Precipitation Rate and Extinction in Falling Snow

Mary Ann Seagraves U.S. Army Atmospheric Sciences Laboratory, White Sands Missile Range, NM 88002

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Abstract

Visible and infrared atmospheric transmittances measured through falling snow have shown a wavelength dependence in which extinction is greater for longer wavelengths. The diffraction component of the energy scattered by the snow crystals causes the spectral dependence because more diffracted energy is detected at shorter wavelengths. A spectral technique for the determination of airborne snow mass concentration and the precipitation rate of equivalent liquid water is proposed and tested. A measure of the particle size distribution obtained from the ratio of visible (0.55 μm) and infrared (10.6 μm) extinction coefficients is used to calculate composite particle fall velocity and effective extinction efficiency which in turn are used in computing mass concentration and precipitation rate. Calculations using this technique agree well with field measurements.

Abstract

Visible and infrared atmospheric transmittances measured through falling snow have shown a wavelength dependence in which extinction is greater for longer wavelengths. The diffraction component of the energy scattered by the snow crystals causes the spectral dependence because more diffracted energy is detected at shorter wavelengths. A spectral technique for the determination of airborne snow mass concentration and the precipitation rate of equivalent liquid water is proposed and tested. A measure of the particle size distribution obtained from the ratio of visible (0.55 μm) and infrared (10.6 μm) extinction coefficients is used to calculate composite particle fall velocity and effective extinction efficiency which in turn are used in computing mass concentration and precipitation rate. Calculations using this technique agree well with field measurements.

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