GATE B-Scale Cloudiness from Whole-sky Cameras On Four U.S. Ships

Abstract

The largest network of surface cameras ever established in the tropics for studies of cloud cover was deployed during the. Global Atmospheric Research Programe's Atlantic Tropical Experiment (GATE) in 1974. Analysis was made of 2572 hourly whole-sky photographs taken aboard four U.S. ships during the daytime hours of nearly every day covering the three phases of GATE. The cloud-cover analyses were made on a grid divided into 100 squares covering most of the overhead sky, much as previously made for Barbados and adjacent Atlantic Ocean cloudiness. Analyzed cloud types include low, middle, high, two kinds of total, and two kinds of combined upper cloudiness. Rainfall duration was obtained from drops impinging an the glass dome covering the whole-sky lens. Average total cloudiness was between 70 and 85% for different ships and individual phases of GATE. From 22 to 56% were low clouds, and the rest were upper clouds. Large standard deviations of means are attributable mainly to frequent values near 0 and 100% cloudiness. Maps of cloudiness amount show a zonal region of active convection, containing maxima in low clouds and rainfall, to be located south of 7°N in Phases I, and near 8°N in Phases II and III. The convective code observed aboard ship shows a consistent positive relationship to low and total cloudiness and precipitation, and an inverse relation to net radiation. There is not a good correlation of upper cloudiness to code because of obscuration by lower clouds. Variations of mean cloudiness at the four U.S. ships, for daylight hours only, show a weak midday minimum in low clouds and a rather strong sunset peak. Total and upper clouds, and rainfall, have consistent afternoon maxima. Statistical analyses of rainfall data indicate significant afternoon maxima in Phases II and III.