WMOD00584

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

J

Presented at the American W~r Resources Association
Symposium on Managing Water Resources During Global Change, Reno, Nevada: Nove~ber 1-5, 1992

USE OF NESTED MODELS TO SIMULATE REGIONAL OROGRAPHIC
PRECIPITATION

David A. Matthews,l Gary Bates,2 and Filippo Giorgi2

ABSTRACT: Reclamation's (U. S. Bureau of Reclamation) GCCRP (Global
Climate Change Response Program) is concerned with the possible impacts
of climate change upon precipitation, evapotranspiration, and streamflow
in the Western United States. This paper presents an assessment of
regional model simulations of the unusually heavy precipitation in the
El Nino of 1983, and the dry 1988-89 period. Comparisons of the
observed winter orographic precipitation and regional model simulated
precipitation indicate good regional predictions; however, summer
convective precipitation was significantly overestimated. Comparisons
of observed and modeled time series of daily cumulative precipitation
indicate that MM4 provides useful climati,c information in the Gunnison
Basin. However, the timing of modeled 24-h precipitation events is
frequently out of phase with that observed, and precipitation spatial
distributions do not have the resolution needed for local-scale
hydrologic modeling. Results clearly show the need for high resolution,
nested regional/local-scale modeling of precipitation. An example from
the Clark model is presented that shows the detailed development of
local-scale precipitation.
KEY TERMS: climate change, El Nino, precipitation, regional numerical
modeling, local-scale nested modeling, water resources management

INTRODUCTION

This study is part of Reclamation's research in its GCCRP, 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 Atmospheric Research) and the GS (D.
s. Geological Survey). The GCCRP is designed to determine 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 determine
streamflow properties and reservoir management needs in present and
future climates. This drainage basin is typical of other headwaters for
the Upper Colorado River and other major river systems in the West.
Therefore, to determine the effects of climate change on precipitation,
Reclamation is modeling the physical mechanisms that produce
precipitation and thereby describe the spatial and temporal evolution of
precipitation from various types of winter and summer precipitation

1 Water Augmentation Group, u. S. Bureau of Reclamation, Denver, CO 80225-0007

2 Climate and Global Dynamics Division, National Center for Atmospheric Research,
Boulder, CO 80307