SENSITIVITY OF SIMULATED SUMMERTIME PRECIPITATION OVER THE WESTERN UNITED-STATES TO DIFFERENT PHYSICS PARAMETERIZATIONS

This paper investigates the effect of different physics parameterizations on summertime precipitation as simulated by the Pennsylvania State University-National Center for Atmospheric Research (NCAR) Mesoscale Model (MM4). The period of simulation is July 1979. The control simulation is carried out with a standard version of the MM4 including a simplified Kuo scheme, a bulk boundary-layer representation, and a force-restore scheme for ground temperature calculation. The parameterizations tested are a modification of the standard MM4 Kuo scheme, which imposes storage and slow release of condensation heat, an explicit moisture scheme, a version of the Arakawa-Schubert scheme, and a relatively sophisticated hydrology package. The standard MM4 strongly overestimates precipitation over mountainous terrain and, in particular, it produces an excessively large number of gridpoint precipitation events in excess of several centimeters ("numerical point storms" or NPS's). These are due to intense vertical motions maintained by large and localized condensation heat release. Both the explicit moisture scheme and the modified Kuo scheme reduce precipitation and number of NPS occurrences, leading to an improvement of the overall precipitation simulation skill of the model. Compared to the Kuo scheme, the Arakawa-Schubert scheme produces less convective precipitation, but still overestimates total precipitation and number of NPS occurrences due to interactions with the resolvable-scale precipitation processes. The inclusion of the enhanced surface physics formulations significantly affects the simulation of surface fluxes, temperatures, and precipitation. Compared to previous wintertime precipitation simulations, the present summertime precipitation results show generally higher biases, lower threat scores, and greater sensitivity to physics parameterizations and local moisture sources.