The Summertime Stratus over the Offshore Waters of California

Abstract

Satellite pictures have been used to investigate the distribution of stratus clouds over the Pacific Ocean near California during one summer. A north-south oriented line of minimum cloudiness, which was particularly well-defined in July, was found off the northern and central California coast. The characteristic is believed to be caused by strong divergence of the surface winds. The clouds east of this minimum cloudiness line are generally “smooth-topped” stratus that form in air which is cooled by the upwelled water adjacent to the coast; in contrast, those to the west of the line are mainly stratocumulus, formed in air streaming southward over warmer water. A strong diurnal variation in low cloudiness was noted: over 18% of the clouds present at 0745 PST dissipated (in the mean) by 1345 PST. The maximum cloud cover occurred near sunrise and the minimum in the late afternoon, suggesting that the net radiative flux by clouds is primarily responsible for the diurnal variation. Divergence and advection decrease the diurnal range along the central California coast but increase it along the southern California coast. Upper-level synoptic-scale flow patterns (i.e., troughs and ridges) appear to be poorly correlated with the low-cloud patterns over the ocean, except in certain extreme cases, such as when offshore flow or an intense cyclone is present.