Dimitrova, Reneta, National Institute of Geophysics, Geodesy and Geography, Bulgarian Academy of Scienc-es, Acad. G. Bonchev, bl. 3, Sofia 1113, Bulgaria; Faculty of Physics, Department of Meteorology and Geophysics, Sofia University “St. K. Ohridski”, 5 James Bourchier Blvd., 1164 Sofia, Bulgaria, Bulgaria