Estimating Wind Velocities in Mountain Lee Waves Using Sailplane Flight Data*

Abstract

Mountain lee waves are a form of atmospheric gravity wave that is generated by flow over mountain topography. Mountain lee waves are of considerable interest, because they can produce drag that affects the general circulation, windstorms, and clear-air turbulence that can be an aviation hazard, and they can affect ozone abundance through mixing and inducing polar stratospheric clouds. There are difficulties, however, in measuring the three-dimensional wind velocities in high-altitude mountain waves. Mountain waves are routinely used by sailplane pilots to gain altitude. Methods are described for estimating three-dimensional wind velocities in mountain waves using data collected during sailplane flights. The data used are the logged sailplane position and airspeed (sailplane speed relative to the local air mass). An algorithm is described to postprocess this data to estimate the three-dimensional wind velocity along the flight path, based on an assumption of a slowly varying horizontal wind velocity. The method can be applied to data from dedicated flights or potentially to existing flight records used as sensors of opportunity. The methods described are applied to data from a sailplane flight in lee waves of the Sierra Nevada in California. * This work is dedicated to Steve Fossett, a true adventurer.

Determining wind fields in atmospheric mountain waves using sailplane flight data

“…Although measurements of either ground velocity and airspeed, or ground velocity and heading, give a one-parameter family of solutions for the wind speed, we have shown that using the fact that the wind field in high-altitude mountain wave varies slowly over the flight path allows estimates of the wind velocity to be determined from these kinds of data. 7,8 However, in the vast majority of cases, the instrumentation installed in most sailplanes records only the GPS position during a flight so that only the ground velocity is known. Determination of the wind velocity is then highly underdetermined and there is a two-parameter family of solutions.…”

“…4(a) shows a histogram of airspeed data measured during the wave flight described in Ref. 7. We fitted a variety of probability distributions to the histogram shown in Fig.…”