WMOD00590

• Title: Analysis and Classification of Mesoscale Cloud and Precipitation Systems
• Date: 8/1/1983
• Weather Modification - Doc Type: Report

90 day period in 1973. Results indicated that convergence fields
formed in the vicinity of precipitation up to 1.5 h before the
precipitation events. Moisture convergence measured in the surface
network was one of the most crucial factors in determining the total
amount of rainfall produced by a given storm. The storm intensity as
measured by rainfall was also directly proportional to the horizontal
gradients and area covered by surface convergence. The surface
convergence and divergence fields of smaller systems nearly achieved
mass balance. However, larger storms had lnw-level convergence in
excess of local divergence, suggesting that to achieve mass balance
there must be net upper-level divergence. The maximum convergence
observed was 1.2 x 10-3/s corresponding to nearly 65 cm/s vertical
motion. Both the area and magnitude of surface convergence were
proportional to observed precipitation.

Doneaud et ale (1981) found that objectively analyzed surface
convergence fields, observed by the mesoscale network at the Montana
HIPLEX field site, were important mesoscale triggers and controls of
convective precipitation. He found large temporal variations in the
variance of the divergence field which was related to rainfall.

Similar analyses by Watson et ale (1981) showed that mesoscale
surface winds and moisture fields could be used to predict convective
rainfall in south Florida with a 0.5 to 2 h lead time. Barnes and
Garstang (1982) have shown that traveling tropical convective systems
were more efficient than stationary systems because they were able to
use fresh supplies of higher energy PBL air. Stationary systems tend
to feed upon their own cool, dry outflow.

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