Precipitation pattern in the Baltic countries under the influence of large-scale atmospheric circulation and local landscape factors

The main objectives of the study are to analyse the mean precipitation pattern in the Baltic countries (Estonia, Latvia, Lithuania) and the influence of local landscape factors on it, to derive a regionalization, i.e. select regions having coherent fluctuations in precipitation, and to analyse relationships between precipitation and characteristics of the large-scale atmospheric circulation. Monthly precipitation data from 123 stations in the Baltic countries from 1966 to 2005 are analysed in relation to 52 landscape variables and 13 circulation variables. The landscape variables characterize landscape around the meteorological stations: land cover, land surface elevation, distance from the sea coast, the proportion of water bodies and of forested area. Circulation is described using several circulation indices and frequencies of the main circulation types according to the Vangengeim-Girs and the Grosswetterlagen classifications. Maps depicting the mean annual and seasonal precipitation are created and analysed in the aspect of spatial and temporal autocorrelation. The Baltic Sea proved to be the main factor determining precipitation pattern in the study region. The presence of different precipitation zones parallel to the coastline is typical for the windward lowland region, whereas the belt of maximum precipitation is located at 10-60 km from the coast. Surface elevation is the second important factor causing higher precipitation on uplands. Rotated principal component analysis enabled to compose a regionalization of precipitation pattern in the Baltic countries. Four main precipitation regions having coherent fluctuations (south-western, south-eastern, north-eastern and north-western) are defined. Correlations between the variables of atmospheric circulation and precipitation are analysed. The intensity of westerlies is significantly positively correlated with the amount of winter precipitation, especially in the belt of maximum precipitation located at a moderate distance from the sea. Precipitation in the Baltic countries is less related to the meridional circulation types.