Ongoing experiments to improve cloud and precipitation forecasts from the WRF NMM modeling system

A series of sensitivity experiments using the Weather Research and Forecasting Nonhydrostatic Mesoscale Model (WRF NMM) will be presented as part of the ongoing effort at the Environmental Modeling Center (EMC) to replace the Eta model with the WRF NMM model as the operational North American Mesoscale (NAM) model from the National Center for Environmental Prediction (NCEP). These experiments are designed to gain a better understanding of how physical parameterizations (esp. cloud microphysics, convection, and surface-boundary layer) affect the fidelity of the temperature, moisture, and wind fields, particularly in regions of strong ascent where the potential for rapid destabilization will produce moist absolutely unstable layers (Bryan and Fritsch, 2000) in the model. Parallel runs of the WRF NMM as of early spring were producing areas of supersaturation with respect to water, namely as a result of cloud microphysical updates occurring less frequently than the fundamental advection time step. Increasing the frequency of physics improves the prediction of in-cloud relative humidities, but these preliminary runs also suggest that a high bias in forecast precipitation is more likely to occur. A variety of strategies will be investigated in an attempt to improve cloud and precipitation forecasts from the NCEP regional modeling system.