Model simulations of extreme orographic precipitation in the Sierra Nevada during the New Year's holiday flood of 2005-06

Forecasting precipitation in mountainous regions is a very demanding and arduous task. Some examples of devastating flooding events in mountainous areas include the Big Thompson Flood of 1976 in the Rocky Mountains (Maddox 1978) and the Piedmont flood event of 1994 in the Alps (Buzzi et al. 1998). The flooding events in the Sierra Nevada during 1997 and 2005 had a major impact on the Truckee River Valley including the Reno metropolitan area, however not much has been documented with either case. Both flooding events in Reno involved the passage of upper-level cold fronts that enabled ageostrophic and diabatic adjustments to create processes that led to flooding rains. It is the juxtaposition of the cold front aloft, tropical air stream from the South Pacific and complex terrain that led to the extreme lee side liquid precipitation accumulation to a significant elevation. This paper focuses on the 2005 event. A verification of the simulation of the 2005 case study employing the Operational Multiscale Environment model with Grid Adaptivity (OMEGA) (e.g., Bacon et al. 2000) was performed. What makes OMEGA a novel approach for modeling research and forecasting is that it employs an unstructured grid that can be used to adapt to certain features like clouds and terrain, therefore enhancing the local resolution of key orographic forcing features. The OMEGA adaptive grid simulations were performed with static grid adaptivity to 1 km resolution over the Sierra Nevada. These simulations were then validated against asynoptic and synoptic observations including Doppler and surface rainfall observations. The goal for this research is to 1) explore whether or not an unstructured, static, adaptive grid will produce accurate simulations of an extreme lee side rainfall event and 2) to aid forecasters in understanding the precursor physical and dynamical processes which cause such an event.