Measurement of Attenuation at C Band in a Convective Storm by a Dual-Radar Method

Abstract

Retrievals of specific attenuation at 5.5-cm wavelength from dual-radar observations of a summer convective storm in Florida are presented. The retrieved specific attenuation is positive except in regions near the radars where the observed reflectivity factors suffered from contamination by ground clutter. The specific attenuations ranged between 0.0 and 2.0 dB km?1; they are small at higher levels of the storm, and high reflectivity cores are generally associated with higher specific attenuations. A plot of the retrieved specific attenuation against reflectivity factor at 10-cm wavelength shows that a majority of the retrieved values agree with those calculated from empirical relationships between reflectivity factor and specific attenuation. A small fraction of the points having high reflectivity factors have smaller than empirically predicted attenuations; these are attributed to dry ice particles. A larger fraction of the points having low reflectivity factors, less than about 30 dBZ, have higher than empirically predicted attenuations; these are attributed to attenuation by cloud liquid water and mixed-phase hydrometeors. A scatterplot of the differential reflectivity factor at 10-cm wavelength against the ratio of the retrieved specific attenuation to the reflectivity factor at 10-cm wavelength agrees generally with a theoretically expected relationship between the two parameters for horizontally oriented deformed raindrops, giving credence to the retrievals. However, the points scatter rather widely around the theoretical curve and the scatter is attributed to 1) signal fluctuations; 2) physical factors, namely, cloud water, ice particles, and mixed-phase particles of various shapes; 3) instrumental factors, namely, ground clutter, finite pulse volume, mismatched radar antenna patterns, and sidelobes; and 4) numerical procedures used in the retrievals, namely, data interpolation, smoothing, and differentiation.