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

JULY 1980

NOTES AND CORRESPONDENCE

1057

suIts in observed cloud base heights in most cases
and that a 2 km initial parcel radius generally re-
sults in model prediction of cloud-top heights similar
to those observed by aircraft or radar echo tops in
the vicinity ofthe rawinsonde observations. Updraft
calculations are then performed in a one-dimen-
sional Lagrangian parcel model which includes the
processes of entrainment, microphysical-dynamic
interactions, and mass continuity between the cloud
and environment. Next, the effects of environmental
subsidence are evaluated. Then precipitation, col-
lection and sub cloud evaporation are computed.
Horizontal mixing of mature cloud and subsided
environment parcels is then performed using a
height-dependent ratio of cloud mass to environ-
ment mass. Next, the sounding which has been
changed by cloud-environment interactions is
examined to see if more clouds can be developed at
this time. If sufficient instability exists, additional
clouds will be developed. This cycle repeats until no
further clouds can be supported during the period
between mesoscale liftings. When no further clouds
may be supported, lifting is applied, then surface
eddy mixing and solar heating, and the search for
convective base and cycle of cloud-environment
interaction repeats until the final time is reached.
Once this time is reached, model computations stop
and a summary of convective development is pro-
duced. The full cycle of computations is repeated as
outlined in Fig. 1.
A study of convective development over the High
Plains by Matthews and Henz (1975) shows that the
magnitudes of convective development, rainfall and
subcloud evaporation predicted by the model are in
reasonable agreement with observations.
When interpreting model analyses of these
processes, one must remember that these are three-
dimensional time-dependent processes which result
in complex dynamic and thermodynamic interac-
tions. The model simply provides a framework
which parameterizes many of these complex inter-
actions and examines the net effect of dynamic
processes such as lifting and subsidence on the en-
vironmental stability and moisture which control
convective development. Hence the model may be
used as an objective analysis of the potential trig-
gering effect of lifting on convective cloud growth.

3. Description of experiments

A series of numerical experiments was performed
using the MESOCU model to simulate effects of
mesoscale lifting on the creation of Potential Buoy-
ant Energy [PBE (Fritsch and Chappell, 1980)]
and the resulting development of convective clouds.
Fritsch and Chappell define PBE as the amount of
buoyant ~nergy which a convective environment
may have iflifted by large-scale forcing. It is a meas-

I
.~

INPUT DATA
SOUNOING - P, T, TO
1I00EL START -STOP TillES
CLOUD RADIUS
IIESO-SYNOPT I C LIFT ING..,

fUEL lAYER DEPTH AND CLOUD COVER
INITIAL SUBSIDENCE CALCULATION
wASS CONTINUITY CHECKS
HYDROST AT! C BALANCE

SUBS I DENCE]

C

PRECIPITATION COLLECTION, SUB-CLOUD EVAPORATION

C=LATERAl ~IXING Of ClOUD+ENVIRON~ENT PROPERTIES

CHECK

CLOUDS

SURfACE

~l SUIolMA'!.Y TABUlATIONSI

FIG, 1. MESOCU model general flow of computations,

ure of potential for buoyant accelerations of a parcel.
PBE may also be interpreted in terms of the amount
of releasable instability, positive area on a thermo-
dynamic diagram (Haltiner and Martin, 1957), which
would exist if the atmosphere were lifted by meso-
scale or synoptic-scale forcing. PBE is a function of
the sounding's static stability lapse rate and vertical
profile of moisture. Soundings which have stable
layers are affected by lifting. Lifting destabilizes
stable layers and advects moisture upward, thereby
creating a more favorable atmospheric structure to
support convection (Hess, 1959). When the parcel
reaches the level of free convection and the energy
from the environment becomes available for buoy-
ant accelerations, it is considered to become Avail-
able Buoyant Energy [ABE (Chappell and Smith,
1975)] .
The numerical experiments simulated the at-
mosphere's response to cases with no lifting, 10
cm S-1 lifting and 20 cm S-1 lifting using the vertical
profiles of lifting shown in Fig. 2. These lifting rates
agree with three-dimensional model simulations of
mesoscale lifting associated with sea breezes
(Pielke, 1974), thunderstorm mesohighs (Fritsch