WMOD00535

• Title: Regional Model Simulation of an Extreme Precipitation Event in the Colorado River Basin
• Date: 1/5/1992
• Weather Modification - Doc Type: Report

the tWo events. MM4 significantly overpredicted the mean
precipitation. However,. comparisons ?e.tween the MM4 and
high elevation cumulatIve mean preCipnatIon ~b~e~atlons.
marked 0, indicated that MM4 cumulatlve preclpltatlOn was
J.bout 10 percent below the observed values. This sug~ests
that the model simulations of precipitation were biased
toward the higher elevations in the mountainous areas and
that higher topographic resolution is needed for more
accurate simulations of mountain precipitation.

4. CONCLUSIONS

The major features that contributed to this extremely
wet period were successfully simulated by MM4. The
primary feature that produced heavy precipitation was the
long-wave trough with polar jet stream located over the
Southwest. This provided a continuous flow of moist air
from the Pacific with a high frequency of storms that passed
through the Colorado River Basin. Combined with late
spring warming, the heavy snowpack melted and ran off at a
record rate. The MM4 simulations accurately reproduced the
details of the mesosynoptic flow across the Western United
States. This rnodel appears to have potential to accurately
simulate similar features within a GCM nested system of
models. The degree to which the GCM is able to accurately
portray the general circulation pattern and location of the
long-wave trough will determine the MM4's ability to
accurately represent future climate. Large spatial and
temporal variations in the regional controls of precipitation
are clearly defmed by the MM4 analysis.

5. FUTURE PLANS

Further analysis and SenSltlVlty testing will be
performed using nested CCM current and future climate
simulations to determine the controlling factors for regional
storms and precipitation. When the modeling system is fully
evaluated and ready for climate change simulations, future
climate scenarios will be examined for western basins using
output from nested MM4 - CCM simulations representing
doubled-C02 and normal conditions. Results from the local-
scale precipitation model will then be used to meet
Reclamation's water resources management requirements.

6. ACKNOWLEDGEMENTS

Computer support from the Climate and Global
Dynamics Division of NCAR is gratefully acknowledged.
The National Center for Atmospheric Research is sponsored
by the U. S. A. National Science Foundation.

7. REFERENCES

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Fig. 8. Cumulative mean precipitation in southwestem
Colorado from 1 October to 30 June for 1982-83 (on) and
1988-89 ( ). Arrows show two storm events on April 21
(1) and May 14-17, 1983 (2). Mode~cumulative ~e~(.m)
and high e:levation observed cumulative mean preCipitatiOn
(0) are shown for the two events.

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