Features of thermodynamic structure of cut-off low systems and their impacts on the weather in Ukraine

Abstract

The paper presents a summary of the proceedings of the middle and upper troposphere cyclones that form when a low is filled with air masses at the surface or southern part of a deep upper-air trough is cut off from the polar cyclone. Cut-off low systems are poorly understood synoptic vortexes, but they always cause a sudden and significant weather deteriorations, so their research is relevant. Feature of these cyclones is the asymmetry of weather conditions: cloudy weather with precipitation observed under the eastern parts of these cyclones; cloudy weather without precipitation registered in the western parts. The purpose of the study is to identify the physical processes that lead to the formation of precipitation zones just under the eastern part of the cut-off lows. To achieve this goal, the thermodynamic structure of cut-off cyclone, associated with heavy rain with thunderstorms and squalls over the western and south-western regions of Ukraine during the period from September 11-13, 2013, is studied. On the basis of objective analysis of wind and temperature fields in the main isobaric surfaces, the spatial distribution of characteristics of the structure of the cut-off cyclones such as the relative vorticity, 2D divergence, potential temperature, potential vorticity and thermal frontal parameter are obtained. The joint analysis of the fields of calculated characteristics, synoptic situation and precipitation maps showed that in the field of the relative vorticity, where the vorticity values are several times as high as vorticity’s order and reaches the values of the planetary vorticity in the upper troposphere, the central parts of the cut-off cyclone and ridges are well detected, which indicates a high activity of atmospheric processes. The positive values of the thermal frontal parameter well represent the location of the most intense areas of fronts at the ground and in the middle atmosphere. The spatial structure of the field of divergence, which is an indicator of the convergence and divergence of atmospheric flows, shows that favourable conditions for the generation of ascending vertical movements and, hence, for the formation of clouds and precipitation, are formed in the eastern part of the cut-off cyclone; in the western part, the conditions are favourable for downward movements that destroy the clouds, if any. The vertical distribution of the potential vorticity and the potential temperature in general confirms this result.