The Fifth Generation Penn State / NCAR Mesoscale Model was utilized to simulate a storm system which affected the IMPROVE 2 study area during 13-14 December 2001. Extensive verification was performed to compare the model depiction with the comprehensive array of observational assets available during the time period, including in situ plane measurements, profilers, upper air radiosonde measurements, P3 Dual Doppler, ground based radar, and surface observations. The comparisons indicate the model requires a 1.33 km grid spacing to accurately capture complex mesoscale forcings related to terrain features. Despite the models relatively accurate portrayal of mesoscale and synoptic forcings a significant overprediction of precipitation occurs on the windward slopes of the central Oregon Cascades which is consistent with previous documented bias of Quantitative Precipitation Forecasts (QPF) in mesoscale models.
To understand the possible cause of the overprediction, certain model microphysical pathways and parameters are assessed and compared with the observed microphysics. Cloud liquid water (clw) is overpredicted on the windward slopes despite the fact the mesoscale forcings appear to be accurately modeled. The impact of this model overprediction of clw on the QPF along the windward slopes and its interaction with other microphysical processes is assessed and compared with observations. In addition, the model's mixing ratios and assumed exponential number distributions for precipitation sized hydrometeors are compared and validated against the in-situ observed hydrometeors at various heights during the precipitation event. Initial comparisons indicate the model is overpredicting the snow mixing ratio and underpredicting the number of larger snow particles at lower heights.