The Alps are a major east-west oriented mountain range, 1000km in length, 200 km across and between 2 and 4 km high. Orographic precipitation events occur when mid-latitude systems bring strong northerly or southerly currents of air against the Alpine ridge. These situations may be characterized by convection, blocking and splitting or stable uplift. Often, currents of dry air descend on the lee slopes, caused either by pseudo-adiabatic drying or upstream blocking and mid-level descent.

In this study, we test mesoscale model predictions of orographic precipitation and airmass drying against balloon data, aircraft/dropsonde data, and hourly surface data from the MAP intensive observing period in the fall of 1999. Four events are considered, three southerly flow cases (Sept 20, Oct 20, Oct 24) and one northerly case (Nov. 6). Several methods are evaluated for determining the precipitation efficiency. Parcel trajectories are traced. The role of gaps and ridge ends are estimated. The new "threshold" theory of precipitation is tested. The origin of the dry air is investigated. The influence of model resolution is evaluated.