Terrain and Multiple-Scale Interactions as Factors in Generating Extreme Precipitation Events

Abstract

The Mediterranean region is often affected by flooding and landslides due to heavy precipitation events. These events have been the subject of specific interest because they represent complex interaction of synoptic-scale upper-level steering flows and local topographic barriers. In the present work, data from a dense network of surface precipitation gauges over northern Italy and a global atmospheric analysis at a coarser scale are combined to develop a multiscale diagnostic model of the phenomenon. Composite maps are formed based on departures from climatology and standard deviation of sea level pressure, 500-hPa geopotential, wind, and water vapor flux. A diagnostic model is built based on the evidence that shows the spawning of secondary mesoscale features in the steering synoptic flow. The mesoscale features draw moisture and energy from local sources and cause extreme precipitation events over adjoining areas. The primary trough system steering the flow often originates in the North Sea and extends over middle Europe. When this system flows across the Alps, a secondary cyclone develops in the western Mediterranean, more frequently over the Gulf of Genoa on the lee side of the Alps barrier. The mesoscale cyclone is evident in the weather charts, and its signature is identifiable in the dense surface gauge network data, but it is not evident in the atmospheric analyses maps owing to their coarse resolution. Further analysis of the ensemble of such cases is made by manual inspection of daily all-Europe weather charts. Two precursor conditions for this mesoscale cyclogenesis are identified. It is shown that longitudinal blocking frequency over a larger region strongly differs from climatology (especially for the second one of these precursors). A low pressure center tracking algorithm is used to follow the evolution of some events. Two cases are presented as illustrative of the patterns identified by the ensemble composites.