SPATIAL VARIABILITY OF THE ASPECT SENSITIVITY OF VHF RADAR ECHOES IN THE TROPOSPHERE AND LOWER STRATOSPHERE DURING JET-STREAM PASSAGES

The aspect sensitivity of SOUSY-VHF-radar oblique-beam echoes from the troposphere and lower stratosphere has been examined for a number of jet stream passages during the year 1990-1992. When the core of the je t is overhead or nearly so, vertical profiles of the aspect sensitivit y display two notable features. First, the distinction between mainly isotropic and strongly aspect-sensitive echoes in the troposphere and the lower stratosphere, respectively, often reported for measurements made during calm conditions, does not necessarily prevail in the vicin ity of the jet stream. Second, echoes obtained at altitudes near the h eight of the horizontal wind maximum are found to be more aspect sensi tive for beams directed parallel to the horizontal flow or nearly so, that for other beam directions. It is demonstrated that time-averaged horizontal wind speeds estimated from the radar data, taking into acco unt the reduced effective oblique-beam zenith angle resulting from asp ect sensitivity, may exceed uncorrected wind speeds by as much as 10 m s(-1) in these circumstances. Implications for wind profiling and for describing the backscattering process are discussed. Doppler spectral widths examined for one jet stream passage are found to be narrower i n a beam aligned with the horizontal wind at heights near the wind spe ed maximum than corresponding widths measured in a beam projected at r ight angles to the jet. The narrowest spectra thus coincide with the m ost aspect-sensitive echoes, consistent with the hypothesis that such returns result from specular backscattering processes.