High resolution simulations and microphysical validation of an orographic precipitation event over the Wasatch Mountains during IPEX IOP3

This paper presents the kinematic flow and precipitation evolution of a winter storm over and upstream of the Wasatch Mountains (IPEX IOP3) using a multiply nested version of the Pennsylvania State University-National Center for Atmospheric Research (NCAR) Mesoscale Model (MM5). Validation using in-situ aircraft data, radiosondes, ground-based radar, and surface observations showed that the MM5, which featured four domains with 36-, 12-, 4-, and 1.33- km grid spacing, realistically simulated the observed partial blocking of the 8-12 m ambient southwesterly flow and development of a convergence zone and enhanced lowland precipitation region upwind of the initial Wasatch slope. The MM5 also properly simulated the advance of this convergence zone toward the base of the Wasatch during the passage of a mid-level trough, despite not fully capturing the westerly wind shift accompanying the trough.

Accurate simulation of the observed precipitation over the central Wasatch Mountains (within 25% of observed at all stations) required a horizontal grid spacing of 1.33 km. Despite close agreement with the observed surface precipitation, the Reisner2 bulk microphysical scheme produced too much supercooled cloud water and too little snow aloft. A model microphysical budget revealed that the Reisner2 generated over half of the surface precipitation through riming and accretion, rather than snow deposition and aggregation as implied by the observations. Using an intercept for the snow size distribution that allows for greater snow concentrations at warmer temperatures improved the snow predictions aloft and reduced the cloud water overprediction. Some microphysical comparisons with the results from the 2001 IMPROVE-2 experiment over the Oregon Cascades are also made.

Sensitivity studies illustrate that the reduced surface drag of the Great Salt Lake (GSL) enhanced the convergence zone and associated lowland precipitation enhancement upstream of the Wasatch Mountains. The presence of mountain ranges south of the Great Salt Lake appears to have weakened the along-barrier flow and windward convergence, resulting in a slight decrease in windward precipitation enhancement. Diabatic cooling from falling precipitation was also important for maintaining the blocked flow.