Radar Reflectivity and Attenuation of Rain

Abstract

The radar reflectivity and attenuation of rain are computed for various empirical drop-size distributions using more accurate and systematic calculations of the back-scatter and attenuation cross sections from Mie theory. Reflectivity-rainfall (Z-R) relations for 3.2 to 10 cm, incorporating deviations from the Rayleigh theory, are found to be not appreciably different from those calculated from the Rayleigh theory. At shorter wave-lengths, however, there is considerable deviation and a single Z-R relation can be used only over relatively small intervals of rain intensity. There is only slight temperature dependence for the Z-R relations. A modified Marshall-Palmer distribution found by Mueller and Jones in Florida showers, with a smaller frequency of small drops, results in Z being directly proportional to R with values greater thin those of the M-P distribution for rain intensities up to about 10 mm hr?1 and less at higher intensifies. Attenuation is roughly proportional to the rain intensity. At 1.24 cm the attenuation is very similar to that found by Gunn and East for the Laws and Parsons drop-size distribution. There is greater deviation at 3.2 cm. Reflectivity measurements at two or more wavelength can provide a representative rain drop size of a spectrum if the shape of the spectrum is known.