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 October 1990) of the Pyrenean experiment was studied using a single VHF stratospheric-tropospheric radar installed 35 km downstream from the Pyrenean chain axis. This instrument obtained time series of the air vertical velocity with a time resolution of 4 min 50 s for altitudes ranging from 2250 to 15 000 m mean sea level with a 750-m step. Maximum amplitudes of 56 m s(-1) were measured between 3750 and 5250 m. Temporal variability of the vertical velocity was observed showing that the wave can be nonstationary at timescales on the order of 5-10 min. A classic signal processing technique using the calculation of height-time cross-correlation coefficient in a 2-h moving window was applied to the radar time series and allowed the wave vertical structure to be monitored with a time resolution of 2 h. The lee wave was found to be trapped in a tropospheric duct having an upper limit of 8000 m. The observed wave activity was found to be well developed from 2300 UTC 14 October 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 obtained from the radar vertical beam data, and interpreted in the frame of the 2D linear theory, were consistent with upstream radiosonde data, numerical simulations by nonhydrostatic models, and measurements made along the chain transect by an aircraft and two constant-level balloons. Thanks to the lee wave horizontal structure obtained by these airborne instruments, it is concluded that the wave activy decrease observed by the radar was mainly due to a progressive shortening of the downstream wave horizontal extent.