A Theoretical Study of the Flow of Air and Fallout of Solid Precipitation Over Mountainous Terrain: Part II. Microphysics

Peter V. Hobbs Dept. of Atmospheric Sciences, University of Washington, Seattle 98195

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Richard C. Easter Dept. of Atmospheric Sciences, University of Washington, Seattle 98195

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Alistair B. Fraser Dept. of Atmospheric Sciences, University of Washington, Seattle 98195

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Abstract

Expressions are derived for the horizontal and vertical components of the wind, the temperature, and the mass of water vapor condensed when air flows over a long mountainous ridge. The growth of solid precipitation particles in the orographic clouds by deposition from the vapor phase, riming and aggregation are considered. The trajectories of these precipitation particles are then computed from their fallspeeds and the airflow model.

The model is used to investigate the effects of the microstructure of clouds on the growth and fallout of solid precipitation over the Cascade Mountains. It is shown that, under suitable conditions, increases in the concentration of ice particles in the clouds from about 1 to 100 liter−1 can cause the solid precipitation to be carried farther downwind and over the Cascade crest, so that snowfall is deposited on the eastern rather than the western slopes of the mountains.

Abstract

Expressions are derived for the horizontal and vertical components of the wind, the temperature, and the mass of water vapor condensed when air flows over a long mountainous ridge. The growth of solid precipitation particles in the orographic clouds by deposition from the vapor phase, riming and aggregation are considered. The trajectories of these precipitation particles are then computed from their fallspeeds and the airflow model.

The model is used to investigate the effects of the microstructure of clouds on the growth and fallout of solid precipitation over the Cascade Mountains. It is shown that, under suitable conditions, increases in the concentration of ice particles in the clouds from about 1 to 100 liter−1 can cause the solid precipitation to be carried farther downwind and over the Cascade crest, so that snowfall is deposited on the eastern rather than the western slopes of the mountains.

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