Influence of atmospheric circulation on the spatial distribution of precipitation in the area of Sofia city

The study aims to reveal spatial distribution of precipitation in the area of Sofia city during the decade 2013 - 2022 and the influence of atmospheric circulation. Statistical methods and cartographic approach are the main tools in this research. The spatial distribution of precipitation is characterized by low amounts (560 mm) in the northern and northeastern parts of Sofia depression and high amounts (760 mm) in the southern part. The main factor for this spatial distribution of precipitation is atmospheric circulation. The relief has a significant modifying effect and affects precipitation through several mechanisms. The most important is the location of mountain slope relative to the main direction of transport of air masses. Leeward slopes receive less precipitation and windward slopes receive more. The second mechanism of influence is anthropogenic relief (high buildings), which is a positive relief form compared to the surrounding plane having respective windward and leeward slopes. This study revealed a third mechanism of relief influence on spatial distribution of precipitation. The large difference in the height of the mountains located south of Sofia creates a significant difference in the air temperature in Sofia depression during a transport of air masses from south and southwest. This is due to the stronger foehn effect of the higher mountain (Vitosha) compared to the foehn effect of the lower mountains (Lyulin, Lozenska Planina), which creates a tongue of higher air temperature northeast of Vitosha, which reaches the southern and southwestern slopes of Stara Planina. The higher temperatures in this tongue create stronger upward air movements, which in turn increase the amount of precipitation. Secondary, but still important factors that affect the spatial distribution of precipitation in Sofia region are the urban heat island and the increased content of aerosols in the air in and over the city.