WMOD00590

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

6-12 h were recognized by Beebe and Bates (1955) as important mecha-
nisms which destroy inversions in the lower and mid-troposphere.
They noted the importance of the superposition of upper-level diver-
gence above low-level convergence fields in locating mesoscale
triggered systems associated with jet stream maxima.

Larger-scale analysis and forecast problems associated with the
initiation of severe moist convection were reviewed by House (1963).
His summary showed the need for improved physical understanding of
synoptic-scale factors which may help to trigger convective storm
development. Severe storms forecasters have recognized for some
time that coupling between zones of upper-tropospheric divergence and
low-level convergence is favorable for aiding the release of latent
thermodynamic instability through the large-scale ascent of convec-
tively unstable air (Beebe and Bates~ 1955; Newton~ 1963; McNulty,
1978). Similarly favorable conditions for realizing potential
instability exist when warm moist air is mechanically lifted while
flowing over upward sloping terrain. Both of these mechanisms are
enhanced in regions of localized boundary-layer convergence. House
(1963) recognized the transient nature of these phenomena and showed
the need for more complete mesoscale observations and analyses.

Newton (1963) showed that downshear of a thunderstorm downdraft
momentum transfer from aloft of 10 mts could increase the surface
pressure change from 0.5 mb in a non-sheared environment to 2.0 mb in
a sheared environment. Thus~ the mechanical lifting associated with
thunderstorm outflow appears to be more effective in sheared environ-
ments associated with jet streak systems. Analyses of the Thunder-

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