The Terrain-Induced Rotor Experiment (T-REX) is a meteorological research program supported by the U. S. National Science Foundation (NSF). The program is investigating the structure and evolution of atmospheric rotors that form in the lee of the Sierra Nevada Range of California over the Owens Valley. Rotors are atmospheric circulations or eddies that form in the lee of a mountain ridge and rotate around a horizontal axis that parallels the ridgeline. These energetic circulations, which are hazardous to aircraft, form in connection with strong background flows that cross the ridgeline barrier. They are usually found in combination with mountain and lee waves.
Rotor flow in lee of a mountain barrier underneath a breaking wave region in a 'hydraulic flow' situation. From Dave Whiteman's book, Mountain Meteorology: Fundamentals and Applications.
A trained observer can identify the presence of a rotor if there is sufficient moisture in the atmosphere to produce the distinctive rotor cloud, which forms in the rising air in the ascending branch of the rotor circulation. The energetic overturning of the flow in a rotor can be seen clearly in a time lapse movie (courtesy of Dr. Stephen Mobbs, U. Leeds, UK) if a cloud is present, but there must be many events when rotors occur and clouds are not present so that the energetic overturning cannot be seen by pilots. There are, nonetheless, a number of phenomena that can give hints to the presence of rotors, including strong flows coming from directions nearly perpendicular to the mountain barrier as indicated, for example, by blowing snow or dust; the presence of a Chinook or Foehn wall cloud over the ridgeline; the presence of lenticular clouds at higher levels of the atmosphere; or an ongoing windstorm on the lee side of the barrier.
Dust being picked up off the Owens Valley floor by strong west winds coming from right to left as we look southward down the Owens Valley. The dust is being lifted into a rotor cloud on the left. A foehn wall cloud is seen over the crest of the Sierra Nevada to the right. Lenticular clouds are present in the atmosphere above. Photo by Robert Symons from the Sierra Wave Project in the 1950s.
The vertical motions in rotors and the associated turbulence can be very strong. In fact, in a previous meteorological research program called the Sierra Wave Project, conducted in the lee of the Sierra Nevada over the Owens Valley in the 1950s a research aircraft (glider) broke up in midair as it traversed a rotor. Fortunately, the pilot was able to parachute to safety. An exciting and very readable account of the 1950s experiments that I would highly recommend for the general reader is the book entitled Exploring the Monster: Mountain Lee Waves: The Aerial Elevator by Robert F. Whelan (available from Wind Canyon Books, Inc.)
The scientific goals of T-REX were laid out in a Scientific Overview Document that was put together for the National Science Foundation by the T-REX Scientific Steering Committee (SSC). Once the Scientific Overview Document was approved by NSF, scientists interested in participating in the research were encouraged to submit research proposals to the NSF. Proposals received by NSF were then sent to other scientists for review, as the first step in the process of selecting the best proposals that could contribute to the T-REX scientific program. Because the goals of the T-REX scientific program focus on a meteorological phenomena that is found in mountain ranges all over the world and because extensive national and international resources would be required to answer some of the scientific questions, some of the scientists have found funding from other agencies and from other countries. The U.S. National Center for Atmospheric Research (NCAR) will play a major role in the program as it provides equipment and organizational resources to scientists funded by NSF and will be supplying some of the expensive research equipment that cannot be supplied by individual scientists. These resources include research aircraft, lidars, radars, and atmospheric sounding systems of various types.
T-REX Scientific Steering Committee members and Advisors at a meeting at Yale University in the summer of 2005. Left to right are Banta, Poulos, Grubisic, Smith, Whiteman and Mobbs. The full membership of the Scientific Steering Committee includes Bob Banta (NOAA), Greg Poulos (NCAR), Vanda Grubisic (DRI), Stephen Mobbs (U. Leeds), James Doyle (Naval Research Laboratory) and Joachim Kuettner (UCAR). Ron Smith (Yale) and Dave Whiteman (U. Utah) are Advisors to the Scientific Steering Committee.
A summary of the T-REX scientific goals follows:
To improve the understanding of:
To achieve improvements in:
The extent to which these goals can be achieved depends on the success of the field experiment and on the scientific plans and productivity of the individual investigators who are funded for participation in T-REX. The results will be reported in peer-reviewed scientific papers authored by the T-REX investigators and published in scientific journals. Preliminary non-peer-reviewed results will be reported at conferences and in annual project meetings.
