WMOD00584

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

Our study focuses on the Gunnison Basin in western Colorado marked
by the bold square in Figure 2 nested within the domain analyzed by MM4.
TheMM4 model provides the large-scale (synopti.c) and regional-scale
(mesoscale) forcing as shown by the summer monsoon flow over the West in
Figure 2a and corresponding 24-h precipitation in Figure 2b. Local-
scale Clark model simulations will examine the detailed evolution of
clouds and precipitation within the 440-, by 440-km domain (dx=10 km)
marked by the bo1d square in Figure 2a. Within this area, the dashed
square defines the nested 5-km resolution, 360- by 360-km domain. Small
dots show each MM4 grid point and large dots indicate the CCM(Conununity
Climate Model) R-15 (rhomboidal' spherical harmonic truncation at wave
number 15) 500-km resolution grid points. Clearly, regional-scale
models are required to capture valuable mesoscale airflow and
thermodynamic information.

Reclamation is using two models to simulate local orographic
precipitation: Rhea and Clark models. Medina (1992) has adapted the
steady-state Rhea model (Rhea, 1977) to simulate local precipitation
. over the Gunnison River Basin in Colorado and has adapted the model to
use initial conditions from soundings derived from MM4. This
approximates a one-way, local-scale nesting. Comparisons between Rhea
model simulations from rawinsondes and MM4-derived soundings are
encouraging; however, convective precipitation is not accurately
simulated in either the MM4 or Rhea models.

Therefore, we are also using the more sophisticated, three-
dimensional, time-dependent, local-scale Clark model to simulate the
local evolution of convective clouds and precipitation over complex
terrain in the centra'l Colorado Rockies. The Clark model is used
extensively throughout the research community to simulate phenomena
ranging from airflow over complex terrain to deep convection (Clark,
1979). The model is an anelastic, finite difference model with one- and
two-way interactive nesting described by Clark (1977), and Clark and
Hall (1991). It uses a terrain-following coordinate system to
accurately describe orographic effects on airflow. It includes warm
rain and ice microphysical parameterizations that explicitly simulate
precipitation development. The Clark model will be used to understand
the complex three-dimensional airflow and evolution of clouds and
precipitation in a limited sample of cases. It will contribute to
better parameterizations of airflow structure for the Rhea model, which
may be run on a large number of cases. Initial Clark model simulations
are examining model sensitivity to initial conditions and increased
resolution of complex terrain in western Colorado using rawinsondes and
MM4 soundings. Later,' a four-dimensional dynamic data assimilation
method, currently under development by scientists at the MMM (Mesoscale
and Microscale Meteorology) Division of NCAR, will. be used to assimilate
the MM4 dynamics into the Clark model initializati.on process.

INITIALIZATION AND ASSESSMENT OF MODE:L SIMULATIONS

Giorgi and Bates (1989) began a validation of the CCM-MM4 nested
model'ing approach using observations for 40 months from January 1982 to
December 1983, and January 1988 to April 1989. They used initial
conditions from ECMWF (European Center for Medium-range Weather
Forecasts) analyses of observed data gridded to a spatial resolution of
2.8 by 2.8. and having temporal resolution of 12 h. MM4' s lateral