WMOD00494 - Laserfiche WebLink

Home Browse Search

"""'. ..;J .'1 .,. .. Similar convective problems occurred in the simulations of the 1988-89 dry period, but cumulative totals were significantly less in the Gunnison. MM4 -model simulations started on January 1, 1988, and continued through April 1989. When comparing 1982-83 with 1988 analyses in Figure 2b, note that results are from different periods when simulations were available. Winter cumulative precipitation was reasonably well modeled; however, from mid-May to September, convective precipitation Wa.$ poorly estimated. Figure la also shows an example of summer simulations of the strong summer monsoon flow predicted by MM4 on July 20, 1983, and the resulting 24-h precipitation field (Fig. 1b) predicted over the West. The maximum of 5.25 cm over Colorado illustrates the overprediction problem in mountainous regions. Precipitation measurements in the Gunnison Basin indicated an area average of 0.2 cm with a maximum of 1.0 cm on July 20, 1983, hence, the need for Clark model simulations to improve convective parameterizations for both MM4 and Rhea models. Summary and Future Studies This paper presents an overview of Reclamation's approach to nested numerical modeling of precipitation in western Colorado for GCCRP research. This collaborative study with NCAR scientists indicates that the regional MM4 model provides useful climate information for regional-scale climate simulations; however, nested local-scale models are needed to resolve local effects of terrain on precipitation. Time series of observed high elevation (> 2700 m msl) and regional model predicted daily cumulative precipitation matched reasonably well during winter months of 1982-83; however, convective periods diverged significantly. This suggests that MM4's regional climate simulations may provide useful estimates of winter cumulative precipitation. A different pattern occurred in 1988 with a good match of model predictions and observations below 2100 m msl during January and February; however, this pattern diverged from March to September. We are currently examining the sources of this problem. Next, we will perform higher resolution nested Clark model simulations and will begin simulations of summer monsoon conditions for more complex convective clouds and precipitation. Acknowledgements The computing was performed at NCAR, which is sponsored by the National Science Foundation. Mr. Ra Aman, Ms. Karen Morris and Ms. Anne Reynolds of Reclamation's Water Augmentation Group, contributed to analysis software development arid data processing. Messrs. John Lease and Curt Hartzell provided helpful suggestions regarding the manuscript. 7 Matthews et al.