WMOD00493

• Title: Nested Model Simulations of Regional Orographic Preocipitation
• Date: 1/17/1993
• Weather Modification - Doc Type: Report

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. Paper presented at the American Meteorological Society's 4th Symposium on Global Change
Studies, Anaheim, €A January 17..22, 1993.

NESTED MODEL SIMULATIONS of REGIONAL OROGRAPIDC PRECIPITATION

David A. Matthews,
U. S. Bureau of Reclamation, Denver, CO 80225-0007

Gary Bates, and Filippo Giorgi
National Center for Atmospheric Research
Boulder, CO 80307

1. INTRODUCTION

This study is part of Reclamation's (U. S. Bureau
of Reclamation) research in its GCCRP (Global Climate
Change Response program), which includes a collaborative
effort described by Dennis (1991), Matthews et al. (1991),
and Medina (1991). This collaboration involves scientistS
from the NCAR (National Center for Abnospheric Research)
and the GS (U. S. Geological Survey). The GCCRP is
designed to detennine the effects of climate change on water
resources in selected western drainage basins; Reclamation
and GS are jointly examining the precipitation and
hydrologic characteristics of the Gunnison River Basin to
detennine streamflow properties and reservoir management
needs in present and future climates. This drainage basin is
typical of other headwatem for the Upper Colorado River and
other major river systems in the West. Therefore, to
detennine the effects of climate change on precipitation,
Reclamation is modeling the physical niechanisms that
produce precipitation and thereby describe the spatial and
temporal evolution of precipitation from various types of
winter and summer precipitation events. This paper briefly
outlines a nested ~odeling approach to examin,e
precipitation over the Rocky Mountains in Colorado and
presents an assessment of a regional model's precipitation
simulations in complex terrain. The paper identifies the
need for a nested regional/local-scale modeling system to
accurately simulate local-scale precipitation processes over
complex terrain.

Reclamation's GCCRP modeling approach
involves two phases. Phase 1 evaluates the nested
regional/local-scale modeling method in current climate to
determine its capability to accurately simulate existing
conditions in well-documented cases. In phase 1, the
regional model bas been iDitialized by observed data sets and
run in a climate simulation mode for 40 months. As
confidence in the large-scale, general circulation model
results improves, phase 2 will apply the nested modeling
approach to future climate simulatioDS that include effects
such as doubled C02. lOOse simulations will use a general
circulation model, a regional model, and a local-scale modeL

In phase 2,the general circulation model will provide
synoptic-scale information that initializes 1the regional
model, wbich will simulate the regional evolution of storm
structure. Then a local-scale model will use the regional
analyses for its initial conditions and simulate local-scale
precipitation with high-resolution (dx=-5 to 10 km)
topography. This paper focuses on research in phase 1 - the
validation of the regional model and early stages of testing a
sophisticated local-scale, time-dependent model.

2. THE NESTED MODELING APPROACH .
Current Climate Simulations

In phase 1, the nested modeling approach uses a
regional model to provide the regional-scale forcing that
largely controls the evolution of local-scale clouds and
precipitation that is simulated by a local-scale model. The
regional model is the MM4 (Pennsylvania State
University/NCAR Mesoscale Model Version 4). The
standard ~4 model is described in Anthes et al. (1987);
however, in this study we use results from an augmented
version for regional climate studies described by Giorgi and
Bates (1989). This version of the MM4 model includes a
sophisticated surface physics/soil hydrology package. The
model is a compressible and hydrostatic model with
equations of motion written in terrain-varying sigma vertical
coordinates.

Our study focuses on the Gunnison Basin in
western Colorado marked by the bold square in Figure 1
nested within the domain analyzed by MM4. The MM4
model provides the large-scale (synoptic) and regional-scale
(mesoscale) foICing as shown by the summer monsoon flow
over the West in Figure la and corresponding 24-h
precipitation in Figure lb. Local-scale model simulations
are examining the detailed evolution of clouds and
precipitation within the 440- by 44O-km domain (dr-:lO km)
marked by the bold square in Figure la. Within this area,
the dashed square defines the nested 5-km resolution, 260- by