WMOD00418

• Title: Sensitivity of Convective Cloud Growth to Mesoscale Lifting: A Numerical Analysis of Mesoscale Convective Triggering
• Date: 7/7/1980
• Weather Modification - Doc Type: Report

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MONTHLY WEATHER REVIEW

VOLUME 108

TIME or L1rTING 12

20 KPA

30 KPA

50 KPA

70 KPA

80 KPA

I 1
0,5 1.0
MAGNITUDE Of LI~T IKMI

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FIG. 2. Two profiles of lifting used in modeling experiments which simulate 10
cm S-1 (dashed) and 20 cm S-1 (solid) lifting at the 70 kPa level (3,1 km MSL),

et al., 1976), radar observations of gravity wave
amplitudes (Gossard and Richter, 1970), and meso-
scale convergence observed in surface. mesoscale
networks in Florida (Ulan ski and Garstang, 1978)
and in Texas.1
In these simulations a measure of the magnitude of
lifting is the vertical distance (km) through which
each parcel in the modeled atmosphere is lifted
during each hour simulated, so that a 10 cm S-1
lifting rate would result in a maximum lifting of 360 m
at the level of nondiyergence each hour. A level of
nondivergence of 70 kP A was assumed for meso-
scale triggering in contrast to the synoptic-scale level
of nondivergence of -60 kP A. This assumption was
based on mesoscale observations indicating more in-
tense low-level forcing. 1
Three-hour sequences of convective development
were simulated using identical initial conditions of
cloud updraft radius (2.0 km) and initial parcel
vertical velocity (2.0 m S-I). The mesoscale lifting
impulse was applied at the end of each hour's con-
vective development such that the subsequent hours
simulated were destabilized by the large-scale forc-
ing which balanced the local stabilizing effects of
cloud-induced subsidence. The arithmetic sum
of the depths of all clouds formed by the one-dimen-
sional steady-state cloud model (Fig. 1) for each 3 h

1 Scoggins, R. S., G, S. Wilson and S, F, Williams, 1979:
Mesoscale characteristics of the Texas HIPLEX area during sum-
mer 1977, Texas Department of Water Resources, Rep. LP. 99,
Prepared under Contract 14-80002 by Texas A & M University
funded by U,S. Department of the Interior and the Texas De-
partment of Water Resources, '

simulation of cloud-environment interaction is taken
as a measure of the thermodynamic potential for
convective cloud growth of each sounding. This total
depth of all clouds in each sequence of growth is
referred to as the convective potential index (CPI).
The CPI is a simple measure of the relative potential
for convective cloud development which may be
realized when synoptic or mesoscale lifting creates
Potential Buoyant Energy that can be converted into
Available Buoyant Energy.
The numerical experiments were performed on
232 rawinsonde observations taken during the sum-
mers of 1975, 1976 and 1977 at Big Spring, Texas,
Goodland, Kansas, and Miles City, Montana (Table
1). The rawinsonde observations for 1976 and 1977
that were used to initialize the model simula-
tions were selected from days with either 1) or-
ganized mesoscale convective clouds, e.g., lines
or clusters, which indicate the existence of a
mesoscale-triggering mechanism that may result in
lifting, or 2) isolated or random convective clouds
or clear skies. GOES-TAP satellite imagery from
the National Environmental Satellite Service was
used to visually classify each day's convective cloud

TABLE 1. Number of rawinsonde cases examined
by site and year.

Site

Number

Year

Miles City, Montana
Goodland, Kansas
Big Spring, Texas
Total

120
56
56
232

1975
1976, 1977
1976, 1977