Orographic precipitation and air mass transformation: An Alpine example

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