We present preliminary results of seven regional climate models participating in the Project to Intercompare Regional Climate Simulations (PIRCS). The goal of PIRCS is to advance the state of regional climate modeling by a series of systematic intercomparisons. PIRCS Experiment 1 includes simulations of the drought of 1988 and the regional flood of 1993 in the central U.S.
Here we focus mainly on preliminary results for precipitation processes during the 1988 drought. Detailed analysis of precipitation is presented for a portion of the Upper Mississippi River basin that provides a contrast between 1988 and 1993, since it includes the peak precipitation area of the 1993 flood. The region also is relatively well resolved from the standpoint of a regional model but marginally resolved in terms of a GCM or the NCEP/NCAR Reanalysis.
All models capture the lateral boundary forcing as indicated by the timing of precipitation episodes. Evaluation of model performance for different types of events gives a perspective on how the models handle different types of forcing (e.g., large-scale versus convective or mesoscale). The early part of the period was dominated by large-scale synoptic events; all models captured the timing of these. A subsequent dry period broken by a mesoscale convective system also was represented by all models. The models occasionally differ on whether precipitation is represented as convective or stratiform. During a two-week period of isolated convective events all models give convective precipitation dominating although timing did not always correspond exactly to observations. These results suggest that large-scale precipitation episodes are predicted deterministically by the models, with all models giving essentially similar results. Scattered convective precipitation is represented more statistically, with variation from model to model in the timing and geographical distribution of precipitation.
Analyses are being conducted of specific precipitation processes in the models. Preliminary results indicate that the models are able to represent organized mesoscale convective systems. Specifically, the low-mid level cyclonic inflow and upper level cyclonic outflow that are characteristic of MCCs were found during precipitation episodes in several models. Comparison with wind profiler data indicates that the nocturnal low-level jet, which is the major environmental influence on summertime precipitation in the target region, is represented much more accurately in the regional models than in the NCEP/NCAR Reanalysis. Further diagnostics include equitable threat and bias scores for quantitative precipitation thresholds as compared with observations.
NOTE: Not all authors could be listed because of the limitations of the electronic modification form. Full author list is R. Arritt, W.J. Gutowski, Z. Pan, R. Silva, E.S. Takle, D. Caya, S. Chen, J.H. Christensen, S.-Y. Hong, J. Katzfey, W. Lapenta, R. Laprise, P. Lopez, J. McGregor, and J. Roads