The use of multi-polarization radars to remotely detect hydrometeors continued
to be demonstrated through a number of observational studies (Aydin et al.
[1990], Bringi et al. [1991], Herzegh and Jameson [1993], and Zrnic et al.
[1993]). A new result is the identification of aggregates using differential
phase shift measurements (Zrnic et al. [1993]). Vivekanadan et al. [1994]
presented new theoretical results on the single scattering properties of ice
crystals as a function of size, bulk density, and shape at microwave
frequencies using discrete dipole approximations and Rayleigh scattering
techniques. They calculated multiparameter radar observables by averaging
single scattering characteristics over a spectrum of sizes, shapes, and
densities of ice scatterers. They used these model computations to
interpret multiparameter radar observables from a winter storm. The model
showed good ability to distinguish between regions of oriented dendrites
(high Z
(dual polarization signal) and low reflectivity), and regions
of large aggregates (low Z
and high reflectivity).
Jameson [1991] showed theoretically the limitations of 9 and 5
GHz (10
HZ) radars in measuring rain. He explored the use of
multi-polarization measurements to remove the effects of attenuation
and showed that residual errors introduced by attenuation and from the
correction scheme itself make the measurement of rainfall at these
frequencies difficult even with multi-polarization corrections.
Knight and Miller [1993] demonstrated that Bragg scattering (due to index of refraction gradients) often dominates the backscattering signal at low reflectivity levels. This implies that one needs to be careful in interpreting early echoes in cumulus clouds because they may be caused by index of refraction gradients rather than initial precipitation particles.