Diurnal Variation of Marine Stratocumulus over San Nicolas Island during July 1987

Abstract

Surface measurements made at San Nicolas Island during the intensive field observation marine stratocumulus phase of the First International Satellite Cloud Climatology Progam Regional Experiment, July 1987, are analyzed to retrieve the average diurnal variation of marine stratocumulus and related surface variables. Cloud thickness and integrated liquid-water content show a clear decrease during the day from sunrise to sunset, increasing thereafter. The average liquid-water density in the cloud is closely related to the cloud thickness, decreasing as the cloud thickness decreases. The cloud-base height has a diurnal range of 150 ± 30 m, rising from sunrise till midafternoon. The cloud-top height has a similar diurnal range of 130 ± 30 m, but the main descent occurs in the late afternoon. Surface air temperature also increases at sunrise, directly in phase with the cloud-base lifting, and has a diurnal range of 2°C. The diurnal behavior of the cloud base appears to be consistent with model-predicted uncoupling of the cloud layer and the subcloud layer as the turbulent flux of moisture is inhibited by solar heating near the cloud base. Similarly, variation in surface air temperature is consistent with the inhibition of the turbulent flux of heat between the two layers, shielding the surface from the effect of longwave cooling from the cloud top. The variation in cloud-top height, however, does not appear to he readily explainable by present diurnal models.