Thursday, 27 October 2005
Alvarado F and Atria (Hotel Albuquerque at Old Town)
In this paper, we examine differential phase (ΦDP
) in the melting layer in winter clouds. Our analysis of the ΦDP
profiles obtained with the polarimetric WSR-88D radar shows two characteristic features: ΦDP
exhibits a positive shift (or perturbation) in the melting layer and slight gradual increase due to a low specific differential phase (KDP
) within the layer. The drop of the cross-correlation coefficient ρhv
within the melting layer causes enhanced statistical fluctuations in the ΦDP
estimates. However, the depth of the ΦDP
perturbations is considerably larger than the one attributed to the sole decrease of ρhv
. Two factors are known that can cause the positive bias of ΦDP
: a) the differential phase upon scattering by large spongy melting particles and b) nonuniform beam filling.
We also consider another factor that can increase ΦDP: the differential phase upon scattering by small (few mm) and highly nonspherical particles. It is demonstrated that such particles (spongy or water-coated) can produce backscatter differential phases of 5..7 deg. Water content in such particles can be as low as 2..3%.
We evaluate and compare possible contributions to ΦDP from all three factors, i.e., backscattering by large spongy snowflakes and /or smaller-size nonspherical particles, and nonuniform beam filling. The impact of beam broadening is estimated from the vertical profiles of the polarimetric variables measured at close distances from the radar. Results of our calculations are compared with radar data collected with the polarimetric WSR-88D radar.
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