WMOD00536

• Title: Release of Potential Instability by Mesoscale Triggering - An Obective Model Simulation
• Weather Modification - Doc Type: Report

. .

,

.

RELEASE OF POTENTIAL INSTABILITY BY MESOSCALE TRIGGERIHG

An Objective Model Simulatton

David A. Matthews
Bureau of Reclamation
Denver, Colorado

1. INTRODUCTION

Organized mesoscale systems tend to produce
large quantities of precfpitatfon and severe
weather. These systems are unquestionably very
important to research scientists working to
understand the cloud physics and dynamics of
precipitation and to operational meteorologists
developing better advanced-warning techniques
to alert the public to 1l11l1fnent dangers.

Smaller mesoscale systems are important in
triggering convective cloud development which
often leads to large quantities of natural
rainfall. These intense mesoscale systems may
be subject to modification to enhance preclp-
it<!ltion in regions of high moisture stress such
as the High Plains.

.

This paper addresses the problem of under-
standing the effects of mesoscale triggering on
organized nonsevere convecthe cloud systems in
the High Plains. Two experiments were deslgn~
to determine if a one-dlmenslon<!ll quasi-time
dependent model coul d 11) detect soundi ngs
whiCh were sensitive to mesoscale triggering,
and '21 discr1min<!lte between cases which h<!ld
mesoscale org<!lnized convection and those with
no organized convection. The MESOCU model
O::reitzberg and Pertey, 1976) w<!lS used to
objecthely analyze the available potential
instability and thermocb'n<!Imic potential for
cloud growth. Available potentfal instabllfty
IAPI) is the <!Imount of releasable instability
Iposlthe area on a thenuodynalllic diagralll)
which woul d exist if the atmosphere were 11 fted
by mesoscale or synoptic scale forcing. API
may be considered as avaflable convective
potential energy. MESCOU was run on rawinsonde
observations which have been stratified accord-
ing to the existence of observed organized
convection (mesoscale triggertng) or isolated
convection and clear days.

.

2. DESCRIPTION OF EXPERIMENT
A numerical experiment was performed using the
MESOCU ~del to si~late effects of mesoscale
lifting on the release of available potential
instability. lifting has an effect of desta-
bilizing stable layers and advecting or
carl')'ing mlsture upward. The numerical
expedlllent sflll.llated the atmosphere's response
to cases with no lifting, 10-cm S.I lifting,
and 20-clII S.I lifting USing vertical profiles
of lifting shown in figure 1. Here the magni-
tude of lifting is the height 0(111) through which
each parcel in the modeled atmosphere is lifted
during each hour simulated, so that a 10-Cln s"
lifting rate would result in a maximum lifting

16.0

1<t.O
12.0 1\
~
<
10.0 ,
,
~ ,
r ,
~ ,
w ,
r '.0 ,
,
,
,
,
'.0 , .
,
,
,
,
,
.. \
,
,
>
-
l.O ~
-
",.

0.0

-~ ~.-.- ~ r
O.
MAGNlTuOL or L II'T IKHI
Figure 1. Two prOfiles of lifting used in
1II0del1ng ej(pertments which sifl'ljlate
10 CIII S.l Idashed) and 20 CIII S.l (solid)
lifting at the 70 kPa level (J.1 kill).

,
LO

0.0

of 360 meters at the level of nondivergence
(lNO). A level of nondhergence of 70 kPa
was assullled for mesoscale triggering in contrast
to the synoptic scale lNO of about 60 kPa. This
assulllption was based on mesoscale observations
indicating IIIOre intense low-level forcing. The
numerical experiment was performed on lOB
rawinsonde observations taken during the summers
of 1976 and 1977 at Big Spring, Texas, and
Goodland, Kansas. These rawi nsondes were
readfly Available in the Bureau of ReclalMtion's
Experilllental Data Neb/ort (Politte et a1.,
1977) to fnitialize the IIlOdel. SU1IIIIIo1rtes of 20
cloud variables for each cloud and four pro-
ftles of total cloud-envi rorllllent effects on
temperature and moisture for each case were
ell,alllined, This paper considers only four of
the cloud variables from the IIIOdel _ cloud-top
height, number of clouds supported, total
rainfall, and total cloud depth. Identical
simulations for modified conditions, using the
dynallllc seeding hypothesis, were performed;
however, these results are not addressed in
this paper (see Matthews, 1977).