Terrain-induced Rotor Experiment - T-REX

Vanda Grubisic, Desert Research Institute, Reno, NV, USA

Terrain-induced Rotor Experiment (T-REX) is the second phase of a coordinated effort to explore the structure and evolution of atmospheric rotors and associated phenomena in complex terrain. Atmospheric rotors-intense low-level horizontal vortices that form along an axis parallel to, and downstream of, a mountain ridge crest in association with large-amplitude mountain waves and windstorms-are known to pose a great hazard to aviation. T-REX has been designed to take advantage of the newest advances in remote sensing technology, atmospheric numerical modeling, and our understanding of boundary-layer processes to study the complex rotor coupled system (Grubisic et al. 2004).

The core scientific objectives of T-REX are focused on improving the understanding and predictability of the coupled mountain-wave/rotor/boundary-layer system. This core objective includes the role of the upstream flow properties in determining the dynamics and structure of the rotor coupled system, wave/rotor dynamic interactions, internal rotor structure, rotor/boundary-layer interactions as well as the upper-level gravity breaking and turbulence. In addition, complementary scientific objectives are focused on understanding the role of mountain waves in stratospheric-tropospheric exchange, structure and evolution of the complex terrain boundary layer in the absence of rotors, and wave cloud phase transitions and layering.

T-REX will take place in March and April 2006, in Owens Valley in the lee of the southern Sierra Nevada (Fig. 1), where also exploratory Phase I (Sierra Rotors) field activities took place in early spring 2004. This portion of the Sierra Nevada is the tallest, quasi two-dimensional topographic barrier in the contiguous United States with a number of peaks above 4 km, including the highest peak in the lower 48 states (Mt. Whitney 4,418 m), and the steepest lee slopes (~30 degrees). The ~3 km high White-Inyo range forms the eastern wall of the valley. The site of the experiment will be the central portion of this valley, near town of Independence.

T-REX experimental program has two main observational thrusts:

1. Comprehensive ground-based and airborne, in situ and remote sensing measurements during strongly perturbed conditions favoring rotor formation, and
2. Comprehensive observations of complex-terrain boundary layer structure and evolution from undisturbed to strongly perturbed conditions.

Measurements will be conducted both upwind and within Owens Valley. Three research aircraft will be involved in T-REX (NSF/NCAR HIAPER, University of Wyoming King Air, UK BAe146). This will be the first field experiment for the new NSF/NCAR HIAPER aircraft. In addition to probes for standard in situ measurements, all three aircraft will be equipped with dropsonde systems, whereas HIAPER and UK BAe146 will be equipped also with atmospheric chemistry instruments and microphysics probes. An array of fixed and mobile ground-based instruments, including Dopper and aerosol lidars, wind profilers, sodars, sounding systems, dense networks of automatic weather stations, microbarographs, temperature data loggers, flux towers, time-lapse photography systems, and an instrumented car, will be used to document the lower portions of the rotor coupled system under strongly perturbed conditions favoring rotor formation as well as the flow and thermodynamic structure of the boundary layer in absence of rotors. Special upstream GPS radiosonde soundings sites will be located in the Central Valley of California. Figures 2 and 3 show the flight-plan schematic, and the planned T-REX ground-based instrumentation. The field operations will be supported by real-time mesoscale model forecasts, and ensuing field research will be tightly coupled with numerical modeling studies.

T-REX participants include investigators from a large number of US universities and agencies, the National Center for Atmospheric Research, and several European universities and research institutes. The major funding for T-REX is being provided by the National Science Foundation (US). A number of other US and European agencies have contributed to funding the T-REX observational and research effort including the Natural Environmental Research Council (UK), Office of Naval Research (US), Federal Aviation Administration (US), and the Austrian Ministry of Science (AU).

For more information on T-REX, contact Vanda Grubisic (Vanda.Grubisic@dri.edu).