SPACE-TIME DESCRIPTION OF NONSTATIONARY TRAPPED LEE WAVES USING ST RADARS, AIRCRAFT, AND CONSTANT VOLUME BALLOONS DURING THE PYREX EXPERIMENT

The third intensive observation period (IOP3) of PYREX. was a case of strong lee waves generated by a southerly wind crossing the Pyrenees c hain. Upstream radiosounds and measurements obtained by aircraft along the chain transect and by constant volume balloons launched near the crest provided spatial characteristics of the lee wave at different ti mes and heights. Values ranging from 7 to 14 km for the horizontal wav elength, and from 3 to 5 m s(-1) for the maximum amplitude of the air vertical velocity, were observed. The lee wave horizontal extent, meas ured from the crest line, reached 30 to 55 km. In addition, two very h igh frequency stratosphere-troposphere (VHF ST) radars, one on the mou ntain mean axis and another downstream in the lee wave field, observed the temporal variations of the vertical profiles of the vertical velo city. The analysis of those observed variations and their vertical dis tribution allowed the stationarity of the wave to be studied. The lee wave was found to be far from stationary during its lifetime, but ther e were some periods, never longer than 1.5 h, during which the wave wa s quasi-stationary. Data obtained by the airborne instruments revealed that the wavelength, amplitude, and downstream wave extent underwent temporal variations. The time evolution of the vertical velocity profi le observed by the radar below the wave field revealed that, most of t he time, the lee wave was trapped, which was found to be consistent wi th radiosounding data. In addition, favorable comparison between airbo rne and radar data added further evidence that direct VHF ST radar mea surement of the Vertical air motion induced by lee waves is not seriou sly affected by beam tilting.