Experimental Forecasts of Mountain Waves For The Terrain-induced Rotor Experiment (T-REX)

Abstract

The main objective of the Terrain-Induced Rotor Experiment (T-REX) is to understand the nature of the coupling of mountain-induced rotor circulations to the structure and evolution of overlying mountain waves and to the underlying boundary layer (Grubisic et al. 2004). T-REX results also aim at increasing the understanding of mountain wave dynamics, including their generation, propagation, and breakdown. The field phase of T-REX took place in March and April 2006 in the Owens Valley region, directly east of the southern Sierra Nevada, the tallest quasi-twodimensional mountain range in the conterminous US. The eastern escarpment from the crest of the southern Sierra to the Owens Valley consists of a drop in elevation of 3km in a distance of approximately 10km. Several groups ran high-resolution non-hydrostatic models in support of either the real-time or research objectives of T-REX. Of particular interest is the ability of the Weather Research and Forecasting (WRF) model to correctly predict mountain waves. The WRF model was developed to serve both research and operational needs. It currently supports two dynamic cores: the Advanced Research WRF (ARW) core developed at NCAR (Skamarock et al. 2005) and the Non-hydrostatic Mesoscale Model (NMM) core, developed at NCEP (Janjic et al. 2001). The WRF model is currently running in several applications at the National Centers for Environmental Prediction (NCEP): The North-American Mesoscale Model (NAM) employs the NMM core of WRF, in a domain that covers all of North America with a grid spacing of 12 km. The Short-Range Ensemble Forecast (SREF) has both NMM and ARW members, with a domain that covers much of North America and adjacent portions of the Pacific Ocean with a grid spacing of 32 km. The Hurricane WRF employs the NMM core. The High-Resolution Window (HRW) WRF has both NMM and ARW cores running with grid spacings of 5.1 and 5.8 km, respectively. Several regional domains are run: eastern CONUS, central CONUS, western CONUS, Puerto Rico, Alaska, and Hawaii. The ARW and NMM cores of WRF differ in a variety of aspects, such as: map-projection, gridstaggering, vertical coordinate, numerical solver, diffusion, divergence damper, and top boundary condition. It is worth noting that, while the NMM has no special treatment of diffusion near the top boundary, the ARW was configured with a 5-km deep layer near the top boundary where extra diffusion was added to reduce wave reflection. The objectives of this study are to compare and contrast the ARW and NMM forecasts, and to compare those forecasts against in-situ aircraft observations collected during the field phase of TREX. Two Intensive Observation Periods (IOPs) were chosen for this comparison: IOP 10 and IOP 13, in which weak and strong mountain waves were observed, respectively. The observational datasets used in this study are in-situ observations taken by the National Science Foundation High-performance Instrumented Airborne Platform for Environmental Research (HIAPER) aircraft and by the United Kingdom Natural Environmental Research Council UK BAe146 aircraft, which is jointly managed the UK Met Office and the consortium of UK universities.