Extended Abstract (108K)P2.6
Low level flow patterns associated with two heavy rainfall events during MAP
James A. Thurman, North Carolina State University, Raleigh, NC; and Y. L. Lin
The Mesoscale Alpine Programme (MAP) was an extensive study of Alpine rainfall. Two such cases studied during the program were Intensive Observation Period (IOP) 2B and IOP-8. Both cases were similar in synoptic conditions but differed dramatically in rainfall totals. IOP-2B had over 200 mm of rainfall while IOP-8 was far less. The PSU/NCAR MM5 model was used to simulate both events. The simulations for both events were initialized with NCEP 2.5º reanalysis data. Two grids were employed for the simulations. A coarse grid of 45 km resolution (85x91) and a nested grid of 15 km resolution (121x121) were used. The IOP-2B simulation was initialized at 0000 UTC 19 September and was 48 hours in length. The IOP-8 simulation was initialized at 12000 UTC 19 October and was 60 hours in length. IOP-2B was associated with an upper level trough and surface low west of Ireland. As the upper level trough and associated jet propagated eastward, a low level jet formed at 850 hPa in the Mediterranean. This jet propagated from the Strait of Gibralter to the western Italian coast where it was impinging directly against the mountains. Rainfall had been moving in from the west and by 1200 UTC 20 September had become more intense in the Lago Maggiore region of Italy. Back trajectories from this area indicate surface air parcels originated along the southern coast of western Italy. Back trajectories at 850 hPa indicated parcels originated in the southern Mediterranean and North Africa. 700 hPa trajectories also originated in North Africa. So it would appear that North African air may have had an influence on the rainfall in Italy. IOP-8 was also associated with an upper level trough and surface low although farther south, in the Bay of Biscay, than IOP-2B. Like IOP-2B, a jet formed at 850 hPa near the Strait of Gibralter and propagated eastward, impinging on the mountains of Italy. Thirty-six hour back trajectories from Lago Maggiore indicated surface parcels originated in Eastern Europe, distinctly different than IOP-2B. Back trajectories at 850 hPa showed parcels originated from the Mediterranean and eastern Europe. From preliminary trajectory analysis, air parcels arriving over Lago Maggiore originated in very different places prior to heavy rainfall development. IOP-2B parcels had some origination in North Africa, implying hot, unstable air, while IOP-8 parcels originated in cooler eastern Europe. Further trajectory analysis, including parcel diagnostics along the trajectory and terrain sensitivity experiments, especially North Africa will aid in further understanding in the role of the flow patterns at low levels on Alpine rainfall development.
Poster Session 2, Orographic Precipitation/Operational and Numerical Weather Prediction (with Coffee Break)
Wednesday, 19 June 2002, 9:15 AM-11:00 AM
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