The distribution of orographic precipitation is dependent on a number of dynamical, thermodynamical, and microphysical processes. In order to examine the importance of each process many different modeling experiments are needed. This paper examines this issue by conducting several sets of two- and three-dimensional idealized simulations at less than 5-km resolution using the Penn State/NCAR mesoscale model (MM5). The idealized approach shown to generate realistic precipitation profiles when compared against IPEX field data over the Wasatch Mountains of Utah.

The first set of 2-D runs were initialized using a bell-shaped mountain with a 25 or 50 km half-width (Lm), a nearly saturated atmosphere (95% relative humidity), and uniform moist static stability (Nm) and flow (U) in the vertical. The freezing-level was specified at either 750 or 650 mb, and ice processes were allowed. One objective is to determine how the cross-barrier precipitation varies as U is incrementally increased. For stabily stratified moist flow (Nm=.01), as the winds gradually increase the orographic precipitation first builds upstream in response to low-level blocking. As the flow becomes unblocked the average precipitation over the barrier and in the lee increases nearly exponentally. At high windspeeds (> 25 m/s), the precipitation builds upstream again with a broader mountain circulation. For less stable flows there is a more linear increase in precipitation over the barrier and in the lee with increasing windspeed. A higher freezing level results in a more narrow peak of precipitation over the windward slope and less spillover into the lee.

Both idealized 2-D and 3-D runs are used to investigate the importance of vertical wind shear above the barrier. Reverse shear, which favors low-level wave-breaking and upstream blocking, enhances upstream precipitation, while forward shear favors more precipitation in the lee. This suggests that other factors besides the flow and stability near crest-level are important in the orographic precipitation distribution. This hypothesis will be tested further by incrementally varying the winds, stability, and moisture at different layers to test their impact on the precipitation distribution. This research has important consequences for ensemble precipitation forecasting in complex terrain.