Analysis and testing of a winter orographic precipitation model
Date
1991-05
Authors
Branson, Mark D., author
Department of Atmospheric Science, Colorado State University, publisher
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Abstract
In the mid-1970's, an orographic precipitation model was developed by J. Owen Rhea in an effort to determine the ability to diagnose the effect of topography on winter precipitation for western Colorado. The model was tested for various time periods for differing wind regimes using upper air data and a fine-mesh topographic grid. The model is two-dimensional, steady state and multi-layer. Computations follow parcels at layer mid-points through topographically-induced moist adiabatic ascents and descents. The Lagrangian coordinate system allows for consideration of precipitation shadowing effects by upstream barriers. The model was originally tested for 13 winter seasons and the results were well correlated to observed values of snowpack water equivalent and spring and summer runoff. Although large discrepancies often existed between model and observations on a daily basis, the model frequency distribution of daily precipitation totals was realistic. This study attempted to update and improve the historical comparisons of model calculations to observations and also investigate the application of the model to current-season snowpack diagnosis and prediction. Model calculations were performed for the most recent 15 years of upper air data in addition to the 12 original seasons previously analyzed by Rhea (1978), and the correlation coefficients for model calculated precipitation values and the three observational types maintained good agreement throughout the 27 year historical period. Model calculations using an extended model winter season for the same 27 year period improved these comparisons for the precipitation gauges but had a slightly negative effect on the snowcourse and streamflow runoff relationships. When pre-model and post-model season observed precipitation data were included in the regression analysis for small basin streamflow runoff, some dramatic improvement in the correlations were noted in a few cases. The application of the model for "real-time" diagnosis of the seasonal snowpack was tested in the 1989-90 season and the results were comparable to the Soil Conservation Service predictions. Model calculations utilizing National Meteorological Center (NMC) gridded data as input were performed as a case study and the results were similar to the model calculations utilizing upper air data as well as to the observed precipitation values. The positive results of this study encourage further use of the model for "real-time" snowpack monitoring. Further case studies should be performed to test the model's ability as a predictive tool. The application of interfacing the model to a hydrological process model coupled with improvements such as the use of finer scale topography might further improve spring and summer runoff predictions.
Description
Includes bibliographical references.
May 1991.
Figure 8 on text page 30 is missing from original.
May 1991.
Figure 8 on text page 30 is missing from original.
Rights Access
Subject
Orographic clouds
Precipitation (Meteorology)