Chakina, N. P. (Hydrometeorological Centre of Russia, Moscow, Russia)
Berkovich, L. V. (same affiliation)
A case of cyclogenesis over South Europe, Mediterranian and Black Seas on March 27 - April 1, 1995, is studied on the basis of objective analysis and numerical model output data. Two small cyclonic eddies originating over Italy at the south periphery of a deep cyclone centered over North Europe caused hazardous weather (heavy snowfalls and rains) in Balkan, Asia Minor, South Ukraine. At low levels, strong frontogenesis in the intense tropospheric frontal zone generated a deep thermally direct circulation with ascending motions in the warm and moist air of Atlantic origin. The potential vorticity field analysis reveals deep tropopause folds in the cyclonically curved tropospheric jet stream, in the circulation cell descending branch in the cold air, while in the warm air, the the tropopause height maximum (above 250 hPa) is formed. Small size of the eddies and significant impacts of orography and convective activity make it difficult to describe the process with operational forecasting models. Diagnostic computations, numerical simulation and weather data show an important influence of sub-grid and mesoscale processes upon the precipitation intensity, while the precipitation zone localization and movement are determined mainly by the tropospheric frontal zone dynamics, especially by the transverse circulation cell evolution. For numerical simulation, a version of hemispheric model with varying boundary layer, orography, radiation and convective activity parameterization schemes and space resolution is used. For diagnostic computations, isentropic potential vorticity analysis and an original objective method of frontal analysis are applied.