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<br />produced by moist downdrafts was postulated as the key to the merger <br />process. Simpson (1980) developed a more complete hypothesis of <br />seeding effects on cumulus mergers. This hypothesis further illus- <br />trates the need for mesoscale analysis of convective triggering <br />mechanisms. <br /> <br />Surface analyses of data collected during the Montana HIPLEX <br />Program by Doneaud et al. (1981) and Viswanath (1982) also showed <br />the importance of mesoscale convergence and moisture flux fields <br />in organizing and maintaining convective precipitation systems. <br />Doneaud's results were similar to those of Ulanski and Garstang <br />(1978); however, he also found large temporal variations in the <br />meso-~ scale convergence structure. In Illinois, Watson et al. <br />(1981) found similarly high correlations between surface area con- <br />vergence and convective cloud development. They found that the <br />convergence reached a threshold of 8 x 10-4/s from 0.5 to 2 h prior <br />to convective rainfall. <br /> <br />Zipser et al. (1981) described the characteristics of a meso-B <br />convective band that developed in GATE. They found that the band <br />differed from the typical tropical squall line described by Houze <br />(1977) in that the band was less intense than the squall line; <br />however, most characteristics were similar in kind but different in <br />intensity. Mesoscale gust fronts played an important role in pro- <br />viding a region of low-level convergence and lifting which initiated <br />the sustained cumulonimbus activity. Mesoscale sinking suppressed <br />convective development ahead of the ascent region so that only <br />cumulus congestus clouds could develop in that region. One- and <br /> <br />18 <br />