The Downwind Spread of an Initially Vertical Column of Particles in a Sheared Environment

Ronald E. Stewart Atmospheric Environment Service, Downsview, Ontario, Canada M3H 5T4

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John D. Marwitz Department of Atmospheric Science, University of Wyoming, Laramie 82071

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Abstract

The effects of particle fallspeeds on the downwind spread of initially vertical columns or curtains are examined in environments with wind shear. Sets of equations describing the column width as a function of time and distance below column top are derived by assuming, first, that the particles fall at a constant rate and, second, that particle fallspeed changes with time. These predictions are compared with measurements of a seeding curtain within a non-turbulent stratus cloud with high wind shear (0.017 s−1). The comparison implies that differential fallspeed effects in a non-turbulent sheared environment can account for much of the spread of the curtains.

Abstract

The effects of particle fallspeeds on the downwind spread of initially vertical columns or curtains are examined in environments with wind shear. Sets of equations describing the column width as a function of time and distance below column top are derived by assuming, first, that the particles fall at a constant rate and, second, that particle fallspeed changes with time. These predictions are compared with measurements of a seeding curtain within a non-turbulent stratus cloud with high wind shear (0.017 s−1). The comparison implies that differential fallspeed effects in a non-turbulent sheared environment can account for much of the spread of the curtains.

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