WMOD00438

• Title: HIPLEX: A Cooperative Program on Rain Augmentation in the High Plains
• Date: 6/1/1984
• Weather Modification - Doc Type: Report

Figure 3.3-Satellite image of convective clouds on May 28.
1979. at 002 G.m.t. showing typical clusters.

within 150 km of Big Spring had minimum echo-top
temperatures in the 0 to -15 0 C range. Nearly 30
percent of these echoes lasted more than one-half
hour. These results suggested that there are a rea-
sonable number of opportunities to examine ice-
phase seeding hypotheses within this region without
having seeding effects masked by abundant natural
ice from colder regions of a cloud.

3.4 Mesoscale Analysis

It has been understood for a long time that
convective showers develop in an unstable atmos-
phere where the temperature decreases with height
at about 5 0 C/km. By the time Texas HIPLEX began.
it was also understood that the occurrence of heavy
convective rainfall requires some organization' of
atmospheric motions on a scale exceeding that of
the local shower but smaller than that of the standard
weather map. It was resolved to make investigations
of these mesoscale systems a major part of the Texas
HIPLEX. To this end. a mesoscale network of rawin-
sonde stations was set up. and the rawinsonde data
were examined along with satellite cloud pictures
and other data.

Rawinsonde data were collected from one site in
1975 and increased to seven sites in 1979 and
1980. During operational days when convective
activity was expected. observations were made
every 3 hours from 1500 to 0300 G.m.t.

Data from the Big Spring and Midland soundings
were generally available for analysis and forecasting
in near real-time (within 2 hours of observation)
through the EDN.

The Texas HIPLEX mesoscale research conducted
during the summers of 1976 through 1980 was
designed to determine factors and environmental
conditions responsible for the initiation. growth.
maintenance. and dissipation of the several classes
of convective clouds. Substantial differences were
found between periods of convection and periods of
no convection. Low-level net horizontal inflow. het
upward transport. and upper-level net horizontal
outflow of total convective energy were found dur-
ing times of convection. By contrast. the reverse was
found when convective activity was absent. Water
vapor transport appears to be the dominant energy
source for convection. supplying both latent and
kinetic energy to the storm. Both depth and intensity
of convective activity wf!te correlated best with the
amount of water vapor transport near the surface
and the degree of vertical transport aloft (Scoggins
et al.. 1979). Horizontal outflow in the upper air was
shown to complement the, low-level convergence
and upward motion. The volume of convective cloud
(depth and area) contributed more to the quantity of
water vapor processed than did the type of organiza-
tion. Comparison with radar and satellite images
showed that mesoscale convergence and lifting are
responsible for most deep convection and that sink-
ing motion suppressed cloud development (Mat-
thews. 1983).

Figure 3.4 shows an example of strong vertical
motion which triggered deep convective clouds and
heavy precipitation. nearly 37 x 106 m3 of rainfall
within the rain gage network. during a convective
line event on July 17. 1979. The mesoscale precip-
itation efficiency of this system was 16 percent. indi-
cating that only a fraction of the total available
moisture was returned in the form of rainfall. This
suggests that there may be potential for improving
the efficiency of these systems.

With the above indications of significant mesoscale
control of precipitation events in west Texas. studies
were initiated to better understand the relationships
involved and their geographic variations. Matthews
(1980) found from analyses of satellite cloud pic-
tures that convection showed different degrees of
organization across the High Plains, Mountain-
generated convective storms. lines. and clusters
were the predominant daily convective types in Mon-
tana. While many isolated convective clouds occur-
red on any given day. days where these clouds were
not associated with larger scale lines and clusters
were few. In Kansas. days' with lines and clusters as
the predominant convective feature were most com-
mon. but days with only isolated cells also occurred.
Days with isolated convective cells were most com-
mon in Texas. but mesoscale lines and clusters were
observed on some days.

15