WMOD00584

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

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boundary conditions were adjusted every 12 h using the ECMWF gridded
data. These initial conditions were located at grid points that Inatched
the CCM T-42 (triangular spherical harmonic truncation at wave number
42) used in climate simulations.

Assessment of the Regional Model Precipitation Predicti.ons

Giorgi et al. (1992a) have validated mean monthly precipitation
predicted by the MM4 model during the 1982-83 and 1988-89 periods.. They
found that the correlation coefficient between observed and modeled
monthly precipitation varied from 0.53 to 0.94 among each of six
subregions of the Western United States. Over the Central Rocky
Mountains, cold season precipitation was reasonably well simulated with
a bias of 40%, however, the warm season bias was 157% in the base runs.
The revised horizontal diffusion method lowered this bias to 39% in the
1988-89 simulations. Giorgi et al. (1992a) defined bias as the sum of
the differences between observed and model predicted daily average
precipitation in each subregion, divided by the total number of station
days for that subregion and season. This model bias is a direct measure
of the model error simulating a given variable over a region. Their
study was based on 390 daily observation stations in an area of ~4.4
million km2 in the West. The station density was about 11,000 km2 per
gauge with few high elevation gauges. We focus on the assessment of
local area precipitation in complex terrain using a higher density of
~930 km2 per gauge with 11 gauges above ~2700 m msl. Modeled
precipitation was compared with, surface precipitation data from NWS
(National Weather Service) offices and cooperative stations, and the
Soil Conservation Service's SNOTEL (snow telemetry) remote high
elevation automatic stations.

Figure 3a shows a time-series comparison between observed high
elevation cumulative precipitation and MM4 predicted cumulative
precipitation in the Gunnison region during the 1982-83 water year
(October 1 to September 30). Predictions by MM4 were averaged over 8
grid points in the vicinity of 31 precipitation gauge sites. Model
daily average values and gauge averages were accumulated to form model
and observed daily cumulative means. This daily cumulative mean gives a
measure of the total precipitation observed from the beginning of a
water year to a given date. Precipitation observations were stratified
by three elevation zones: < 2100 m, 2100 to 2700 m, and> 2700 m msl
and similarly accumulated into three daily cumulative means. Figure 3a
shows the time series of daily cumulative precipitation observed in
three elevation zones from October 1, 1982, to May 15, 1983. The
striking difference between the high elevation SNOTEL data and the low
and moderately high observation sites illustrates the problem of model
verification over complex terrain. In the model validation by Giorgi et
al. (1992a), few precipitation observations were available at high
elevations such as the SNOTEL sites; consequently, their analyses may
have overestimated the model's positive bias (overprediction of
precipitation) in these regions. Figure 3a shows that MM4 predicted the
high elevation cumulative precipitation remarkably well through 'winter
and spring in 1982-83; however, it significantly overpredicted warm
season convective precipitation.

Similar convective problems occurred in the simulations of the