The IMPROVE event was associated with tipped-forward lower tropospheric front with strong westerly flow (> 80 kts) just above crest level. There was little flow blocking observed during the IMPROVE event. In contrast, IPEX had weaker cross barrier flow (to 25 kts), blocked low-level flow, and a southerly barrier jet to 10 m/s below mid-mountain, which resulted in an upward convergence boundary and sloping ascent 20-30 km upwind of the barrier. Simulations using the Penn State/NCAR Mesoscale Model down to 1.33-km grid spacing suggest that both the reduced friction over the Great Salt Lake enhances the windward convergence boundary and the diabatic cooling effects from falling precipitation enhance the windward convergence boundary. There was large amounts of precipitation spillover for both cases, likely because of the strong flow in IMPROVE and the narrow Wasatch terrain in IPEX.
The MM5 at 1.33-km grid spacing overpredicted precipitation for both the IMPROVE and IPEX over some of the steeper terrain areas. For IMPROVE several BMP sensitivity studies were completed to test parameters such as the slope intercepts for the snow and graupel number concentration, cloud water autoconversion, CCN concentrations, and snow fallspeeds. For each simulation the cloud ice and water amounts were verified using in situ aircraft data and a microphysical budget was completed. For IMPROVE problems of excessive snow and too little cloud water over the crest in this event are enhanced by using a fix sloped intercept for snow and slower fallspeeds. These IMPROVE microphysical results will be compared with IPEX during the presentation.