Abstract : F.2
An observational and numerical study of a regional-scale downslope flow in northern Arizona

Sharon Zhong, Crosby Savage
Michigan State University

During the month-long METCRAX field campaign in northern Arizona near Winslow in Oct. 2006, observed wind profiles revealed a frequent presence of a near- surface wind maximum on nights with relatively quiescent synoptic conditions. The development of jet-like profiles usually began shortly after sunset with a shift from whatever wind directions in late afternoon to a southwesterly flow and continued through the night. The peak wind speed was typically between 4-6 m s-1 and was within the lowest 50 m above the ground surface with wind speed decreasing with height. The observational data from various measurement platforms were used to characterize the structure and evolution of this phenomenon. The data included 15 min-averaged vertical profiles of wind and temperature from near surface to approximately 350 m above from a sodar with radio acoustic sounding system (RASS), rawinsonde soundings launched every three hours between 3 pm to 9 am on seven nights, hourly wind and temperature profiles from 100 m to more than 2000 m from a 915 MHz radar wind profiler/RASS, and surface weather data from a 10-m tower. To help identify the forcing mechanisms, high resolution numerical simulations were performed using the Regional Atmospheric Modeling System (RAMS). The model results, which compared reasonably well with the observations, suggest that this low-level wind maximum represents a regional-scale downslope flow developed over the Coconino Plateau that slopes up towards southwest of the observational site near Winslow. Several sensitivity simulations were performed to help understand the role of various forcing mechanisms play in the development of this flow and the interactions of this regional-scale terrain-induced flow with synoptic as well as local forcing.