An evaluation of the NCEP RSM for regional climate modeling

In this study, the National Centers for Environmental Prediction (NCEP) Reg ional Spectral Model(RSM) has been evaluated as a means of enhancing the de piction of regional details beyond that which is capable in low-resolution global models. Three-month-long simulations driven by the NCEP-National Cen ter for Atmospheric Research 40-yr reanalysis data are conducted with a hor izontal resolution of about 50 km over the United States, for the two winte rs and summers. The selected winter cases are December-February (DJF) 1991/ 92 (warm eastern Pacific SST anomalies) and DJF 1992/93 (normal eastern Pac ific SST anomalies). Summer cases are May-July (MJJ) 1988 (a drought in the Great Plains) and MJJ 1993 (a Flooding). Overall, the results from the model an very satisfactory in terms of the pr ecipitation distribution for different seasons as well as the representatio n of large-scale features. Evaluation of simulated large-scale features rev eals that the model does nor exhibit a discernible synoptic-scale drift dur ing the 3-month integration period. irrespective of the seasons. Surprising ly, the model simulation is found to correct some biases in the large-scale fields that exist in the reanalysis data. This bias reduction is attribute d to the improved depiction of physical processes within the RSM. This find ing indicates that one should rake special care in the interpretation and v alidation of simulated results against the analyzed data. Evaluation of the RSM simulated precipitation for the winter and summer cas es generally agrees with results obtained from previous studies. For instan ce, the skill for simulated precipitation in the winter cases exceeds that of the summer cases by a factor of 2. Comparison of simulated precipitation with observations reveals the 3-month-long RSM simulated precipitation to be more skillful than that obtained from the reanalysis data (the 6-h forec ast from the data assimilation system). In addition to seasonal variations in precipitation. daily variation in the simulated precipitation is quite g ood. However, detailed analysis point:, to the need for further RSM develop ment, particularly in physics. In the summer cases the grid-resolvable prec ipitation physics simulate excessive precipitation over the northern United States. A more serious problem is found in the diurnal cycle of the simula tion precipitation, in that the model initiates convection too early. Despi te these deficiencies, it is concluded that the NCEP RSM is a very useful t ool for regional climate studies.