Jg. Yoe et al., SPATIAL VARIABILITY OF THE ASPECT SENSITIVITY OF VHF RADAR ECHOES IN THE TROPOSPHERE AND LOWER STRATOSPHERE DURING JET-STREAM PASSAGES, Annales geophysicae, 12(8), 1994, pp. 733-745
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.