East Asia and Western Pacific METEOROLOGY AND CLIMATE

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climatology of the MM4 precipitation for these five cases corresponds much more closely to the
observed average January precipitation over the western U.S. than does the CCM precipitation
climatology, and that the individual storms simulated by MM4 are much more realistic than those
in the CCM.
Using the same CCM-MM4 system, Giorgi and Bates (1989) simulated the period 1-31
January 1979, in which 9 Pacific storms moved across the western U.S. The MM4 captured most
regional features of the orographic forcing of precipitation. Compared to station data,
precipitation amounts tended to be overpredicted. Daily precipitation threat scores for various
precipitation thresholds varied between 0.315 and 0.385. However, the threat scores for the
monthly precipitation, more indicative of the model's ability to simulate regional climatological
precipitation, were higher, ranging from greater than 0.8 for light precipitation to 0.5 for
moderate to heavy precipitation. Snow depths predicted by the model also showed realistic
regional features.
In the first known application of a mesoscale model to paleoclimate studies, F. Giorgi, S.
Hostedtler, and L. Benson (personal communication) have simulated the effect of two large
prehistoric lakes, Lake Bonneville and Lake Lahoutan, which existed in the western U.S. during
18,000 B.P., on the wintertime precipitation of the region. The hypothesis was that the enhanced
precipitation resulting from the large lakes would increase runoff from the winter snows and help
maintain the high lake levels during this period. Fig. 4 shows the differences in the January 1979
precipitation simulated by MM4 between model runs with and without the enhanced lakes.
Maximum differences of more than 2.5 cm exist in two locations in the vicinity of the lakes,
suggesting that the lake-effect snowfalls could have contributed significantly to the precipitation
of the region during this time.
SIMULATED> JANUARY 1979 PRECIPITATION
DIFFERENCES LAKES-CONTROL
CONTOUR INTERVAL OF 1.0 cm
Fig. 4: January
1979
precipitation
differences
between
simulation
containing Lake
Bonneville and
Lake Lahoutan as
they were 15K BP
and the present.
Contour interval
is 1.0 cm. (Giorgi
1989)
6.0 SUMMARY
These results of many studies using MM4 show that a general mesoscale model with
realistic treatment of surface conditions and physical processes and initialized with good largescale
conditions is capable of simulating and predicting a large variety of synoptic and mesoscale
phenomena in different parts of the world. In a significant number of cases, realistic mesoscale
features develop during the simulation even though only large-scale data are used to initialize the
model. The model simulations also provide high-resolution dynamically consistent data sets
which are useful in understanding the physical behavior of complex mesoscale systems.
In addition, the MM4 has been coupled with the NCAR Community Climate Model (CCM) to
study the regional climate of the western United States. Five-and thirty-day simulations of the