Some Comparisons Between IMPROVE-2 and IPEX Kinematic and Precipitation Structures and Bulk Microphysical Verification

Several field studies (MAP, PACJET, IPEX, IMPROVE) have occurred recently to better understand orographic precipitation processes and improve bulk microphysical parameterizations. Each of these experiments occurred over different ambient environments and terrain geometries, so it is important to compare results to better understand the phase space of orographic precipitation and mesoscale model capabilities. This paper will compare some of the results of IMPROVE for a heavy precipitation event over the Cascades on 13-14 December 2001 and a heavy precipitation event during IPEX on 12 February 2000 over the Wasatch.

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.