Space–Time Description of Nonstationary Trapped Lee Waves Using ST Radars, Aircraft, and Constant Volume Balloons during the PYREX Experiment

Abstract

The third intensive observation period (IOP3) of PYREX was a case of strong lee waves generated by a southerly wind crossing the Pyrenees chain. 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 times and heights. Values ranging from 7 to 14 km for the horizontal wavelength, and from 3 to 5 m s?1 for the maximum amplitude of the air vertical velocity, were observed. The lee wave horizontal extent, measured from the crest line, reached 30 to 55 km. In addition, two very high frequency stratosphere-troposphere (VHF ST) radars, one on the mountain mean axis and another downstream in the lee wave field, observed the temporal variations of the vertical profiles of the vertical velocity. The analysis of those observed variations and their vertical distribution allowed the stationarity of the wave to be studied. The lee wave was found to be far from stationary during its lifetime, but there were some periods, never longer than 1.5 h, during which the wave was 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 profile observed by the radar below the wave field revealed that, most of the time, the lee wave was trapped, which was found to be consistent with radiosounding data. In addition, favorable comparison between airborne and radar data added further evidence that direct VHF ST radar measurement of the vertical air motion induced by lee waves is not seriously affected by beam tilting.