Impact of the windward Cascade ridges and Oregon coastal range on the precipitation distribution during 4-5 December 2001 of IMPROVE-2

Numerous ground-based and aircraft observations were collected over the central Oregon Cascades during the second Improvement of Microphysical Parameterization through Observational Verification Experiment (IMPROVE-2). The most analyzed IMPROVE-2 IOPs to date occurred on 28-29 November 2001 and 13-14 December 2001, both of which featured strong (30-40 m s-1) cross barrier flow and a well-defined landfalling baroclinic wave. This presentation compares some of the results from these two IOPs with an orographic precipitation event on 4-5 December 2001, in which the cross barrier flow and precipitation rates were about half as large. It will be shown that the weaker flow on 4-5 December resulted in less gravity wave activity over the Cascades and less lee side spillover of precipitation over the Cascade crest. NOAA P-3 observations suggest that there was also less riming over the windward ridges at 2-4 km ASL on 4-5 December as compared to13-14 December. For the 13-14 December IOP, it has been documented that the mountain waves over the windward ridges helped to enhance the total windward precipitation by 10-20% relative to a smooth slope simulation. An outstanding question is whether these same ridges can impact the net precipitation on 4-5 December 2001.

The 4-5 December event was simulated down to 1.33-km grid spacing using the Weather Research and Forecasting (WRF V2.1) model using the NCEP GFS for initial and boundary conditions, Mellor-Yamada-Janjic PBL, Kain-Fritsch convective parameterization on the 36- and 12-km grids, and the Thompson microphysical scheme, which includes graupel processes. The 1.33-km WRF realistically simulated the three dimensional thermodynamic and kinematic fields over the barrier, including ~1 m s-1 vertical velocities produced by the narrow Cascade ridges. The impact of these ridges on the net precipitation was quantified by replacing the Cascades in the WRF with a smooth slope (no ridges) of similar crest height as the control run. The ridges increased the net precipitation over the windward slope by 5-10% relative to the smooth topography. The ridge impact is therefore noticeably (5-10%) less than the 13-14 December IOP, which was associated with stronger mountain waves. A simulation without the Oregon coastal range during 4-5 December is shown to have a much larger (20-30%) impact (reduction) of the precipitation over the windward Cascades, since some of the available moisture was depleted over the coastal range.