Aspect sensitivity of VHF radar echoes observed in the middle and upper troposphere during the passage of a cut-off low

The aspect sensitivity of VHF radar signals is related to the enhancement of the echo power and the narrowing of the spectral width at vertical incidence relative to the off-zenith directions. The strong aspect sensitivity is usually associated with stable atmospheric layers. In this paper the aspect sensitivity measured between an altitude of 4 and 15 km during the passage of a tropopause folding linked to a cut-off low is analyzed and discussed. The study mainly uses data acquired during a 2-day period with a three-beam VHF radar with relatively broad beam widths (6.8 degrees). The angular dependence of the echoes is investigated using the power ratio of vertical to oblique beam and the spectral width obtained at vertical incidence. These two diagnostic parameters provided a nearly identical depiction of the stratospheric descent associated with the upper frontal zone. However, a discrepancy is observed between the location of the stratospheric subsidence and the tropopause height offered by the radar and that deduced from the meteorological analysis produced by an operational forecasting numerical model. Owing to the aspect sensitivity of the echoes, an underestimate amounting to 20% was found in the horizontal wind measurement. Finally, a comparison of simultaneous observations of two closely spaced VHF radars (100 km) has shown a good spatial coherence of the aspect sensitivity structures.