The Mesoscale Alpine Programme (MAP) examined orographic precipitation processes on the Mediterranean side of the Alps. The Alps are a long barrier some 3-5 km in height. MAP was conducted in the autumn, when heavy rains and sometimes floods occur on the windward slopes of the Alps. The southerly flow of maritime air at low levels, ahead of baroclinic troughs, impinges on the mountain barrier, and, if the air is slightly unstable, enormous rainfall can occur. One of the key instruments deployed in MAP was the NCAR S-Pol dual-polarization radar. The S-Pol measurements in MAP allowed particle types to be estimated in the radar echoes of major rain events. The S-Pol data have been analyzed in combination with vertically pointing S-band radar measurements, showing fine-scale vertical and horizontal structure, and airborne and ground-based Doppler radar observations of the flow field. The S-Pol data provided measurements that indicated that coalescence and riming at relatively low altitudes plays a major role in producing large quantities of orographic precipitation, especially over the first major peak of terrain encountered by the air flowing toward the mountains. The vertically pointing S-band radar indicated that cellular convection embedded in the orographic airflow contributed to growth by coalescence and riming.

In the present study, we compare the MAP analyses of precipitation mechanism over Alps to analyses of precipitation processes over the Oregon Cascade Mountains. This comparison is made possible by data collected in the project IMPROVE II (the second phase of the project Improvement of Microphysical PaRametrization through Observational Verification Experiment). This field experiment was conducted over the Oregon Cascades from 26 November-22 December 2001. The comparison between the two experiments MAP and IMPROVE II is feasible since the crucial instruments deployed during MAP were deployed again and in a similar way in IMPROVE II. The NCAR S-Pol collected precipitation and airflow data, as well as hydrometeor type information at a location near the base of the Oregon Cascades in IMPROVE II. This radar site was similar to the location of the S-Pol near the base of the Alps in the Lago Maggiore region on the Mediterranean side of the Alps. The radar observations were made in conjunction with airborne Doppler radar measurements and airborne in situ particle sampling. Particle sampling was also carried out on the ground.

Westerly flow ahead of baroclinic troughs impinges on the Cascade Mountain barrier, similar to the southerly flow impinging on the Alps. However, the westerly flow from the Pacific is colder and more stable than the Mediterranean airstream. The ridge-line of the Cascades rarely rises to only about 2 km, contrasting with the Alpine crest around 3-5 km. The differences in temperature, stability and mountain height between the Alps and the Cascades produce differences in the precipitation growth and fallout mechanisms over the Cascades. An important question is whether coalescence and riming play the same crucial role in orographic precipitation enhancement in the Cascades that they do in the Alps. By pursuing the answer to this question we seek to determine whether the processes accounting for orographic precipitation enhancement in MAP are also operative in the Cascades.