The Diurnal March of Convective Cloudiness over the Americas

Abstract

Based on nine years (1983?91) of infrared data from geostationary satellites (the B3 ISCCP product), several features of the diurnal march of the frequency of convective cloudiness over the tropical and subtropical Americas are documented with 3-h temporal resolution and 0.5° ? 0.5° latitude?longitude spatial resolution. The frequency of convective cloudiness in each grid box is defined in terms of the fraction of temporal samples that exhibit cloud-top temperatures colder than 235 K. The effect of varying the threshold is explored and selected results are compared with rainfall estimates based on microwave (SSM/I) imagery. Convective cloudiness over most land areas exhibits a coherent diurnal march with relatively clear mornings, a rapid afternoon buildup, and a more gradual nighttime decay. The highest, coldest convective clouds peak a few hours earlier than those with lower tops. Morning to noontime maxima tend to be prevalent over offshore waters that experience significant convection such as the Gulf of Panama, with the highest convective clouds leading the lower ones by 6 h or more. During the austral summer, the strongest diurnal march is observed over the Andes Mountains, in a band just inland of the northeast coast of South America and in two intermediate, parallel bands over Amazonia. Between these bands, weak night and morning maxima are observed in some areas. During the boreal summer the strongest diurnal march is observed over Central America and extending northwest along the continental divide as far as the southwestern United States. In comparison to the microwave imagery, the features in convective cloudiness tend to be somewhat larger in spatial scale, and cloudiness over land tends to be biased toward the late afternoon and evening hours. Regions with strong late afternoon/early evening maxima in convective cloudiness tend to experience more diurnal-mean convective cloudiness than those with weak (or morning) maxima. Although the evidence is less conclusive, it appears that the same may be true of rainfall. On the basis of these results it is suggested that over regions of relatively flat terrain such as Amazonia, the dynamics of the diurnal march may play a role in determining the spatial distribution of climatological-mean rainfall.