EARLY RADAR ECHOES FROM SMALL, WARM CUMULUS - BRAGG AND HYDROMETEOR SCATTERING

Studies of small cumulus clouds in Florida using X- and S-band radar ( 3- and 10-cm wavelengths) reveal both hydrometeor and Bragg scattering signals. Turbulent mixing between cloudy and drier environmental air can produce centimeter-scale variations in refractive index that can l ead to strong mantle echoes around the sides and tops of the clouds. W hen the environmental air-is-exceptionally dry, the S-band Bragg scatt ering signals are as strong as 10 dBZ at cloud boundaries, with weaker echoes in the cloud cores where hydrometeor scattering is also presen t. The Bragg signal at S-band is typically about 19 dB stronger than t hat at X-band, as expected from theory. However, there is in many case s an unexplained, Bragg-like return from the clouds at S-band that cor relates with the X-band echo but is only about 10 dB stronger. The X-b and echo is often dominated by backscattering from the cloud droplets: and shows adiabatic ascent within the cloud cores fairly often up to at least 1 km above cloud base. In these cases, the radar echo profile s can be used to estimate the adiabatic droplet concentration, given r ough knowledge of the cloud-base height and temperature. The first pre cipitation shafts often occur before the cloud tops reach the 0 degree s C level, are narrow, and probably consist of low concentrations of d rops several millimeters in diameter.