The Microstructure of California Coastal Fog and Stratus

Abstract

Detailed vertical profiles of the microphysical properties of West Coast advection fog were made over theSan Francisco peninsula during the summers of 1974 and 1975. The sampling platform was the 250 m highMount Sutro tower which, during much of the summer, protrudes through the coastal marine layer intothe subsidence inversion. The microphysical structure was found to vary systematically as a function of airparcel trajectory and air mass history. Maritime trajectories had an average droplet concentration of 89 cm-3, contrasting sharply with continental trajectories averaging 265 cm-3. The effect of nuclei depletiondue to washout processes as the air parcels travel through stratus decks was found to reduce the numberof nuclei available for condensation by as much as 30%. Despite large variations in their initial structure,each sample displayed a remarkable consistency in its microphysical development. Over the peninsula, themean droplet diameter and liquid water content increased with height in every case. Without exception,broader drop size distributions were observed near the inversion interface; the distribution also broadenedthroughout the entire layer during the onshore portion of the fog history. In all cases drizzle drops formed inthe uppermost portions of the stratus layer which precipitated downward with time.