Orographic precipitation and air mass transformation: An Alpine example

摘要: A case of orographic precipitation in the Alps on 20 September 1999 was studied using several models, along with rain-gauge and radar data. The objective study is to describe transformation an air mass, including multi-scale aspects. Several new some conventional diagnostic quantities are estimated, drying ratio, efficiency, buoyancy work, condensed-water residence time, parcel changes heat, moisture altitude, dominant space- time-scales. For considered, ratio about 35%. Precipitation efficiency values ambiguous due repeated ascent descent over small-scale terrain. sign work changed during event, indicating a shift from stratiform weak convective clouds. Cloud-water times different for two mesoscale models (400 compared 1000 s) cloud–physical formulations. agree that spatial-scale lifting 10 km; smaller than scale main Alpine massif. Trajectory analysis crossing casts doubt classic model fohn. Few parcels exhibit pattern moist followed by dry descent. Parcels gain latent heat descend only briefly, before rising into middle troposphere. lee slope, originate troposphere little, or even lose, transit. As seek their proper level downstream, surprising scrambling mass occurs. Radar data confirm prediction rainfall field tightly controlled local terrain scales as small km, rather full 100 km cross-Alpine scale. curious pulsing seen, either drifting anomalies convection. Copyright © 2003 Royal Meteorological Society.