Atmospheric patterns for heavy precipitation in Bulgaria

Abstract

This paper presents a classification of the atmospheric circulations producing extreme precipitation events in Bulgaria. Heavy precipitation data set from the National Institute of Meteorology and Hydrology, Bulgaria, atmospheric fields of geopotential height at 1000 hPa, at 500 hPa and temperature at 850 hPa of the ECMWF operational analysis are used to determine the atmospheric patterns (AP). Other atmospheric fields such as geopotential height at 850 hPa, at 700 hPa and relative humidity at 700 hPa are also depicted to analyze the AP. Two statistical methods are used to obtain the AP. Principal Component Analysis (PCA) was applied to reduce the number of variables. Then, Cluster Analysis (CA) was performed and four main AP were obtained. For two AP, heavy precipitation is associated with a low-level cyclone. They can occur in all seasons. For the cold season (October to March), the trajectories of the cyclones are represented. Another pattern, which occurs mainly in the warm season (April to September), depicts an upperlevel cyclonic disturbance associated with heavy precipitation. The last AP represents a weak cyclonic circulation. Finally, a more detailed nine-cluster classification has been also obtained by adding some regional and seasonal features of the heavy precipitation events.