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upward motion (Fig. Ib) of air with high equivalent potential temperature. The model predicted a
48-h rainfall maximum of 213 mm over the Sichuan Basin, in good agreement with observations.
Diagnostic analysis of the model results showed that the formation of the SW vortex was
induced by the strong variations in terrain elevation. As the southwesterly monsoon current
impinged upon the mesoscale Yun-Gui Plateau, which extends from the southeastern corner of the
main Tibetan Plateau, the low-level flow was blocked. The air aloft descended into the Sichuan
Basin to the lee of the Plateau, creating cyclonic relative vorticity through vertical stretching.
Differential surface friction and diabatic effects were not responsible for the initiation of
the vortex. However, both latent heating and surface fluxes of sensible and latent heat were
essential to the full development of the SW vortex.
Li ejLaL (1987) simulated another case of extreme rainfall over China. During the period
1200 UTC 19 June to 1200 UTC 20 June 1982, heavy rain fell over the Yangtze Valley, with more
than 300 mm falling near the city of Wuhan. The numerical simulations of this case showed that
the interactions of the latent heat release and the low-and upper-level jets were critical to the
maintenance of the slowly moving convective rainfall systems.
2.4 Simulation Of The Johnstown, Pennsylvania Flood Of 1977 And
Other Mesoscale Convective Systems.
Many successful simulations have been obtained with versions of MM4 that use horizontal
grids of 50-100 km and a relatively simple parameterization of cumulus convective effects.
However, the simulation of many mesoscale convective phenomena requires higher horizontal
resolution and inclusion of more complete physical processes such as downdrafts and
microphysical processes. Da-Lin Zhang and his colleagues at The Pennsylvania State University
developed a nested-grid version of MM4 for study of some of these complex moist convective
events. The modified version of MM4 features a two-way interactive nested grid, (Zhang et al..
1986) with a fine mesh of 25 km and a coarse mesh of 75 km; a version of the Fritsch/Chappell
convective parameterization scheme on the fine mesh; and a Kuo-type convective parameterization
on the coarse mesh. Variations of this model have been used to study the Johnstown flood of 19-20
July 1977 (Zhang and Fritsch, 1986), a mesoscale convective complex over Oklahoma on 7-8 July
1982 (Zhang and Fritsch, 1988c), and a squall line that developed over Oklahoma during 10-11
June 1985 (Zhang, filaL, 1989).
In all of these simulations the model, initialized with conventional observations,
reproduced well many (though not all) observed meso-beta scale features of the convective
systems, supporting the hypothesis that under certain synoptic-scale conditions, mesoscale
convective systems can be predicted using observations from a synoptic-scale network if
appropriate model resolutions and complete physical parameterizations are used.
2,5 Simulations Of The Effect Of "Lids" And Soil Moisture On The
Development Of Mesoscale Convective Systems
The importance of elevated mixed layers and the associated capping inversions ("lids") in
the initial suppression, then initiation of severe convective storms is well known (Carlson eiaL
1983). Idealized simulations by Lanicci elaL (1987) and real-data simulations by Lakhtakia and
Warner (1987) showed that numerical models can accurately simulate the development of these
"lids" and that horizontal variations in soil moisture are often critical to the formation of the lid.
Fig. 2 shows a west-east vertical cross section of an elevated mixed layer and lid from a 9-h model
simulation of the development of the environment of the severe convective storms of 9-10 May
1979. The model simulated well the dry line between 101 and 102 W longitude, the elevated
mixed layer, and the lid. Lakhtakia and Warner's studies showed that differential surface heating
at the edge of the lid was the most important single factor in initiating undemmning of moist
boundary layer air and the subsequent development of convective precipitation in this case.