Periodicity of precipitation in Estonia. Estonia. Man and Nature (Eds. T. Kaare et al). Estonian Geographical Society, Tallinn, 43-53 Jaagus J (1996) Climatic trends in Estonia during the period of instrumental observations and climate change scenarios. Estonia in the system of the global climate change. Institute of Ecology. Publication, 4:35–48 Jaagus J, (2006) Climatic changes in Estonia during the second half of the 20th century in relationship with changes in large-scale atmospheric circulation. Theor Appl Climatol 83:77–88 Jaagus J, Briede A, Rimkus E, Remm K (2010) Precipitation pattern in the Baltic countries under the influence of large-scale atmospheric circulation and local landscape factors. Int J Climatol 30:705–720 Jaagus J, Tarand A (1988) Territorial distribution of precipitation in Estonia. Yearbook of the Estonian Geographical Society 24:5-16 (in Estonian, summary in English) Karl RT, Knight RW (1998) Secular trends of precipitation amount, frequency, and intensity in the USA. Bull Amer Meteor Soc 79:231–241 Kažys J, Rimkus E, Bukantis A (2009) Heavy precipitation events in Lithuania in 1961–2008. Geografija 45(1):44–53 (in Lithuanian, summary in English) Keevallik S (2003) Changes in spring weather conditions and atmospheric circulation in Estonia (1955-95). Int J Climatol 23:263–270 Keevallik S, Post P, Tuulik J (1999) European circulation patterns and meteorological situation in Estonia. Theor Appl Climatol 63:117–127 Kiktev D, Sexton DMH, Alexander L, Folland C (2003) Comparison of modelled and observed trends in indices of daily climate extremes. J Climate 16:3560–3571 Kivi K (1998) Dangerous weather phenomena. Estonian Meteorological and Hydrological Institute, Tallinn (in Estonian) Klein Tank AMG, Können GP (2003) Trends in indices of daily temperature and precipitation extremes in Europe 1946-99. J Climate 16:3665–3680 Łupikasza E (2010) Spatial and temporal variability of extreme precipitation in Poland in the period 1951–2006. Int J Climatol 30:991–1007 22 Merilain M, Post P (2006) Heavy rainfall – is it only a feature of recent years’ summers in Estonia? Publ Geophys Univ Tartuensis 50:144–153 (in Estonian, summary in English) Moberg A, Jones PD (2005) Trends in indices for extremes in daily temperature and precipitation in central and western Europe, 1901-99. Int J Climatol. 25:1149–1171 Mätlik O, Post P (2008). Synoptic weather types that have caused heavy precipitation in Estonia in the period 1961-2005. Estonian Journal of Engineering 14:195–208 Osborn TJ, Hulme M, Jones PD, Basnett TA (2000) Observed trends in the daily intensity of United Kingdom precipitation. Int J Climatol 20:347–364 Päädam K, Post P (2011) Temporal variability of precipitation extremes in Estonia 1961–2008. Oceanologia 53:245–257 Remm K, Jaagus J, Briede A, Rimkus E, Kelviste T (2011) Interpolative mapping of mean precipitation in the Baltic countries using landscape characteristics. Est J Earth Sci 60:172–190 Rimkus E, Kažys J, Bukantis A, Krotovas A (2011) Temporal variation of extreme precipitation events in Lithuania. Oceanologia 53:259–277 Salmi T, Määttä A, Anttila P, Ruoho-Airola T, Amnell T (2002) Detecting trends of annual values of atmospheric pollutants by the Mann-Kendall test and Sen’s slope estimates – the Excel template application MAKESENS. Finnish Meteorol Inst Rep 31. Scaife AA, Folland CK, Alexander LV, Moberg A. Knight JR (2008) European climate extremes and the North Atlantic Oscillation. J Climate 21:72–83 Schmidli J, Frei C (2005) Trends of heavy precipitation and wet and dry spells in Switzerland during the 20th century. Int J Climatol 25:753–771 Sen Roy S, Balling RC (2004) Trends in extreme daily rainfall indices in India. Int J Climatol 24:457– 466 Stone DA, Weaver AJ, Zwiers FW (2000) Trends in Canadian precipitation intensity. Atmos Ocean 38:321–347 Tammets T (2007) Distribution of extreme wet and dry days in Estonia in last 50 years. Proc Estonian Acad Sci Eng 13:252–259 Tammets T (2010) Estimation of extreme wet and dry days through moving totals in precipitation time series and some possibilities for their consideration in agrometeorological studies. Agronomy Research 8:433–438 Valiukas D (2011) Dry periods in 1891-2010 in Vilnius. Geography 47(1):9–18 (in Lithuanian, summary in English) Wibig J (2009) Variability of daily precipitation totals in Poland (1951-2000). Geographia Polonica 82:21–32 Zhang X, Hogg WD, Mekis E (2001) Spatial and temporal characteristics of heavy precipitation events in Canada. J Climate 14:1923–1936 Zolina O, Simmer C, Kapala A, Bachner S, Gulev S, Maechel H (2008) Seasonally dependent changes of precipitation extremes over Germany since 1950 from a very dense observational network. J Geophys Res 113(D06110). doi:10.1029/2007JD008393 Zolina O, Simmer C, Belyaev K, Kapala A, Gulev S (2009) Improving estimates of heavy and extreme precipitation using daily records from European rain gauges. J Hydrometeorol, 10:701– 716 Zolina O, Simmer C, Gulev S, Kollet S (2010) Changing structure of European precipitation: longer wet periods leading to more abundant rainfalls. Geophys Res Lett 37(L06704). doi:10.1029/2010GL042468 Zolina OG (2011) Changes in the duration of synoptic rainy periods in Europe from 1950 to 2008 and their relation to extreme precipitation. Doklady Earth Sciences 436:690–695 Zou XK, Zhai PM, Zhang Q (2005) Variations in droughts over China: 1951–2003. Geophys Res Lett 32(L04707). doi:10.1029/2004GL021853 23