CURRENT ISSUE: November 2005
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Sierra Rotors As Big As Dinosaurs
By Dr. Vanda GrubisicDesert Research Institute
Chair, T-REX Scientific Steering Committee
The Terrain-induced Rotor Experiment (T-REX) is the second phase of a multi-year effort to explore the properties of atmospheric rotors and associated phenomena in complex terrain (1). Atmospheric rotors are 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, similar to a tornado on its side. High-levels of turbulence in atmospheric rotors can pose a great hazard to aviation. The initial, exploratory phase was the Sierra Rotors Project, whose observational period was completed in spring 2004. The internationally supported T-REX will take place in March and April 2006 in the same location of the Sierra Rotors field activities, in Owens Valley in the lee of the Sierra Nevada in California.
Figure 1. Rotor during Sierra Rotors Project, spring 2004. Photo by Vanda Grubisic, http://www.mdml.dri.edu/iops/iop5.html
The western wall of the valley, the High Sierra, is the tallest, quasi two-dimensional topographic barrier in the contiguous United States with a number of peaks above 4 km, including Mt. Whitney at 4,421 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 near Independence, where mountain waves and attendant rotors have been known to reach particularly striking amplitude and strength. This was also the site of the 1950’s Sierra Wave Project (2). Climatological studies show that the months of March and April have the highest frequency of rotor events, including many days with conditions favorable for generation of mountain waves and rotors, and also many days when it will be possible to document terrain-induced boundary-layer circulations in the valley under more quiescent conditions.
Recent numerical, theoretical, and observational studies of rotors show that rotors are strongly coupled to the structure and evolution of overlying mountain waves and the underlying boundary layer. Consequently, 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 objective includes elements of the dynamics and structure of rotors and their interactions with higher mountain waves and the lower boundary layer. In addition, complementary objectives are focused on understanding the role of mountain waves between atmospheric levels, the atmospheric conditions without rotors, and wave cloud evolution.
In order to achieve its main scientific objective, the T-REX program has two main observational foci:
Figure 2. Rotor during Sierra Rotor Project, spring 2004. Photo by Alex Reneicke, http://www.mdml.dri.edu/iops/iop14.html
The first observational focus measurements will be conducted both upwind and within the valley. T-REX plans include three research aircraft equipped with: a cloud radar, chemical instruments, microphysics probes, and dropsonde systems. This will allow documentation of the properties and evolution of the wave/rotor part of the coupled system over the valley as well as the structure of airflow through the lower atmosphere up- and down-stream of the Sierra Nevada. An array of fixed and mobile ground-based instruments to document the lower portions of the rotor coupled system under 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 in the Central Valley, together with the NOAA wind profilers located upwind of the study area, will provide critical information on the upstream conditions. This comprehensive set of measurements has been designed to document the three-dimensional nature of the coupled mountain-wave/rotor/boundary-layer system in unprecedented detail.
All field activities will be actively supported by a real-time dedicated numerical modeling effort. In addition to advancing the main scientific objective, T-REX data sets and findings are expected to yield further significant improvements in mesoscale and microscale modeling, and prediction of aviation hazards, downslope windstorms, and aerosol transport and dispersion.
References:
1. Grubišic, V., J. D. Doyle, J. Kuettner, G. S. Poulos, and C. D. Whiteman, 2004: Terrain-induced Rotor Experiment (T-REX) Overview Document and Experiment Design. 72 pp. Available at http://www.joss.ucar.edu/trex/.
2. Grubišic, V., and J. M. Lewis, 2004: Sierra Wave Project revisited: 50 years later. Bull. Amer. Meteor. Soc., 85, 1127-1142.
Other: WRCC Sierra Rotors surface network page: http://www.wrcc.dri.edu/trex/
Figure 2. Jones Tract levee after repairs, August 24, 2004. Credit: DWR, http://www.publicaffairs.water.ca.gov/newsreleases/2004/08-26-04floodpics.cfm
Page last updated 12/15/05.