MEASUREMENT OF ATTENUATION AT C-BAND IN A CONVECTIVE STORM BY A DUAL-RADAR METHOD

Retrievals' of specific attenuation at 5.5-cm wavelength from dual-rad ar 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 cont amination 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, a nd high reflectivity cores are generally associated with higher specif ic attenuations. A plot of the retrieved specific attenuation against reflectivity factor at 10-cm wavelength shows that a majority of the r etrieved values agree with those calculated from empirical relationshi ps between reflectivity factor and specific attenuation. A small fract ion of the points having high reflectivity factors have smaller than e mpirically predicted attenuations; these are attributed to dry ice par ticles. A larger fraction of the points having low reflectivity factor s, less than about 30 dBZ, have higher than empirically predicted atte nuations; these are attributed to attenuation by cloud liquid water an d mixed-phase hydrometeors. A scatterplot of the differential reflecti vity factor at 10-cm wavelength against the ratio of the retrieved spe cific attenuation to the reflectivity factor at 10-cm wavelength agree s generally with a theoretically expected relationship between the two parameters for horizontally oriented deformed raindrops, giving crede nce to the retrievals. However, the points scatter rather widely aroun d the theoretical curve and the scatter is attributed to 1) signal flu ctuations; 2) physical factors, namely, cloud water, ice particles, an d mixed-phase particles of various shapes; 3) instrumental factors, na mely, ground clutter, finite pulse volume, mismatched radar antenna pa tterns, and sidelobes; and 4) numerical procedures used in the retriev als, namely, data interpolation, smoothing, and differentiation.