Sunday, 10 August 2003
Doppler spectra of copolar and cross-polar signals
Two modes of operation have been implemented on the NSSL's polarimetric WSR-88D. One is Simultaneous transmission and reception of Horizontally and Vertically (SHV) polarized waves and the other is Transmission of Horizontally (TH) polarized waves and reception of both copolar H and cross-polar V waves. Time series data have been obtained in both modes and Doppler spectra of these data are examined. In the TH mode Doppler spectral widths of the cross-polar (V) signal are often two to three times wider than the widths of the copolar signal. Further, the spectra in the copolar channel have more pronounced peaks. In contrast to the TH mode, in the SHV mode the spectra of the H signals are very close to the spectra of the V signals; this is a consequence of high correlation between these two signals. Vertical cross-sections of spectral moments obtained in the SHV and TH mode are presented. The cross-sections of spectral widths reveal well pronounced zones of enhancements in the cross-polar channel. Several processes may contribute to the spectral widening in the cross-polar channel: wobbling of the scatterers, drops' oscillations, multiple scattering, antenna sidelobes, and receiver noise. We show that wobbling has little influence on the spectral difference. Further we identify and thus exclude from analysis signals with possible contaminations through antenna sidelobes. Our calculations show that either one or both, oscillations of drops and multiple scattering can explain observed wide spectra in the cross-polar channel. Although we have no independent means to verify which mechanism causes this broadening, by using a simple scattering model we can exclude multiple scattering as a contributor in some regions of the storm. We are thus left with the question: are drop's oscillations responsible and can these be identified in the spectra of the cross-polar signals?