9.3
Prefrontal and frontal precipitation events during MAP IOP 5
Joze Rakovec, University of Ljubljana, Ljubljana, Slovenia; and G. Gregoric, T. Vrhovec, and N. Pristov
During MAP SOP the NE Italy (Friuli - VG) and the West Slovenia (Posocje) were chosen as secondary target area for MAP P1 - Heavy precipitation as in this part of the Southeast Alps is the area of the climatological precipitation maximum for the whole Alps and the Central Europe. In the area that covers 80 x 80 kilometers there are several operational networks of meteorological stations, operational radar and two radio soundings (14015 Ljubljana and 16044 Udine). Additionally a special network of 25 recording pluviographs was deployed in Posocje.
The most interesting precipitation event during MAP SOP was IOP5 (Oct. 3rd and 4th 1999). During this period the maximum MAP daily precipitation amount was recorded for this area. The MAP IOP 5 was divided in two parts. During prefrontal stage the precipitation was mostly limited to the mountain ranges of the Julian and the Karnic Alps. With SW advection of moist and warm air conditional instability was released when flow was forced to ascend over the southern slopes of mountains creating a continuous series of thunderstorms that persisted for about 15 hours (during the night from Oct. 3rd to 4th) and attributing to the most of the precipitation of IOP5. Several mesoscale thunderstorm bands were observed before fontal passage and a strong low-level jet was diagnosed. Frontal passage was noted by a squall line moving from the Veneto towards the East with strong radar echoes but with relatively little precipitation and an upper levels jet. At the same time a strong flow of cold air from the East arrived in the area, as the majority of cold air moved ageostrophically around the east edge of the Alps at elevations bellow 2000 m. After the passage of squall line there was a rapid clearing of clouds in target area in the Alps due to strong NE foehn wind while precipitation was still intense further to the East. Strong bora was measured near the northern Adriatic Sea coast.
The synoptic situation and mesoscale features of the MAP IOP5 event are described with the use of radar images, time-height cross-sections using radio sounding data and surface precipitation and equivalent potential temperature fields. For the frontal squall line also some aircraft radar date were used. The data are used for verification of forecasted parameters from four different numerical weather prediction models setups: operational ALADIN/SI model, ALADIN/SI with assimilation of additional local data, MM5 model and MM5 with the assimilation of local radar data. The results show that models initialized just with the operationally available data explain less variance of the precipitation and surface equivalent potential temperature; especially ageostrophic flow around the Alps is delayed in operational model's setups. As squall line was marked by strong radar reflectivity its precipitation was assumed to be high also in model initialization. As it was a narrow line the surface precipitation accumulation during its passage was small comparing it with the prefrontal events precipitation. A conceptual model of flow regime during IOP5 event for the region is proposed.
Session 9, Orographic Precipitation IV
Wednesday, 19 June 2002, 8:00 AM-9:15 AM
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