Non-Rayleigh Scattering Effect in Rain Observations by an X- and Ka-band Dual-Wavelength Radar

Abstract

The effect of non-Rayleigh scattering appearing in a rain observation experiment using a dual-wavelength rain radar, is investigated. The non-Rayleigh scattering effect causes a small difference between the effective radar reflectivity factor and the actual radar reflectivity factor that is defined as the summation of the sixth power of raindrop diameter in a unit volume. This difference causes errors in the rain retrieval algorithm based on attenuation. A simultaneous rain observation by a dual-wavelength radar and a C-band Doppler radar was performed to confirm the existence of the non-Rayleigh scattering effect. Raindrop-size distributions are estimated from Doppler spectra at C band measured by the vertically pointing Doppler radar. The difference between effective radar reflectivity factors at X and Ka band was determined in two different ways. One way was to estimate the difference in the effective radar reflectivity factors at these wavelengths based upon raindrop-size spectra determined from the Doppler spectra of the C-band radar. The second was to use the measured values of the effective radar reflectivity factors obtained directly from the X- and Ka-band radar. It is found that both differences vary according to the raindrop-size distribution variations and have a good correlation in a time scale of less than a few minutes. This fact confirms that the effective radar reflectivity factors are affected by the non-Rayleigh scattering.