A research project in California’s Sierra Nevada mountains is attempting a major leap forward in unraveling the mysteries surrounding one the biggest atmospheric hazards in modern aviation — a wind phenomenon known as a “mountain rotor.”
Both rotors and a related phenomenon, mountain waves, regularly form on a mountain’s lee side, or on the side opposite the direction from which strong winds typically blow. Just such a site is in Owens Valley, Calif., which is between the Sierra Nevada and the city of Fresno. There, from March 1 to April 30, a series of airborne and ground-based readings are being taken for the “terrain-induced rotor experiment” (T-REX), says the lead researcher, Vanda Grubisic, of the Desert Research Institute (DRI) based in Las Vegas and Reno, Nevada. Primary funding is coming from the National Science Foundation in Washington, D.C.
Around the world, there are 60 principle researchers and another 40 interns and technical staff who are starting to analyze the new data, Grubisic adds. For the scientific community, it’s a chance to revisit these atmospheric phenomena whose existence have been known for a long time. But now, researchers are coming back to them with the latest technology and research methods, and the chance over the next three years to develop numerical models from the T-REX data.
“Despite numerous investigations during the last 70 years concerning lee waves, the structure and dynamics of rotors remains largely unknown,” René ˆeise, a meteorologist in Germany with the Mountain Wave Project (MWP), tells Air Safety Week.
Mountain waves, which in form are something like the waves at a sea shore, result from oncoming air coming against a mountain’s face that is then forced up and over the crest. On the other side, gravity suddenly pulls the air down and the waves form. Rotors develop right below the waves, and resemble a whirlwind or vortex tilted to the horizontal.
But little is known so far about the “whys and hows” of rotor formation, Grubisic tells Air Safety Week. T-REX will be an important step in understanding these phenomena better, but much more research probably will remain to be done after the current project is over.
In aviation, mountain waves and rotors have long been recognized as significant dangers. The Australian Transport Safety Bureau cites a 1968 incident when a BOAC Boeing 707 was ripped apart by a mountain wave as the craft flew near Mt. Fuji in Japan. Also, in 1968, a Fairchild F-27B lost parts of its wings and empennage, and a Douglas DC-8 lost an engine and wingtip in 1992, in wave-related accidents.
Rotors, specifically, have been cited as contributors to accidents in commercial, military, and general aviation (GA), Grubisic says. Experienced pilots know about them and avoid them. But rotors and waves remain particularly dangerous to pilots who are unaware of them.
As the FAA has aptly put it, “Your first experience flying over mountainous terrain (particularly if most of your flight time has been over the flatlands of the Midwest) could be a never-to-be forgotten nightmare [italics in original] if proper planning is not done and if you are not aware of the potential hazards.”
Besides the aviation dangers that rotors pose, Grubisic says “we’re doing this because it’s one of the unsolved problems in atmospheric research.” The outstanding questions for her team includes not only why rotors form, or how they do, but also how they often get so strong. It seems, she adds, that rotors pick up their “intense rotation” from the “boundary layer” of air next to the earth’s surface. But explaining exactly how this happens has been “one of the more puzzling questions,” and has become one of the principle research aims.
Moreover, the numeric modeling of certain atmospheric conditions from T-REX could lead to better forecasting of rotors and waves. Indeed, another question occupying researchers’ minds is just how predictable rotors will come to be.
Current attempts at numerical simulations are not the best because they use “idealized assumptions” of atmospheric conditions, MWP’s Heise explains. Moreover, the lack of “sufficient empirical data” makes it difficult to develop certain parameters, a problem that T-REX’s more precise measurements should ameliorate. This should lead to better forecasting of rotors and waves, and enhanced flight safety.
Steve Nelson, NSF’s program director for physical and dynamic meteorology, agrees that there are a lot of unknowns with mountain rotors, adding that T-REX eventually could have significant implications for aviation safety. He draws an analogy between the scientific inquiry into the dynamics of rotors and an older inquiry into two other troublesome atmospheric phenomena–downbursts and microbursts. Some years ago, the scientific knowledge base for these second two was similar to what exists today for rotors, he tells Air Safety Week. But a long stream of related research projects led to better radar and wind-detection systems at airports, greatly reducing the hazards. If a similar research stream gets going in the wake of T-REX, rotors someday may subject to far more accurate forecasting and become much easier for pilots to avoid.
The Sierra Nevada mountains are especially ideal for studying rotor and wave formation because they are “the tallest, steepest, quasi two-dimensional topographic barrier in the contiguous United States,” according to the T-REX Web site (at http://www.joss.ucar.edu/trex). Thus, rotors and waves grow particularly large and strong there. Additionally, prior research shows that they are especially frequent in the Sierra Nevada in March and April.
There also are two aspects to T-REX’s current phase of data collection, Grubisic explains. One involves the data being read by several ground-based stations. The second involves the readings coming from three aircraft. One is a Beechcraft King Air turboprop, owned and operated by the University of Wyoming. It can take readings from 500 ft. to 28,000 ft. above ground, and is flying for T-REX while based at Bishop, Calif. The craft is doing about 25-30 flights for the study. On the mountains’ lee side, Bishop also is the T-REX operations center.
Above the range of the Beechcraft at altitudes reaching 35,000 ft. is a British Aerospace BAe 146. It’s based in Fresno and is making about 10 flights for the study.
The third craft, which can take readings at up to 45,000 ft., is the new Gulfstream V HIAPER, which stands for “high-performance instrumented airborne platform for environmental research”. NSF developed and modified the craft specifically to enhance its environmental research needs in the coming years (and indeed, T-REX also is expected to yield data to help fight environmental pollution). The HIAPER is being operated and maintained for NSF by the National Center for Atmospheric Research (NCAR) in Boulder, Colo. The craft will make its dozen-or-so data-gathering flights from a base just south of Boulder in Jefferson County (which is part of metro Denver). T-REX also represents the craft’s maiden use for scientific research.
The HAIPER is especially suited for its role in T-REX because it’s the only craft that can reach such heights while deploying GPS Dropsonde technology and other instruments to measure certain meteorological parameters, says Jim Huning, program officer for NSF’s Lower Atmospheric Observing Facilities. Dropsonde, which was developed at NCAR, drops a sensor below the craft that is equipped with a little parachute to get measurements of such factors as atmospheric pressure, horizontal wind, and moisture. Coupled with GPS, those readings can now be tied to very specific points in space and time.
“It gives a very accurate idea of what’s going on,” Huning tells Air Safety Week, and should help get more precise measures of rotor dynamics. The National Oceanic and Atmospheric Administration (NOAA) has found the technology very useful recently in hurricane research.
The mid-altitude BAe is also deploying the GPS Dropsonde sensors, while the low-altitude Beechcraft King Air is equipped with a special Dopplar radar sensor for studying clouds. Not only will its readings reveal where the clouds are, but the wind velocities within the clouds.
>>Contacts: Vanda Grubisic, DRI, (775) 674-7031, [email protected]; Steve Nelson, NSF, (703) 292-8521, Jim Huning, NSF, (703) 292-4703, [email protected]; René ˆeise, MWP, Rene.Heise@t-online<<