A Squall Line in Southern Germany: Kinematics and Precipitation Formation as Deduced by Advanced Polarimetric and Doppler Radar Measurements

Abstract

A multiscale analysis of a squall line system is reported in this paper. It is shown that the squall line was initiated as part of a synoptic-scale frontal zone. The main emphasis then is on the polarimetric and Doppler radar measurements which give insight into the meso- and microscale structure of the kinematics and the precipitation microphysics especially within the new cells growing ahead of the squall line, and within the main precipitation system. The principal polarimetric measurements considered are the differential reflectivity (ZDR) and a related new derived parameter termed the difference reflectivity or ZDP which is useful in detection of rain-ice mixed phase precipitation. A limited amount of time series data have been analyzed to derive the specific differential phase (KDP) and the backscatter differential phase (δ) between horizontal and vertical polarizations. A brief overview of the microphysical interpretation of these parameters is provided. The newly grown clouds are identified as positive ZDR columns, i.e., regions of low reflectivity and unusually large ZDR. Within the high reflectivity part of the squall line, intense precipitation in the form of raindrops mixed with small, melting hail may be inferred. The radar observations were shown to be in good agreement with a hail melting model. A conceptual model of the squall line is provided based on the Doppler and polarimetric data. It demonstrates the internal circulation structure as well as the contribution of melting ice particles to the cold pool dynamics.