18th Conference on Weather and Forecasting, 14th Conference on Numerical Weather Prediction, and Ninth Conference on Mesoscale Processes

Thursday, 2 August 2001
Sensitivity of orographic precipitation to changing ambient conditions: An idealized modeling perspective
Brian A. Colle, SUNY, Stony Brook, NY
In order to improve orographic precipitation forecasts, we need better understanding of the sensitivity of orographic precipitation to incremental changes in upstream ambient flow (U), moist static stability (Nm), terrain height (Hm), and mountain half width (Lm). This paper examines this issue by conducting idealized simulations at 4-km horizontal resolution using a 2-D version of the Penn State/NCAR mesoscale model (MM5V2). All model runs were initialized using a bell-shaped mountain with a 25 or 50 km half-width, a nearly saturated atmosphere (95% relative humidity), and uniform Nm and U in the vertical. The freezing-level was initially specified at 750 mb, and ice and graupel processes were allowed.

One objective was to determine how the total surface precipitation (6-12 h) averaged across the barrier varies as U is incrementally increased. For stabily stratified moist flow (Nm=.01) and Hm > 1000 m, the total precipitation increases exponentially as U increases from 5-15 m/s; however, there is little precipitation increase between 15 and 20 m/s. This bifurcation at 15-20 m/s is related to mountain wave breaking at mid-levels over the immediate lee, which results in weak subsidence aloft over the upper windward slopes and more limited precipitation near the crest. There is a dramatic precipitation increase from 20-25 m/s since the wave-breaking region is advected downstream and there is less flow blocking. In contrast, for weaker stabilities (N=0-.005/s) and Hm>1000 m, there is a more linear increase in orographic precipitation amounts for windspeeds between 5-30 m/s.

The spatial distribution of precipitation was also investigated for various upstream winds and stabilities. As the upstream flow is slowly increased for Hm > 1000 m and Nm > 0, the maximum precipitation first occurs near the crest, then shifts upwind due to terrain blocking, and then back towards the crest as the flow becomes unblocked. For Hm > 1000 m and N=.01/s, there is an exponential increase in lee-side precipitation as the flow becomes unblocked (Froude number, U/HmN, is > 1); however, for N=.005/s, this unblocked flow/spillover transition is delayed until the Fr > 2. Because of variations in static stability across the barrier from diabatic effects and lee side drying, the upstream Fr has limitations predicting spillover. Lee side spillover also depends on the mountain wave structure as determined by the local Nm over the barrier, the mountain half width (Lm), and how much suspended ice is generated over the barrier.

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