The Mesoscale Structure of Severe Precipitation Systems in Switzerland

The structures of severe mesoscale precipitation systems (MPS) in Switzerland have been classified by analyzing radar images obtained over a 5-yr period. Severe MPSs were defined to be those producing most of the damage on days on which at least 5 (out of 2400) communities reported water and/or at least 20 reported hail damage. Of 94 MPSs selected, 82 had radar reflectivity of 47 dBZ or greater and were referred to as mesoscale convective systems (MCS). The 12 remaining MPSs consisted of less intense, long-lasting, and widespread frontal or orographic rainfall. Subclasses of MCSs were defined according to their internal arrangements of cell complexes (CC). A CC was defined as an echo contour of 40 dBZ surrounding echo maxima of at least 47 dBZ. Four general categories of organization were found: isolated CC, a group of CCs, and a broken or continuous line of CCs. All categories can be purely convective at the mature stage, or the CCs may be juxtaposed with a stratiform precipitation area, usually behind moving convection. The stratiform region often developed as a decaying convective area. These categories were examined in relation to sounding, surface mesonet, synoptic weather type, and severe weather information. In 26 cases, the MCS had “leading line-trailing stratiform” structure. These MCSs were graded according to a classification scheme previously used to characterize spring rainstorms in Oklahoma. Only moderately and weakly classifiable storm systems occurred in Switzerland. The mountain barriers apparently interfered with the airflow such that MCSs were prevented from having enough time and space to develop to a higher degree of organization as is possible over the relatively flat terrain of Oklahoma. In addition, the instability and the wind shear in the Swiss storm environment was found to be weaker.