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

1056

MONTHLY WEATHER REVIEW

VOLUME 108

Sensitivity of Convective Cloud Growth to Mesoscale Lifting: A Numerical Analysis
of Mesoscale Convective Triggering

DAVID A. MATTHEWS AND BERNARD A. SILVERMAN

Office of Atmospheric Resources Management. Water and Power Resources Service, Denver, CO 80225
7 January 1980 and 25 March 1980

ABSTRACT

Numerical model simulation of mesoscale lifting of the convective environment indicates that the ability
of the environment to develop deep moist convection increases as mesoscale lifting increases. Mesoscale
lifting produces increases in modeled cloud development (cloud depth, cloud-top height, number of clouds,
etc,) in most of the samples of232 summer soundings observed in the High Plains as part of HIPLEX (High
Plains Cooperative Program). These increases were statistically significant at the P = 0,001 level in most
cases, The effect of lifting was found to vary geographically from north to south over the High Plains,
On days when convective cloud lines and clusters were observed in satellite imagery, model simulations
produced deep convection only when mesoscale lifting released the conditional instability. On days when
isolated convective clouds or clear skies prevailed, model simulations produced less intense convective
development; however, lifting was often required to supplement surface heating to produce clouds on
these days, These results suggest that the model may be used to determine a convective potential index of
the effects of lifting on c1@ud development, provided there is a means for determining the magnitude of
mesoscale lifting velocities.

1. Introduction

Convectively driven mesoscale, weather systems
on the order of 20-200 km (Orlanski, 1975) produce
large quantities of precipitation (Caracena et al.,
1975; Fujita, 1963; Ulanski and Garstang, 1978) and
severe weather (Williams, 1948; Magor, 1958). These
mesoscale systems are very important to research
scientists working to understand the cloud physics
and dynamics of precipitation (Simpson and Wood-
ley, 1975) and to operational meteorologists de-
veloping better advanced-warning techniques to
alert the public to imminent dangers of severe
weather (Maddox et al., 1978; Endlich and Man-
cuso, 1968). This paper discusses the role of meso-
scale lifting in the development of organized (non-
severe) convective cloud systems in the High Plains.
The source of lifting may be a thunderstorm's gust
front, a local convergence zone, a mountain range or
a small ridge, a sea breeze, or an upper level fea-
ture such as a jet streak (Uccellini and John-
son, 1979).
Experiments were conducted with a one-dimen-
sional quasi-time-dependent model as an analytical
tool to 1) detect soundings which were sensitive to
mesoscale lifting and determine the effect of lifting
on cloud development; 2) discriminate between cases
which had observed organized mesoscale convec-
tion and those with no observed organized convec-
tion; and 3) determine the effect of lifting on the

dynamic modification potential of resulting clouds
due to ice phase seeding. Analyses of 232 soundings
were performed which show the natural variability
of mesoscale lifting effects in different regions of
the High Plains. Two case study examples of model
simulations which show the important effects oflift-
ing on different environments are also presented.

2. General description of model processes

The MESOCU model developed by Kreitzberg
and Perkey (1976) that was used in this experiment
is a simple, fast, one-dimensional model capable of
being used operationally in weather modification
projects and in forecasting. This model simulates
several fundamental processes associated with the
development of convective clouds including lifting,
subsidence induced by convective development,
mixing of the cloud into the environment, entrain-
ment, surface eddy mixing, solar heating and sub-
cloud evaporation.
The model is initialized by a vertical profile of
temperature and moisture interpolated to 250 m grid
levels from significant level data observed in a
rawinsonde. In these simulations, a convective base
is determined from the initial sounding and an im-
pulse of 2 m S-1 is applied to a parcel at this level.
The initial parcel radius is specified as 2 km. Model
sensitivity studies (Matthews and Henz, 1975) in-
dicate that 2 m S-1 initial parcel vertical motion re-

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