18th Conference on Weather and Forecasting, 14th Conference on Numerical Weather Prediction, and Ninth Conference on Mesoscale Processes

Wednesday, 1 August 2001
Numerical simulations of heavy rainfall during the Mesoscale Alpine Program (MAP)
James A. Thurman, North Carolina State University, Raleigh, NC; and Y. -. L. Lin and J. J. Charney
Poster PDF (408.4 kB)
To assess the characteristics of heavy precipitation over the Lago Maggiore region of Northern Italy, we selected two IOPs from the MAP Field Study of Fall 1999. The IOP-2B (19 September) and IOP-8 (19-21 October) events were selected for intensive study because they represent two precipitation events that exhibit similar low-level flow patterns and precipitation distributions, yet differ in the amount of observed precipitation.

The precipitation in IOP-2B developed in association with a cold frontal passage across the southern Alps as a trough moved from western Europe into northern Italy. Heavy rainfall developed on the western side of Lago Maggiore as moist inflow from the south and southwest over the Mediterranean Sea impinged upon the steeply sloping terrain of the southern Alps. A numerical simulation using the MM5 model was developed to study the evolution of the precipitation. A triple-nested simulation (45km, 15km, and 5km resolution grid) captured the evolution of a surface low pressure system off the west coast of Ireland and a deep trough over the eastern Atlantic Ocean. A broad 300hPa jet was over Spain, with diffluence over the Alps. Throughout the simulation, precipitation was collocated with the low-level southerly surface flow ahead of the frontal system. Analysis of the 850 hPa theta e indicated a tongue of high theta e air of North-African origin entering the Lago Maggiore region. A low-level jet oriented perpendicular to the Alps transported high theta e air into the region, producing considerable rainfall. As the front propagated eastward, the winds shifted, moist inflow was cut off, and rainfall subsequently dissipated.

Precipitation in IOP-8, in contrast to the convective environment of IOP-2B, developed in a more stable environment ahead of a front. A second MM5 demonstrated the precipitation development as 850hPa southeasterly flow overran a surface cool layer. Eventually, a trough developed to the west of the Lago Maggiore which, along with a 300 hPa jet entrance region, contributed to the formation of a broad area of precipitation in the region.

Both IOP-2B and IOP-8 exhibited similar low-level moist inflow from the Mediterranean. However, differences arise from the fact that in IOP-2B, inflow was from the southwest, which allowed warmer, dry air from North Africa to mix with Mediterranean moisture. This contributed to more unstable conditions and heavier precipitation when the southerly flow was lifted by orography. In contrast, southeasterly inflow from IOP-8 led to cooler, more stable conditions. Greater instability, more moisture, warmer temperatures, and stronger southerly inflow contributed to stronger uplift in the mountains and more precipitation in IOP-2B than in IOP-8. The simulations also indicate the presence of upper-level PV streamers that contribute to the development of strong upslope winds and heavy precipitation. These simulation results are compared against the observations from the IOPs to assess the accuracy of the PV streamer predictions. The 5km simulation resolves the finer scale orographic influences on the local distribution of precipitation in the mountains. The extent to which mountain valley circulations and environmental characteristics (moist Froude number) affect the evolution of the precipitation is explored.

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