The T-REX field program was run during March and April 2006 in the southern Sierra Nevada in and upwind of California's Owens Valley. The March/April period, called the Special Observing Period (SOP), was chosen based on the climatology of rotor events, which have a maximum during these months (but also in November). A smaller rotor research program, the Sierra Rotors Project, was conducted in this region in the years 2004 and 2005 to install initial ground-based meteorological instruments and to address both operational and initial scientific questions to pave the way for T-REX. The Sierra Wave Project of the 1950s, in contrast, studied the development of mountain waves in the lee of the Sierra, without a focus on the rotors. The present focus on rotors is supported by great advances in meteorological instrumentation and numerical modeling that now allow, for the first time, the possibility of completing a successful study on this phenomenon.
California's Owens Valley - site of the Terrain-Induced Rotor Experiment (T-REX). Base map from the Color Landform Atlas of the US.
The instrumentation that can now be used to investigate rotors is quite extensive. A map of the ground-based instrumentation, as obtained from a map server at NCAR, is shown below. In addition to this equipment, three research aircraft were used to make in situ meteorological observations.
T-REX ground instrumentation. Click image for full resolution.
The Sierra Nevada-Owens Valley area is perhaps the best-known location in the world for producing rotors. The roughly north-south orientation of the Sierra crest is nearly perpendicular to the prevailing westerlies of the mid latitudes so that strong jet stream flows often cross these mountains. The shape of the mountain profile (a ramp up the windward side and a sudden drop-off in elevation into the Owens Valley on the leeward side of the crest) is thought to be especially conducive for forming mountain waves and rotors. It is also thought that the vertical temperature structure on the windward side plays an important role in producing waves and rotors. Unraveling this mystery is a key goal of the experiments.
The shape of the cross-mountain profile of the Owens Valley. The cross section shown is through Fresno, California. From Holmboe and Klieforth, 1957.
Dr. Vanda Grubisic at Desert Research Institute provides the scientific leadership for T-REX with input from a Scientific Steering Committee (see above), special assistance from Dr. Joachim Kuettner (NCAR) and Dr. Ron Smith (Yale University), and with support from Dr. Steve Nelson, the Project Officer at NSF. A listing of all the Principal Investigators (i.e., funded scientists) involved in T-REX and their institutions is given on the T-REX Home Page. International collaborators include scientists from Germany, Austria, and the United Kingdom.
Funding for T-REX supports not only the attainment of research goals that will improve understanding of the rotor and related phenomenon, but also supports the education of students. University students in higher education degree programs will participate in the field experiment as well as in the research. Experience from previous research programs of this nature shows that students gain a great deal of experience and knowledge by participating in such a program and that these students often become the nation's scientific leaders in later years.
The University of Utah's participation in T-REX is funded through a National Science Foundation grant. The grant funds a collaborative proposal submitted by Dr. Dave Whiteman (UU) and Dr. Sharon Zhong (U Houston) entitled "Collaborative research: Boundary-Layer Influences on Mountain Waves and Rotors". We are especially keen to use the large quantities of meteorological data from the T-REX SOD to look at boundary layer processes in the Owens Valley to determine how the boundary layer evolves during both disturbed and undisturbed conditions. Specifically, we will be investigating the following atmospheric boundary layer topics using both analysis and modeling.
We contributed field equipment to the T-REX field experiment, including lines of temperature data loggers (UU), a flux tower (UH) which measures all components of the surface energy balance, and a device called a sodar (UH) which collects continuous vertical profiles of horizontal winds using a technique in which an acoustic transducer emits bursts of sound upwards into the atmosphere and receives the sound scattered back by the atmosphere. We placed these meteorological instruments at locations in the Owens Valley as shown on the map below. We asked land management agencies in the Owens Valley (the Los Angeles Department of Water and Power, the U. S. Forest Service and the Bureau of Land Management) for permission to install and operate these instruments during the T-REX SOP. The flux tower and sodar were installed on the grounds of the Owens Valley Radioastronomy Observatory which is operated by the California Institute of Technology.
Our plans are to address the scientific issues in collaboration with other T-REX scientists. The T-REX boundary layer sub-group, in particular, has worked together to list a number of boundary layer hypotheses that can be tested with the T-REX data.
We are also planning to participate in a model intercomparison with other investigators with the goal of determining the effects of model dynamical cores and physical parameterizations on the simulation of thermally-forced slope and valley winds. The experimental design for this research can be found in the attached pdf file.
Click here to go to our data page.
Click here to go to our scientific results page.
Separate web pages have been established for photographs of the equipment sites, maps of the instrument deployments and candid photographs from the field experiment.