Some Aspects of Tropical Oceanic Cloud Populations

Abstract

Oceanic cloud populations were photographed from the R. V. Crawford stationed near 13N, 55W during a 22-day period in August?September 1963. A panoramic series of six pictures per hour was taken with a Hasselblad camera around the complete horizon during daylight. These pictures were reduced by photogrammetry to yield oceanic cumulus height distributions related to the synoptic state and time of day, information that previously has not been obtained often for one location in the tropics away from land influences. Such quantitative data are useful for several types of budget studies and should be applicable for adjacent portions of the tropical North Atlantic Ocean and Caribbean Sea. Within 18 n mi of the ship it was found that half the hours had one or more cumulus clouds over 9000 ft and 15% of the hours had clouds over 18,000 ft. Other height percentages were obtained for disturbed and undisturbed hours, and also for hours with 3-cm radar echoes near the ship. Both maximum and average hourly cloud tops were obtained. It was estimated that during as many as 60% of the hours within 18 n mi of the ship there was at least one cloud which precipitated during its lifetime. No cumulus were seen above 40,000 ft during the entire period and tops exceeded 30,000 ft during less than 2% of these daytime hours. Clouds were measured to be highest at 0600 and 0700 LST and decreased somewhat throughout the day. Similar cumulative percentages for days rather than hours showed that on half the days during daylight at least one cumulus was above 20,000 ft. No quantitative nighttime population could be obtained from the data. Radiosonde data at the ship were analyzed to find simultaneous relationships between the cloud height population and sounding parameters. Compared to normal conditions during the period, highest cumulus clouds tended to occur during hours with a) lower cloud base than normal, b) moister soundings from 1000?700 and 700?500 mb, c) less stable soundings from 1000?700 and 400?200 mb, and d) more stable soundings from 700?500 and 500?400 mb. A combination of these sounding parameters comprised an index which was fairly well correlated with the cumulus height measured simultaneously. Attempts were made to calculate vertical motion and divergence, but values were unreliable for several reasons and could not be included in the index. From this study it is apparent that temporal and spatial scales of the dynamic and thermodynamic variables smaller than the synoptic scale must be examined to explain many aspects of a tropical cumulus population.