The IMPROVE field study along the Washington coast and the central Oregon Cascades gathered a unique observational dataset to verify and improve the bulk microphysical parameterizations (BMPs) in numerical models. In order to fully utilize this dataset, more understanding of the sensitivities within BMPs are needed in order to quantify their importance on the microphysical pathways associated with heavy precipitation.

In this paper several BMP sensitivity studies are completed using the Penn State/NCAR Mesoscale Model (MM5) down to 1.33 km grid spacing for a heavy precipitation event over the Cascades on 13-14 December 2001. The simulations were setup to test several parameters within the Reisner2 BMP, 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. It will be shown that 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.

Most previous BMP sensitivity studies in the literature simply compare the mixing ratios aloft and surface precipitation. The second goal of this paper is to complete a full microphysical budget within Reisner2 to determine the dominate microphysical pathways that lead to the surface precipitation. The microphysical processes are calculated for an average box over the windward slope, spatially in a cross section across the barrier, and for falling hydrometeors in the model. It will be shown that the cloud water accretion and riming pathways contribute most to the windward surface precipitation. Most of the snow growth aloft via deposition is advected over to the immediate lee. The hydrometeor trajectories illustrate that rapid snow and graupel growth occurs above some of the narrow windward peaks. The microphysical budget is repeated for a few of the sensitivity experiments to illustrate their impacts on the processes and resulting precipitation.