Geoscience Reference
In-Depth Information
It is tempting to try to get rid of the Doppler snake by using a less aggressive Doppler filter.
However, amount of residual clutter may increase. Finding the compromise between too
aggressive and too weak filter is threading on a fine line, and the selection should be tested
in different wind and temperature conditions. Temperature inversions (typical for cloudfree
winter days or nights) affect amount of clutter by causing anomalous propagation, which
then leads to the increase of ground or sea clutter. Wind affects sea clutter but may also
cause blowing snow. Especially the blowing snow falling from trees in hilly areas may give
false alarms of ground clutter - it is real snow flying in real wind and hence immune to most
clutter cancellation techniques, even though it is not precipitation falling from clouds.
Case of sea clutter is especially annoying. In summer we have nocturnal inversions and
anomalous propagation mainly when it is not raining. In winter it is very likely to have on
continent cold weather and inversion, and simultaneously rigorous lake effect snow (see
section 6.2) over the water areas. This is possible, because the propagation is affected by
temperature near the radar, not at the measurement location. Sea clutter is immune to
Doppler filtering, but dual polarization measurements can reveal it, as is seen in Fig. 4.
Fig. 4. Sea clutter and lake effect snow seen with polarimetric parameter RhoHV. RhoHV
over 0.98 in snow, less than 0.8 in sea clutter. The range ring indicates 100 km from the
radar.
5. Dual polarization
While one of the most popular applications of dual-polarisation technology enables one to
distinguish the types of the hydrometeors measured (Straka et al., 2000), dual polarization
can be used for much more. On the other hand, some applications developed for rain, such
as KDP-based algorithms for quantitative precipitation estimates do not work in snow.
