The Improvement of Microphysical PaRametrizations through Observational Verification Experiment (IMPROVE II) was conducted over the Oregon Cascade Mountains from 26 November - 21 December 2001. The objective of the experiment was to collect a large enough sample of meteorological data over a quasi two-dimensional mountain range to evaluate and improve the performance of mesoscale numerical models in predicting orographic precipitation. The simultaneous mapping of the flow and precipitation by radars and aircraft allow a detailed study of the orographic precipitation mechanism. The NCAR polarimetric S-Pol radar and a vertical pointing S-band radar obtained precipitation and radial velocity information with high temporal and spatial resolution. The NOAA P3 aircraft obtained flight track data, 2D imagery of microphysical particles, and Doppler radar data.

IMPROVE II documented 16 cases of storms that produced large precipitation amounts over the Cascade Mountains. Over the course of the experiment, a station located on the windward slopes of the Cascade Mountains, measured three times as much precipitation than a station located in the Willamette Valley. One particularly strong event occurred on 13-14 December 2001 in association with the passage of a baroclinic trough.The S-Pol radar, deployed at the foothills of the Cascade range, ~70 km west of the crest, measured large cross-barrier radial velocities. The vertically pointing S-band radar, located on the windward slopes of the Cascades, showed a continuous deep layer of precipitation with a well-defined bright band. The cold frontal precipitation exhibited small scale alternating up- and downdrafts just above the melting layer. The radial velocity pattern observed by the S-band during the Dec 13-14 case suggests the presence of turbulence. The vertical wind shear associated with the radial velocity jet observed by the S-Pol reached ~30 m/s over 2 km or 0.015 s-1 at times, which could be a generating mechanism for the turbulence. The upstream flow was stable, and the Richardson number was estimated at 0.2-0.3. The vertical motion in the S-band-observed updrafts (~ 2 m/s or more) was strong enough to promote the formation of large hydrometeors. Nearly collocated with the intense updrafts, the NOAA P3 aircraft detected at an altitude of 2 km vertical velocities ~3 m/s of alternating signs. In this region, the liquid water content reached ~0.6 g/m3. The crystal imagery collected by the 2D probes on board of the P3 aircraft showed very large aggregates and some rimed particles. The S-Pol showed at an altitude of 2 km an horizontal layer of particles with polarimetric observables that are consistent with the large aggregates and rimed particles observed by the 2D probes.

These observations suggest that turbulence played an important role in enhancing the orographic precipitation over the mountain slopes. This hypothesis will be explored in this study.