Spatial structure of rainfall in mid-latitude cold front systems

Experimental evidence and physical interpretation of rainfall spatial distribution associated with cold front systems in an inland region (2282 km2) of the Mediterranean area are provided. By using a representation on the meso-local scale, postfrontal and prefrontal storm ensembles consisting of thirty-two and seven components, respectively, were considered. For postfrontal storms the distribution of storm total depth was found to be mainly determined by small mesoscale precipitation areas of random nature with orographic effects that in the average were represented by a ratio of 1.5 between rainfall depth over the hills and in a lowland area just upwind of them. However, despite the basic role of random effects it was possible to divide the study region into a restricted number of homogeneous zones whose structure appears to be linked with orography. For prefrontal storms the orographic enhancement was more considerable with the above ratio expressed by a value of 2.5. The enhancement was limited by the absence of contributions to surface rainfall due to a seeder-feeder process. The occurrence of an effect of inhibition of forced uplift, which accompanied both the storm types and was caused by fairly modest orography, is also discussed. Such a mechanism frequently produced a weakening or dissipation of pre-existing precipitation and may take on an important role in problems of rainfall analysis over real basins.