11th Conference on Satellite Meteorology and Oceanography

Monday, 15 October 2001
Temporal Scales of the Areal Coverage and Precipitation of Monsoonal Convective Cloud Systems Over the Tropical Indian Ocean
Eric M. Wilcox, SIO/Univ. of California, La Jolla, CA
Poster PDF (549.0 kB)
The production of precipitation and the growth of extended decks of radiatively active anvil and cirrus cloud are tied together through the lifecycle of tropical convective cloud systems. Yet in general circulation models, convection, microphysics and cloud coverage are parameterized separately. Observations of the evolution of cloud systems can provide important constraints that are useful for evaluating whether models can reproduce clouds with proper spatial and temporal scales. In this study quantitative relationships are sought between the amount and duration of precipitation produced during tropical convection, and the areal extent and temporal duration of the entire cloud system producing the rain, including the anvil and extended cirrus cloud attached to the system. Geosynchronous satellite images from METEOSAT-5 are blended with TRMM Microwave Imager precipitation measurements and an automated cloud-tracking algorithm in order to observe the lifecycle characteristics of wintertime monsoonal cloud systems over the tropical Indian Ocean during January and February 1999. Infrared window and water vapor channel brightness temperatures from METEOSAT-5 images are used to identify boundaries between overcast pixels and areas of clear sky and broken clouds. Cloud systems comprising adjacent overcast pixels are identified in the images. Their areal coverage is evaluated and their evolution is tracked through their lifecycle by making use of consecutive half-hourly images. The temporal scales of cloud systems are observed ranging from a few hours or less for clouds with horizontal scales less than 100 kilometers to a week or more for giant overcast decks spanning 1000s of kilometers and associated with deep precipitating convection. The area and the rate of area change of high cloud-top features within the cloud systems are used to infer the surface rain rate averaged over the entire area of the cloud. The relationship between high cloud-top features and cloud-averaged rain rate is calibrated using independent measurements from the TRMM TMI. This analysis results in a large ensemble of clouds spanning the wide range of spatial and temporal scales achieved by clouds from which statistics can be generated relating precipitation, areal coverage and temporal duration.

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