Mesoscale diagnostics of prefrontal and frontal precipitation in the Southeast Alps during MAP IOP 5

Summary

In terms of heavy precipitation, the MAP IOP 5 was a two-phase event. During the first phase – on 3 October 1999 – there was strong precipitation in the Lago Maggiore MAP target area, while the prefrontal precipitation was mainly limited to the mountain ranges of the MAP mission area in the Julian and the Karnic Alps involving a series of thunderstorms developing continuously for about 15 hours and contributing most to precipitation levels. During the second phase – on 4 October – the main precipitation was limited to the Julian and the Karnic Alps where a frontal passage was noted by a squall line moving from Veneto region towards the east, accompanied by a strong SW upper-level jet. At the same time, a strong low-level cold flow invaded the region to the north of Adriatic Sea from the east as a significant amount of cold air moving ageostrophically around the eastern edge of the Alps was arriving in the area. To study MAP IOP 5 in detail, we describe the development for mesoscale features of the event’s radar images, time-height cross-sections and estimates of Convective Available Poteintial Energy (CAPE) based on radio-sounding data, and how surface-measured precipitation offers some smaller scale information. Surface potential temperature and winds are also studied. Very large precipitation accumulation gradients are diagnosed (150 mm per day/25 km in S–N direction) and time distributions of hourly precipitation shows completely diverse regimes in the Friuli plain and in the Alps with peak intensities in the Julian Alps. The mesometeorological mechanisms for high precipitation rate in the SE Alps are diagnosed and some characteristics of the squall line are discussed.