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 o f the echo power and the narrowing of the spectral width at vertical incide nce relative to the off-zenith directions. The strong aspect sensitivity is usually associated with stable atmospheric layers. In this paper the aspec t sensitivity measured between an altitude of 4 and 15 km during the passag e 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-be am VHF radar with relatively broad beam widths (6.8 degrees). The angular d ependence of the echoes is investigated using the power ratio of vertical t o oblique beam and the spectral width obtained at vertical incidence. These two diagnostic parameters provided a nearly identical depiction of the str atospheric descent associated with the upper frontal zone. However, a discr epancy is observed between the location of the stratospheric subsidence and the tropopause height offered by the radar and that deduced from the meteo rological analysis produced by an operational forecasting numerical model. Owing to the aspect sensitivity of the echoes, an underestimate amounting t o 20% was found in the horizontal wind measurement. Finally, a comparison o f simultaneous observations of two closely spaced VHF radars (100 km) has s hown a good spatial coherence of the aspect sensitivity structures.