Radar Depolarization Signatures of Rain in Cumulus Clouds Measured with a Dual-Frequency Air-Borne Radar

In this paper experimental data are presented on dual- wavelength scattering characteristics of sub-tropical rain observed during the CaPE experiment in Florida in July, 1991. Linear de- polarization ratios (LDRs) are calculated at both X-band and Ka- band channels. In stratiform rain the cross-polarized return signal is generally below the noise level except within the melting layer where the maximum LDR at both frequencies is typically between -12 and -10 dB. In convective rain, however, the range profiles of the LDR in X-band and those in Ka-band do not coincide. The X- band LDR values in convective rain typically remain smaller than -25 dB at high altitudes and take somewhat larger values of -25 to -15 dB at low altitudes. The Ka-band LDR values, in contrast, typically increase with radar range, sometimes reaching a level of -3 dB just before the cross-polarized signal falls below the noise level. Such high values of LDR in Ka-band are most likely attributable to multiple scattering effects. become more highly aligned with symmetry axes close to the ver- tical. At Ka-band the maximum reflectivity in the melting layer is neither as strong nor as well defined as in the X-band case because non-Rayleigh scattering is significant in all three regions. Near the top of the precipitation echo, ZX and ZK. take ap- proximately the same values because the particles are sufficiently small that the Rayleigh scattering theory is applicable for which the reflectivity factor is independent of the wavelength of the electro- magnetic wave. The situation is similar just below the bright band when the precipitation is weak (ZX < 25 dB) and the Ka-band signal is free from the attenuation. The cross-polarization components can be observed only at the bright band region and from the surface reflection in stratiform rain. In other regions the cross-polarized returns are too weak to be de- tected. It is sometimes more physically meaningful to work with the linear depolarization ratio (LDR) than the cross-polarized return power itself or the cross-polarized radar reflectivity factor. LDR is defined as the ratio of the cross-polarized return power to the co- polarized return power. In what follows LDR in X-band is denoted by LDR(X) and LDR in Ka-band is denoted by LDR(Ka). The LDR in both frequencies increases to -12 to -10 dB in the bright band. The maximum of LDR(X) occurs at approximately 150 m below the ZX maximum. The location of the maximum LDR(Ka) is almost the same as that of the maximum LDR(X), but sometimes appears slightly (50m to 100m) higher than the maximum position of LDR(X) and the LDR(Ka) itself is sometimes 1 to 2 dB higher than the LDR(X) at the maximum. The most noticeable differ- ence between LDR(X) and LDR(Ka) occurs below the bright band where the cross-polarized component of X-band signal falls below the noise level while that of the Ka-band first decreases to -18 to -17 dB just below the bright band and then gradually increases with increasing radar range. The most probable cause for the rel- atively high LDR values in this region is depolarization caused by multiple scattering. Another interesting feature of the LDR measurements in strat- iform rain is that the LDR(X) of the mirror image of the bright band is almost the same as the LDR of the bright band itself. This is not the case, however, in convective rain.