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Parameterization of Droplet Terminal Velocity and Extinction Coefficient in Fog Models

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  • 1 Air Force Geophysics Laboratory, Hanscom AFB, MA 01731
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Abstract

Fog droplet spectra data were obtained in several advection fogs at the AFGL Weather Test Facility at Otis Air National Guard Base, Massachusetts, using two PMS forward-scatter spectrometer probes (FSSP-100). Approximately 90 h of droplet data were recorded simultaneously at 5 m and 30 m above the ground throughout the life cycle of 11 fogs during 1980 and 1981.

This large data set was used to develop new parameterizations of the extinction coefficient and the mean droplet terminal velocity in terms of liquid water content and droplet concentration for fogs with liquid water contents greater than 0.018 g m−3. The advection fog data show an excellent correlation of 95% between the extinction coefficient and liquid water content. The regression line for these two quantities is more nearly linear than found by other investigators. The correlation between the mean droplet terminal velocity and various microphysical parameters is not as high as that for the extinction coefficient and liquid water content. The best correlation (75%) is found between mean droplet terminal velocity and a power function of the ratio of liquid water content and concentration of droplets greater than 2.5 μm in diameter. However, since the concentration of such droplets is highly variable, and not easily measured, it is suggested that a constant mean droplet terminal velocity of 1.9 cm s−1 be used in numerical fog prediction models.

Abstract

Fog droplet spectra data were obtained in several advection fogs at the AFGL Weather Test Facility at Otis Air National Guard Base, Massachusetts, using two PMS forward-scatter spectrometer probes (FSSP-100). Approximately 90 h of droplet data were recorded simultaneously at 5 m and 30 m above the ground throughout the life cycle of 11 fogs during 1980 and 1981.

This large data set was used to develop new parameterizations of the extinction coefficient and the mean droplet terminal velocity in terms of liquid water content and droplet concentration for fogs with liquid water contents greater than 0.018 g m−3. The advection fog data show an excellent correlation of 95% between the extinction coefficient and liquid water content. The regression line for these two quantities is more nearly linear than found by other investigators. The correlation between the mean droplet terminal velocity and various microphysical parameters is not as high as that for the extinction coefficient and liquid water content. The best correlation (75%) is found between mean droplet terminal velocity and a power function of the ratio of liquid water content and concentration of droplets greater than 2.5 μm in diameter. However, since the concentration of such droplets is highly variable, and not easily measured, it is suggested that a constant mean droplet terminal velocity of 1.9 cm s−1 be used in numerical fog prediction models.

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