Terrain-induced Rotor Experiment (Invited)

The Terrain-induced Rotor Experiment (T-REX) is the second phase of a coordinated effort to explore the structure and evolution of an atmospheric rotor coupled system. Atmospheric rotors are intense low-level horizontal vortices that form along an axis parallel to, and downstream of, a mountain ridge crest, and are strongly coupled to both the overlying mountain waves as well as the underlying boundary layer. The field campaign of the Terrain-induced Rotor Experiment (T-REX) took place in March and April 2006 in Owens Valley, CA, which was also the location of the Sierra Rotors Project, the initial phase experiment in March and April 2004. Owens Valley lies to the east of the southern Sierra Nevada, which is the tallest, steepest, quasi two- dimensional topographic barrier in the contiguous United States. Mountain waves and attendant rotors are known to reach particularly striking amplitude and strength there.

The main scientific objective of T-REX is a comprehensive study of the coupled mountain-wave--rotor--boundary-layer system. Complimentary scientific issues include stratospheric-tropospheric exchange (STE) and structure and evolution of the complex-terrain boundary layer in the absence of rotors. Resulting from the need to document atmospheric conditions and processes from the Earth surface up to the upper-tropospheric/lower-stratospheric altitudes, the T-REX field campaign had both a comprehensive ground-based and an airborne observing program. The ground-based observing platforms were located on both sides of the high Sierra Nevada, with the majority of instrument platforms positioned near the town of Independence in Owens Valley in the Sierra Nevada lee. These include Doppler and aerosol lidars, wind profilers, sodars, networks of automatic weather stations, soil moisture and temperature sensors, flux towers, and radiosonde operations. The three aircraft involved in the airborne program were NSF/NCAR HIAPER, UK FAAM BAe146, and the University of Wyoming King Air, which were involved in a number of two- and three- aircraft coordinated missions.

An anomalous weather pattern that persisted throughout March and April 2006 resulted in an unusually large number of West coast weather systems, which generated conditions conducive for mountain wave and rotor formation over Owens Valley. Fifteen Intensive Observing Periods that documented the coupled rotor and boundary layer system were carried out during the two-month field campaign. In addition, five Enhanced Observing Periods were conducted to observe the quiescent boundary layer evolution in the absence of rotors. The comprehensive T-REX data set represents a unique opportunity to address basic scientific questions motivated by T-REX, and will provide crucial observations for numerical model validation and ultimately will enhance the capability of mesoscale and microscale modeling of processes in complex terrain. The results of the T-REX research effort are expected to be instrumental in achieving further improvements in prediction of aviation hazards, downslope windstorms, and aerosol transport and dispersion.