WMOD00489

• Title: Modeling Regional Controls of Watershed Precipitation for Climate Change Studies
• Date: 1/13/1991
• Weather Modification - Doc Type: Report

includes the radiative tramrer scheme developed for
the CCM to facilitate the nested coupling of MM4
with the CCM. This package performs separate
calculations of atmospheric beating rates and surface
Ouxes for solar and infrared radiation under dear and
cloudy sky conditions (Kiehl et aI. 1987). The infrared
radiative transfer calculations include the contributions
of COl , 0) , HlO, and douds. Preliminary nested
model results from feasibility studies have been
described by Dickinson et aI. (1989) and Giorgi
(1990). These results were encouraging. indicating
reproduction of realistic regional climate detail over
the Western United States. Details of the nested
model design are described in the above papers.

In order to evaluate the MM4 performance,
especially with respect to regional orographic
precipitation, the model was run with European
Center for Medium Range Forecast's (ECMWF)
large-scale analysis of rust Global Atmospberic
Research Project - Global Experiment (FGGE) data
for January 1979. The ECMWF analysis was used as
initial and lateral boundary conditions for MM4. It
bad a temporal resolution of 12 bours and a spatial
resolution of 1.875" latitude and longitude. The MM4
model domain is that adopted by Dickinson et aI.
(1989). It is centered over the Great Basin at 38" N.
and 1160 W. with a size or 3000 x 3000 kIn', having a
horizontal grid spacing or 60 kIn and 16 sigma levels
in the simulations presented bere. Figure 1 shows this
domain and topographic features used in the
simulations. Giorgi and Bates (1989) describe the
details of the MM4 simulations for January 1979,
which are used in the feasibility tests reported on in
this paper. They sbow that the MM4's predictions of
regional wintertime precipitation and bydrologic
budgets were quite realistic.

The next section shows examples of important
regional features analyzed by MM4 wbicb control the
intensity, duration, and location of precipitation.

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Fig. 1. Topographic field used by the MM4 model with a
grid point spacing of 60 kin. Units are meters above sea
level and the contour interval is 2SO m.

t.

3. MESOSCALE CONTROLS OF
PRECIPITATION - AN EXAMPLE

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During JanlW)' 1979, nine storms moved
through the Western United States producing
widespread precipitation in all watersheds of interest.
The MM4 model simulated the evolution of these
systems and generally described the distribution and
quantity of precipitation reasonably well (Giorgi and
Bates 1989). This section focuses on one storm which
affected watc:rsbeds in Arizona and illustrates the
model's abi1i~y to simulate the mesoscale forcing that
largely controls local precipitation. These examples
are provided ito illustrate the importance of defining
the regional structure of storms in climate simulations.

Two :mesoscale short-wave perturbations
moved through the Southwest from January 16 to 19,
as a vigorous low-pressure system moved inland from
the Pacific passing over Arizona on January 19, 1979.
Figure 2 sho\l\'5 the wind field at 700 mb on January 18
at 0000 UTC (all times are given in Universal
Coordinated Tune) during the period of maximum
moisture flux and precipitation. This storm system
produced bea'V)' precipitation over the Verde and Salt
River Basins in central Arizona. Strong southerly flow
with moist ad~ection ahead of the storm produced a
significant regjon of lifting and condensation upwind
of the Mogollon Rim. Note the detailed structure
analyzed by MM4 between the CCM grid points,
which are shown by large dots in Figures 2 and 3.

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Fig. 2. Airflow field from the MM4 on January 18, 1979,
at 0000 me showing wind vectors at grid points with
contours ofv~)mponents of the wind (m 5-1) for 700-mb
level (a) and 500 m above the ground (b). CCM grid point
locations are shown by the large dots.