Analysis of extreme precipitation in different time intervals using moving precipitation totals Tiina Tammets 1, Jaak Jaagus 2 1 Estonian Meteorological and Hydrological Institute 2 Department of Geography, University of Tartu

Outline Introduction, precipitation regime and extremes in Estonia Overview of the characteristics used for description of precipitation extremes Objectives of the study Using of moving precipitation totals as characteristics of precipitation regime Extreme precipitation in Estonia in dependence of a number of days and months in observed period Extreme precipitation in Estonia in case of very wet and dry conditions during Trends in precipitation extremes in Estonia during Conclusions

Annual curve of monthly mean, maximum and minimum precipitation in , mean of the meteorological stations in Estonia

Precipitation in July and August , mean of the meteorological stations in Estonia

JUNE JULY AUGUST PRECIPITATION 2009 IN ESTONIA

Characteristics used for description of precipitation extremes quantiles of precipitation amounts; maximum number consecutive wet days (R>= 1 mm; 10 mm); maximum number of consecutive dry days with the threshold of 0.1 mm; 1 mm precipitation; number of heavy precipitation days in a month or year (R>=10 mm); number of very heavy precipitation days (R>=20 mm); number of dry and wet days in a month or in a year with chosen threshold; mean wet-day and dry-day persistencies; hydrothermical coefficient.

hydrothermical coefficient (HTK) : HTK =  Precipitation / 0,1*  Temperature

to show how much has been rained till this day not to show how much it rains in a day but For many purposes it is essential A day will be extreme day, if the amount of precipitation till this day has been too small or too large.

Objectives To define characteristics describing continuous dry and wet spells with various duration, which doesn’t divide time into months or 10-day periods To elaborate a method characterising climatology of extreme precipitation totals for any time periods (number of following days, months or years using moving totals) To detect the most severe wet and dry spells in Estonia during last years and to analyse trends in days with precipitation extremes

Mathematically the sequence of moving totals (averages){s j (n),1  j  N-n+1} is derived from a sequence {a i, 1  i  N} obtained by taking the totals (averages) of the subsequent n terms: s j (n) = a j (by moving averages s j (n) = a j ),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 drought and wet days by calculating s j (n) with time period n for each day i in the time series and choosing the days with values of s j (n), that are smaller or larger than the given threshold t. CALCULATING MOVING TOTALS (AVERAGES) To find the number of extreme days we have to calculate the moving total or average of precipitation time series

Counting of moving total n p n N days

Presentation of the extreme precipitation in any number of days, months or years

Extreme totals of precipitation for any number of days during has been found in Tallinn, Vilsandi, Väike-Maarja, Võru Tartu and Pärnu stations Precipitation stations in Estonia

Dependence of maximum and minimum precipitation on the number of successive days at six stations in Estonia in

Dependence of maximum and minimum precipitation on the number of successive months at six stations in Estonia in

In agrometeorological studies of Estonia, the criterion of extremely wet conditions - mean daily precipitation 10 mm or more during successive 10 days is used. If the moving average for a 10-day period s j (10) >= 10 mm, then the last day of the period is regarded as a wet day. Extremely dry conditions for field plants mean that there is no precipitation during successive 20 days; then s j (20) = 0 and the last day of the period has been counted as a dry day. Extremely wet and dry days for Estonia

criteria: extremely wet conditions: when the mean daily amount of precipitation is 10 mm and more during 10 consecutive days extremely dry conditions: no precipitation during 20 consecutive days

Monthly relative number of wet days and days with precipitation ≥10 mm during , mean of the meteorological stations in Estonia

Monthly relative number of dry days and days without precipitation during , mean of the meteorological stations in Estonia

Maximum number of dry days in Estonia from May to August during

Number of wet and dry days (mean of 56 stations) and relative number of extreme (wet+dry) days in

The method of moving precipitation totals allows to 1.present maximum and minimum precipitation in different periods (from 1 day to 4-5 years) on graph, which gives complete information about precipitation extremes of a station. It allows also to compare extreme precipitation amounts in different stations for every time period. 2.connect precipitation extremes with the dynamics other characteristics of hydrological regime (for example soil moisture content, ground water level, river runoff etc.) and find the best predictants of precipitation regime for extreme situations of environment 3.extract the time intervals of extreme precipitation events to relate them to atmospheric circulation Conclusions

Using of the method of moving precipitation totals for the analysis of precipitation extremes in Estonia demonstrated that maximum and minimum precipitation in case of any number of successive days and months on the coast of open sea (Vilsandi) is significantly lower than in the continental Estonia occurrence of wet days is the highest in July and August while the number of dry days is maximal only in August and much lower in July maximum number of days without precipitation has been observed in May total number of extreme (wet and dry) days in Estonia has increased significantly during