WMOD00584

• Title: Use of Nested Models to Simulate Regional Orographic Precipitation
• Date: 11/1/1992
• Weather Modification - Doc Type: Report

1988-89 dry period, but cumulative totals were significantly less in the
Gunnison. MM4 model simulations started on January 1, 1988, and
continued through April 1989. Modeled cumulative precipitation from
January 1r to May 15, 1988; was 64% of that in the same period of the
wet 1983 El Nino. On May 15, 1988r the cumulative precipitation
observed above 2700 m msl was 73% of that observed in 19,83. Winter
cumulative precipitation was reasonably well modeled; however, after
mid-May, convective ~recipitation was poorly estimated. This crossover
point in late spring marks the MM4 change from mesosynoptic prediction
of orographic precipitation to orographically triggered convective
precipitation (see Figure 3arb). It clearly shows MM4' s limited
ability to simulate convective precipitation. Figure 2 shows an E~xample
of summer simulations of the strong 'summer monsoon flow predicted by MM4
on July 20, 1983r and the resulting 24-h precipitation field predicted
over the West. The maximum of 5.25 em over Colorado illustrates the
overprediction problem in mountainous regions. Precipitation
measurements in the Gunnison Basin indicated an area average of 0.2 cm
with a maximum of 1. 0 em on July 20, 1983r hencer the need for Clark
model simulations to improve convective parameterizations for both MM4
and Rhea models.

Modeled cumulative precipitation matched cumulative observations
much better than daily 24-h predictions. Daily predictions were often
shifted out of phase with observations and had a large scatter showing
the difficulty in predicting point and local-scale precipitationr which
is beyond the intent of these climate simulations. However, 5-day
running mean analyses of the observations and MM4 predictions indicated
that the model was able to capture the regional precipitation
characteristics of storms; but, the phasing was not exact. Paired
scattergram comparisons of observed and modeled cumulative daily
precipitation suggested a nearly linear relation with correlation
coefficients greater than 0.94 for all data. Figure 4 compares observed
and predicted cumulative precipitation for 1982-83 and 1988 periods.
MM4 performed better in 1982-83 than in 1988. Daily cumulative
precipitation observed above -2700 m msl from October to mid-May (Figure
4a) displayed a nearly 1: 1 pattern with that modeled (slope 0.99,
intersect 19.1). However, summer precipitation was grossly
overpredicted -- note the change in slope to 0.14 from May to September
1983 in Figure 4b. In 1988, modeled cumulative precipitation matched
that observed above -2700 m better than that observed below -2100 m
levels (Figure 4c,d). Note that the excessive convective precipitation
over mountainous terrain was reduced in the 1988 simulations by an
improved horizontal diffusion method (Giorgi et al'r 1992b).

To verify MM4 dynamic and thermodynamic performance, we compared
the GJT (Grand Junction,' Colorado), NWS rawinsonde temperature, dew
point, and wind profiles with those from representative grid points in
the MM4 model simulations. Modeled and observed vertical profiles were
overlaid on a thermodynamic diagram. The comparisons showed similar
modeled and observed vertical structure; however, MM4 failed to capture
local effects, particularly during moist convective periods. Medina
used these sounding sets to initialize the Rhea model for local-area
precipitation simulations. Results are discussed by Medina (1992) in a
companion paper. Previous studies by Matthews et al.(1991,1992) showed
that MM4 simulated reasonably well the mesoscale-synoptic structure of
pressure height, wind, and moisture fields.