Supersaturation and Droplet Spectral Evolution in Fog

H. Gerber Naval Research Laboratory, Washington, D.C.

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Abstract

Droplet sizes, larger than expected, and transient water vapor supersaturations were measured in radiation fog. Nongradient turbulent mixing of saturated air parcels at different temperatures and the release of excess vapor by molecular diffusion at the interface between the mixing parcels are suggested as the mechanisms causing the large supersaturations. Approximate agreement is found between calculated rates of change of supersaturation during nongradient mixing and the supersaturation measurements. A stochastic mixing model, based on the supersaturation and other measurements in the fogs, is used to estimate if nongradient mixing and transient supersaturations cause the appearance of large droplets. The model predicts a broadening of the droplet spectra to include no larger than midsized droplets. This study concludes that a form of nonlocal turbulence closure may be required in models to accurately describe microphysics in fogs and clouds when nongradient mixing is important. This mixing causes droplet broadening and activation of cloud condensation nuclei within fogs and clouds; the effect is both proportional to the temperature difference of mixing saturated air parcels and inversely proportional to the droplet integral radius.

Abstract

Droplet sizes, larger than expected, and transient water vapor supersaturations were measured in radiation fog. Nongradient turbulent mixing of saturated air parcels at different temperatures and the release of excess vapor by molecular diffusion at the interface between the mixing parcels are suggested as the mechanisms causing the large supersaturations. Approximate agreement is found between calculated rates of change of supersaturation during nongradient mixing and the supersaturation measurements. A stochastic mixing model, based on the supersaturation and other measurements in the fogs, is used to estimate if nongradient mixing and transient supersaturations cause the appearance of large droplets. The model predicts a broadening of the droplet spectra to include no larger than midsized droplets. This study concludes that a form of nonlocal turbulence closure may be required in models to accurately describe microphysics in fogs and clouds when nongradient mixing is important. This mixing causes droplet broadening and activation of cloud condensation nuclei within fogs and clouds; the effect is both proportional to the temperature difference of mixing saturated air parcels and inversely proportional to the droplet integral radius.

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