LEE WAVE VERTICAL STRUCTURE MONITORING USING HEIGHT-TIME ANALYSIS OF VHF ST RADAR VERTICAL VELOCITY DATA

The strong lee wave event of intensive observation period 3 (14-15 Oct ober 1990) of the Pyrenean experiment was studied using a single VHF s tratospheric-tropospheric radar installed 35 km downstream from the Py renean chain axis. This instrument obtained time series of the air ver tical velocity with a time resolution of 4 min 50 s for altitudes rang ing from 2250 to 15 000 m mean sea level with a 750-m step. Maximum am plitudes of 56 m s(-1) were measured between 3750 and 5250 m. Temporal variability of the vertical velocity was observed showing that the wa ve can be nonstationary at timescales on the order of 5-10 min. A clas sic signal processing technique using the calculation of height-time c ross-correlation coefficient in a 2-h moving window was applied to the radar time series and allowed the wave vertical structure to be monit ored with a time resolution of 2 h. The lee wave was found to be trapp ed in a tropospheric duct having an upper limit of 8000 m. The observe d wave activity was found to be well developed from 2300 UTC 14 Octobe r until 0830 UTC 15 October and slowly decreasing after. This behavior was related to large timescale nonstationarities, that is, changes in the upstream conditions observed every 6 h. All those results obtaine d from the radar vertical beam data, and interpreted in the frame of t he 2D linear theory, were consistent with upstream radiosonde data, nu merical simulations by nonhydrostatic models, and measurements made al ong the chain transect by an aircraft and two constant-level balloons. Thanks to the lee wave horizontal structure obtained by these airborn e instruments, it is concluded that the wave activy decrease observed by the radar was mainly due to a progressive shortening of the downstr eam wave horizontal extent.