Prof. Dr. Jaak Jaagus, University of Tartu
Dr. Piia Post, University of Tartu
Michael Kunz, Institute for Meteorology and Climate Research, Karlsruhe Institute of Technology, Germany
Thunderstorms are hazardous and potentially damaging weather events which are among the major causes of weather-related damages and economic losses in mid-latitudes. The thesis investigates thunderstorm and lightning climate and its long-term changes in the Baltic countries and northern Europe. The first complex overviews of thunderstorm climate in the Baltic countries as well as of lightning climate in northern Europe are presented. The annual average ground flash density varies from 0.01 flashes km−2 y−1 in northern and western Norway to 1.08 flashes km−2 y−1 in southwestern Sweden and in the Baltic countries. The average annual number of thunderstorm days varies from 2 days in northern Norway to 29.5 days in southern Lithuania. Thunderstorms are most abundant during summer afternoons when solar heating is the most intense. Thunderstorms are clearly more frequent over land which is a warmer surface than the sea. Many local thunderstorm maxima in the Baltic countries are associated with uplands which force the air to rise and initiate convection. Thunderstorms are most frequent in case of warm southerly or southeasterly airflow and least frequent in case of cool northerly airflow. Thunderstorm frequency in the Baltic countries has decreased during 1950-2004 and the main descent has occurred between 1960 and 1990. This trend was found to be associated with long-term changes in the warm season atmospheric circulation in northern Europe. During the last 20 years, a controversial trend has appeared and thunderstorm frequency has increased again. Results indicate that thunderstorm climate in the study area is similar to that in the surrounding countries. Results allow the estimation of lightning risks in northern Europe.