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John T. Ball, Stephen J. Thoren, and Marshall A. Atwater


The spatial and temporal characteristics of cloud coverage during the 21-day GATE Phase 111 period were studied using SMS-1 infrared hourly digitized data and standard hourly meteorological surface observations taken on 18 ships positioned within the A/B and C scale regions. Results were obtained for the entire sample and for portions stratified according to enhanced (E) or depressed (D) convective activity. Separate areal analyses based on ship data and on satellite data were obtained for total coverage, low-cloud amount including cumulonimbus, middle-cloud amount and high-cloud amount. Both the satellite and the ship data indicate that the average coverage for the GATE A/B array area during Phase 111 is close to 80%. The zone of maximum cloudiness exhibits a basic east-west orientation and is centered near 8°N.

Analysis of hourly variations yields a nighttime maximum of total coverage near 0300 GMT on D days while on E days the maximum occurs in late afternoon or early evening. Analysis of low clouds showed a double maxima at 0400 and 1500 GMT. The early afternoon maximum predominates on E days while only the nighttime maximum is present on D days. On days of significant activity, the high cloud maximum occurs in the late afternoon (1900 GMT), ∼4 h later than the low-cloud maximum. The results of this study emphasize that generalizations about the diurnal variations of clouds, convective activity and precipitation over tropical oceans must be carefully evaluated in terms of regional location and prevalent degree of convective activity.

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John T. Ball, Marshall A. Atwater, and Stephen J. Thoren


A radiation model was used to simulate daily incoming solar radiation at four ships during Phase III of GATE. The accuracy of the simulations from several different cloud analyses based on ship or satellite data was estimated by comparison with measured values. The cloud-data source, objective analysis characteristics and analysis grid interval affected the accuracy of the radiation computations.

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