WMOD00499

• Title: An Objective Forecasting Technique for High Plains Thunderstorm Development
• Date: 10/16/1978
• Weather Modification - Doc Type: Report

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The horizontal gradient, cross-isentropic winds
and vertical isentropic gradient for each element
are then entered into~n ~lgorithm which estimates
the average vertical veloc1ty cross-isentropic
~ponent. Figure 4 offers examples of the resur
tant f1eld for two cases, Th$-Yertical velocities
are then used as input to MESOCU which d' ses
the resu 1ng a1 a e tential instabi 1
The mo e re ates convective cloud
.development.

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Figure 4. Computed vertical velocities
and grid element mid-point positions for 2 cases.

4.

NUMERICAL MODEL EXPERIMENT

A relatively simple quasi-time depen-
dent numerical model of cloud-environment inter-
action ~SOC!n. developed by Krei tzberg and Perkey
~is used in this study to diagnose convec-
tive potential for cloud growth. The model simu-
lates cloud growth by Lagrangian parcel dynamics
using Stommel (1951) entrainment with the effects
of water loading and cloud-environment mixing to
damp thermal buoyancy, It includes the environ-
mental controls of solar heating and surface eddy
mixing in the planetary boundary layer~ and
the effect of meso-synoptic scale lift1ng on
convective thermodynamics. The model also simu-
lates the local microphysical impact of precip-
itation - evaporation in the dry sub-cloud layer
which modifies the environment for further con-
vection by COOling and mOistening the@L:' )

A numerical experiment was performed
using the MESOCU model to simulate effects of
mesoscale lifting on the release of available
potential instability. Lifting has an effect of
destabilizing stable layers and advecting or carry-
ing moisture upward. The numerical experiment sim-
u~at:d the atmosE~er:'s,response to case~lwith no
Ilftlng, 10 em s 11ftlng, and 20 cm s lifting
using vertical profiles of lifting, Here the mag-
nitude of lifting is the height (km) through which
each parcel in the modeled atmosphere is lifte~
during each hour simulated, so that a 10 em s-
lifting rate would result in a maximum li~ of
360 meters at the ~E!l_.9!....n.ond.iv,er.gence ~LND1J
A level of nondivergence of 70 kPa was assumed for
mesoscal~ triggering in contrast to the synoptic
scale~of about 60 kPa. This assumption was
based on mesoscale observations indicating more

intense low-level forcing. The numerical exper-
iment was performed on 3 rawinsonde observations
taken dur ffig the summers of T977'ancl-T9~ --'
~dland, Kansas. These raw1nsondes were ava11-
able in real time in the Bureau of Reclamation's
Environmental Data Network (Politte et al., 1977)
to initialize the model. Summaries of 20 cloud
var iables for each cloud and four pl:ofiles of
total cloud-environment effects on temperature and
moisture for each case were examined. This paper
considers only four of the cloud val: iables from
the model - cloud-top height, number of clouds
supported, cloud base height and total cloud depth.
Identical simulations for modified conditions,
using the dynamic seeding hypothesis, were per-
formed; however, these results are not addressed
in the paper (see Matthews, 1977),

The rawinsonde observations used to
initialize the model simulations wel:e selected
from three typical convective day. illh eitheL- (1)
esosca e organlzed convection, which indicates
'tl; existence of a mesoscale-tr1ggel:ing mechanism
likely to result in lifting, or (2) isolated ~
random convective clouds or clear skies. GOES-TAP
(Corbell, 1976) satellite imagery was used to vis-
ually classify each day on a 150 km grid (fig. 4).
Then all model. results were verified according to
observed cloud types and mesoscale triggering.

One example of a mesoscale-triggered
.case is presented to show important effects of
lifting on the release of available potential in-
stability and show how this isentropic analysis
teChnique may be used with the model to quanti-
tatively forecast thunderstorm development.

5.

RESULTS AND VERIFICATIrn~

5.1

General Summary

Six cases were examined for this pre-
liminary study. Three of the cases were used in
I1ESOCU analysis. The days are summarized by
exc:erpts from the operations log:

May 24, 1977

The intensive thunderstorm activity
occurred after 1800 MDT (0000 GMT) l"h,m a cluster
of cells developed in southeastern Colorado. The
cells were moving toward the northe;~sl:. New cells
were growing along the southeast qu,~dl.ant of the
cluster and were developing on the ,,,indward side
of the low-level (surface) flow.

June 2, .1977

Thunderstorm cells developed north-
northeast of Limon, Colorado and moved eastward
after 1500 MDT (2100 GMT). The cells gradually
moved toward Goodland and were within 40 km by
2000 MDT (0200 GMT). The cluster intensified at
this time and continued to move slm}l? eastward
throughout the nighttime hours.

July 2, 1977 (f1ESOCU Stu,~ I

Isolated small cumulus fo.:med 50 to
100 km west of Goodland at approxim~t~ly 1500 MDT
(2100 GMT). These cells did not grow substantially.
Major activity to the southwest after 1800 i4DT

L81

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