WMOD00491

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

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Figure 2. .. Two aircraft cross-section observation of the arc cloud observed at altitude of
1300 and 1450 m m.s.l. The variables presented are from top of figur'e to bottom. I True air
speed ms-1 (TAS), flight altitude m (m.s.l. ALT), vertical velocity ms-1 (VERT VEL),
Turbulence IT (TURB), equivalent potential temperature oK (THETA E), dewpoint oc :(DEW pT),
and temperature oc (TEMP). I

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Penetration of the meso-front occurred at 2315 and
2316.5 G.m.t. Even though the meso-high is over
4 h old, the cold, moist features of the original
downdraft are still discernible. The temperature
is 2 Oc colder and the dewpoint 4 oc higher at
the lower level, while the upper level shows
differences of 1 and 3 oc, respectively. The
"frontal" zone varies from 3 to 8 km in width and
is characterized by a very poorly mixed ainnass,
while the region within the meso-high is well
mixed and very stable. Measurements of the speed
of this portion of the front give 7 ms-1 toward
the southwest, which compares favorably with
those determined by satellite and radar.

A significant factor in the development of convec-
tive activity is the frontal lifting that occurs at
the boundary of the meso-high. On these two pene-
trations, which were flown entirely in clear' air,
vertical velocities were frequently above 3 ms-1
and on one occasion was in excess of 5 ms-1 for
10 s, or 800 m across. Calculation of the vertical
velocity from aircraft altitude changes during this
time period give similar results.

Another interesting feature which is apparent in
the aircraft data is the erosion of the leading
edge of the high by what appears to be entrain-
ment of warmer, drier air behind it. This can be
seen by the twi n-peak structure of the dewpoi nt
trace. The most significant downdrafts occur in
this region, with some reaching 6 ms-1. It is
assumed that at this stage in the life of the
meso-high the leading edge of the ainnass is thin

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enough to allow thE! downdrafts ;to penetrate to
the surface. This mechanism w~uld result in the
erosion and breakup of the mesq-high.

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4. MODEL SIMULATION OF LIFTING EFFECTS

Thermodynamic changles caused b~ mesoscale lifting
were simulated in al one-dimensional-quasi-time-
dependent numeri cal cloud modell (MESOCU) developed
by Kreitzberg and Perkey (1976)1. The model
simulates (1) the E!ffect of lining on release of
available potential instability (API), (2) surface
heating and edqy mixing, (3) t~e one-dimensional
Lagrangian parcel thennoqynamids, (4) mixing
between cloud and envi ronment, :( 5) 1 oca 1 envi ron-
ment subsidence which compensa~es cloud growth, and
(6) subcloud evapor'ation of pr~cipitation.

A seri es of mode 11 rig experiment's was made to deter-
mine the effect of lifting on ~he release of
AP I usi ng the ai rcraft-observed verti ca 1 moti on.
Above the two ai rcr'aft fli ght ':eve 1 s, four di fferent
lifting profiles were tested to detennine the
re 1 ative importanCE! of the sha~e of the 1 ifti n9
profile (figure 4). These pro~iles were used to
simul ate the therm(lqynamic response for the 1200
G.m.t. and 1800 G.m.t. soundings at Goodland on
August 19, 1977. The results i ndi cate that. the
shape of the 1 i fti 119 profi 1 e aFlJd its verti ca 1
extent are important.

Different model re!;ponses were diagnosed for each
sounding, showing the importance of representa-
tive soundi ngs. A 24-percent increase in t.he