Extended Abstract (640K)8.1
Orographic Precipitation and Airmass Transformation: An Alpine Example
Ronald B. Smith, Yale University, New Haven, CT; and Q. Jiang, M. G. Fearon, P. Tabary, M. Dorninger, J. D. Doyle, and R. Benoit
A case of orographic precipitation in the Alps on 20 September 1999 was studied using models and raingauge and radar data. The objectives of the study are to describe the orographic transformation of an airmass and test the accuracy of mesoscale models. For the case considered, the ratio of precipitation to incoming water vapor flux was about 35%. Precipitation efficiency values are ambiguous due to repeated small scale ascent and descent. Cloud water residence times are different for two models (400 compared to 1000 seconds) due to different microphysical formulations. Delay times of this order work well in a simple upslope-delay model. Radar data confirms the model prediction that the rainfall field is tightly controlled by local terrain on scales as small as 10km. On this scale, orographic precipitation occurs by the amplification of drifting features over the hills. Low level trajectories crossing the Alps integrate these small-scale processes to develop net warming and drying. Starting at mid-level, some parcels descend on the lee slopes giving a foehn behavior. Aloft, parcels are moistened and cooled by the sublimation of snow.
Session 8, Orographic Precipitation III
Tuesday, 18 June 2002, 1:30 PM-3:00 PM
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