WMOD00491

• Title: A Multisensor, Three-Dimensional Analysis of a Meso-High's Development
• Weather Modification - Doc Type: Report

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A Mul ti sensor" Three-Dimensional Analysis
of a ME!so-High's Development

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by

John C. Lease

and

David A. Matthews
Bureau of Reclamation
Denver, Colorado

1. INTRODUCTION

Mesoscale triggering mechanisms are ve~ important
to the initiation of organized convective cloud
development, locally heavy precipitation, and
severe weather events. One type of mesocale
triggering mechanism is the mesoscale cold front
associated with the meso~high created by moist
downdrafts from decaying thunderstorms (Fujita,
1956). The meso-cold front is often delineated by
an "arc cloud" in satellite image~. The intersec-
tion of two arc clouds has been associated with
severe weather and triggering of convection (Oliver
and Purdom, 1974).

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In the central and southern High Plains, arc clouds
appear to be an important mechanism for organizing
convection and producing large meso-scale systems
(Matthews and Koshio, 1977). Detailed analysis of
mesoscale surface observations by Maddox showed
that a meso-high was associated with the Johnstown
Flood in 1977 (Maddox and Chappell, 1978). Meso-
high arc clouds may also be important in promoting
cloud mergers, and recent results from the Florida
Area Cumulus Experiment (Simpson and Woodley, 1975,
and Cunning et al., 1977) suggest that total rain-
fall from large mesoscale systems resulting from
cloud mergers may be from one to two orders of
magnitude greater than that from single isolated
convective clouds or thunderstorms.

On August 19, 1977, near the Goodland, Kansas, High
Plains Cooperative Program (HIPLEX) field site, we
had an opportunity to make multisensor observations
of a meso-high arc cloud. This paper discusses the
results of the multisensor (satellite, radar, air-
craft, rawinsonde, and numerical model) analysis
of this arc cloud. The structure, strength, and
effect of mesoscale triggering on the release of
available potential instability are presented.

2. SATELLITE AND RADAR OBSERVATIONS

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A mesoscale cluster of cumulonimbus clouds 45 km
wide developed on August 19, 1977, at 1900 G.m.t.
125 km northeast of Goodland, Kansas. This cluster
produced a distinct meso-high arc cloud in a field
of cumulus clouds at 2030 G.m.t. In regions were
sufficient instability and moisture exist, a series
of clouds will form along the leading edge of this
meso-scale cold front. These clouds will be seen
on satellite imagery as a distincit arc. The arc
generally moves radially outward from the gener-
ating cumulonimbus clouds.

The arc cloud moved southwestward from the cloud
cluster with a mean speed of 7 ms-1, while the
cloud cluster moved southward at 6 ms-1. This
relatively weak arc cloud had a cord length of
nearly 95 km and duration of more than 4 h

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(see figure 1). Note the clear ~egiOn behind the
arc cloud associated with the cold, moist, stable
air produced by the thunderstorm downdraft and
local cloud-environment compensating subsidence.
Several large cumulus congestus S'ouds developed
along the arc; however, the dry subsidence'
associ ated wi th the synopti c-sca l:e hi gh pressure
in Nebraska suppressed further development of
clouds along the arc.

The formation and development of ithe meso-high and
its associated arc cloud was also: observed with t.he
Bureau of Reclamation's 5.4-cm (C~Band) radar.
Thi s radar is located at Goodl and" and is operated
conti nuously whenever there are clouds wi thi nits
150-km range. It normally operat:es in a volume-
scan mode, which consists of one 13600 azimuth rot.a-
tion for each 10 elevation angle between 1 and 120.
In this mode, the first 3600 scan~ requires 34 s
and the remaining scans require 1;7 s each, with the
entire volume scan completed in 51 min. On this
date radar data were recorded continuously through-
out the life of the meso-high. :
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The cumulonimbus cloud cluster whiich produced the
meso-high is clearly visible in f~gure (1). As the
arc cloud moved southwestward, the mesco-cold front
and the associated cells which de~eloped along it
were readily discernible on the raw video display
and were recorded as digital datal for later analysis
(fi gure 2). The average speed ofl the front was
6-7 ms-1 toward the southwest. .

3. AIRCRAFT OBSERVATIONS

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Two aircraft extensively instrumerted for cloud
phsics measurements operated fromiGoodland in
support of the HIPLEX program. Measurements of the
basic state parameters, turbulence, vertical
velocity, doppler winds, and liqujd-water content
were recorded at 1-s intervals. In addition of
the spectra and concentrations oflaerosol, cloud,
and precipitation particles were recorded at
1-s intervals. I

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To investigate the structure of the arc, rada."
information was used to establishlfli9ht patterns
that would result in penetrations made perpendicu-
1 ar to the front. The ai rcraft f'l ew at 160 and
320 m above ground level, with th~ lower airCl"aft
(Meteorology Research, Inc., Navajo) approximately
1 km behind the upper aircraft (Upiversity of
Wyoming, Queen Air). This resulbed in a time
separation of approximately 12-15: s. Figures 3
shows data for two passes through: the arc for
each ai rcraft. In these fi gures 'the ai rcraft fl ew
out of the meso-high, turned arou~d, and reentered
it. The data as presented have been binomially
averaged over a period which is lhnger than the
response time of any of the instrlJmentation.