Evaluation of high-resolution WRF ARW model simulations of atmospheric river events during HMT-2006

Significant precipitation events in California during the winter season are often caused by land-falling “atmospheric rivers” associated with extratropical cyclones from the Pacific Ocean. Atmospheric rivers are elongated regions of high values of vertically integrated water vapor over the Pacific and Atlantic oceans that extend from the tropics and subtropics into the extratropics. The high values of integrated water vapor are advected into the extratropics within the warm sector of extratropical cyclones. The warm sector is characterized by warm temperatures, high moisture content and strong winds at lower altitudes. When an atmospheric river makes a landfall on the coast of Califronia, the northwest to southeast orientation of the Sierra Mountain chain will exert forcing on the low-level flow in the warm sector of approaching extratropical cyclones. As a result, sustained precipitation will be enhanced and modified by the complex terrain. Due to the steep terrain and soil characteristics in the area, a high risk of flash flooding and landslides can be associated with even moderate precipitation events. The importance of a good understanding and numerical prediction of these events is further amplified by the fact that the lower elevations of the Central Valley are areas of a large urban and infrastructural expansion. To support and facilitate observational planning for field experiments to better understand and forecast precipitation in the Central Valley, the National Oceanic Atmospheric Administration (NOAA) has established the Hydrometeorological Testbed (HMT) which designs and supports series of field experiments. One important aspect of the HMT research is to understand microphysical processes behind the orographicaly-induced/enhanced precipitation and improve the microphysical parameterizations in numerical weather prediction models. The main goal of the present study is to evaluate the performance of the WRF ARW numerical model using four different microphysical options in the case of five intense precipitation events. For this purpose, high resolution (3-km horizontal grid spacing and 32 vertical levels) WRF ARW model simulations will be used. Four different microphysical options include Lin, WSM6, Thompson and Schultz schemes. The events used in the study occurred during the 2005-2006 HMT winter field project. For the model performance validation, several different observational datasets are going to be used. Model produced rainfall will be compared to data obtained from the rain gauge systems in the area of interest. The model ability to simulate bright band versus non-bright band precipitation will be evaluated by comparing the model reflectivity to 2-minute reflectivity data from the S-band vertically pointing radar. For the analysis of hourly averaged wind profiles, data obtained from 915-MHz wind profilers will be used. Preliminary results of the study indicate that there is significant disparity among the various schemes. Analysis of the reasons behind the disparity will be provided at the presentation.