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profiles of optical Extinction Coefficients Calculated from Droplet Spectra Observed in Marine Stratus Cloud Layers

V. Ray NoonkesterOcean and Atmospheric Sciences Division, Naval Ocean Systems Center, San Diego, CA 92152

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Abstract

Airborne measurements of droplet spectra n(r) where r is droplet radius were made in stratus cloud layers over the ocean 130 km southwest of San Diego. Optical extinction coefficients σe were calculated at selected wavelengths (λ= 0.53, 3.75 and 10.59 μm) from these n(r) and were used to construct average vertical profiles of σe between 200 m above cloud base to 250 m below cloud base. An approximation to σe a power function of the liquid water content w was found to be superior compared with approximations involving linear functions of w or the cross-section area. The horizontal variability of σe in the clouds was examined. The average horizontal scale size of σe was estimated to be 3 km and the horizontal measuring distance required to obtain values of σe with a relative accuracy of 10% was found to be in excess of 30 km.

Abstract

Airborne measurements of droplet spectra n(r) where r is droplet radius were made in stratus cloud layers over the ocean 130 km southwest of San Diego. Optical extinction coefficients σe were calculated at selected wavelengths (λ= 0.53, 3.75 and 10.59 μm) from these n(r) and were used to construct average vertical profiles of σe between 200 m above cloud base to 250 m below cloud base. An approximation to σe a power function of the liquid water content w was found to be superior compared with approximations involving linear functions of w or the cross-section area. The horizontal variability of σe in the clouds was examined. The average horizontal scale size of σe was estimated to be 3 km and the horizontal measuring distance required to obtain values of σe with a relative accuracy of 10% was found to be in excess of 30 km.

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