Extended Abstract (1.4M)6.2
Orographic precipitation processes over the Wasatch Mountains during IPEX IOP3
W. James Steenburgh, NOAA Cooperative Institute for Regional Prediction and University of Utah, Salt Lake City, UT; and J. A. W. Cox, J. C. Shafer, D. E. Kingsmill, and B. A. Colle
The kinematic and precipitation structure of a winter storm over the Wasatch Mountains of northern Utah is described using observations collected during IOP3 of the intermountain Precipitation Experiment (IPEX). The event featured the passage of a mid-level (700-500 hPa)trough followed 3 h later by a surface trough. Prior to and during passage of the mid-level trough, southwesterly large-scale flow with a Froude number of ~0.75 impinged on the Wasatch Mountains. Low-level confluence was observed between this southwesterly flow and partially blocked, terrain-parallel southerly flow within 20-40 km of the Wasatch Mountains. This confluence zone, which collapse to near the mountain slope during passage of the mid-level trough, was accompanied by enhanced precipitation in the lowlands upstream of the Wasatch Mountains. Within the partially blocked flow region, dual-Doppler analysis revealed the presence of a shallow terrain-prallel jet near or just above valley level, which was surmounted by southwesterly cross-barrier flow at mid- and upper-mountain levels. The latter produced ~2 m/s ascent within 1 km of the windward slopes of the Wasatch Mountains, where precipitation was roughly double that observed over the lowlands 10-15 km upstream. Flow deflection and splitting was also observed near isolated terrain features, such as Ben Lomond and Ogden Peaks. Strong subsidence (2-4 m/s) was found to the lee of the Wasatch, and, based on radar imagery, appeared to modulate leeside hydrometeor spillover aloft. Processes contributing to the evolution of the near-barrier flow field, including topographic blocking, surface friction contrasts, and diabatic effects, are discussed.
Session 6, Orographic Precipitation I
Wednesday, 25 June 2003, 8:30 AM-10:15 AM
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