Early Radar Echoes from Small, Warm Cumulus: Bragg and Hydrometeor Scattering

Charles A. Knight National Center for Atmospheric Research,* Boulder, Colorado

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L. Jay Miller National Center for Atmospheric Research,* Boulder, Colorado

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Abstract

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 lead to strong mantle echoes around the sides and tops of the clouds. When the environmental air is exceptionally dry, the S-band Bragg scattering signals are as strong as 10 dBZ at cloud boundaries, with weaker echoes in the cloud cores where hydrometeor scattering is also present. The Bragg signal at S-band is typically about 19 dB stronger than that at X-band, as expected from theory. However, there is in many cases an unexplained, Bragg-like return from the clouds at S-band that correlates with the X-band echo but is only about 10 dB stronger. The X-band 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 profiles can be used to estimate the adiabatic droplet concentration, given rough knowledge of the cloud-base height and temperature. The first precipitation shafts often occur before the cloud tops reach the 0°C level, are narrow, and probably consist of low concentrations of drops several millimeters in diameter.

Corresponding author address: Dr. Charles A. Knight, National Center for Atmospheric Research, P.O. Box 3000, Boulder, CO 80307-3000.

Email: knightc@ncar.ucar.edu

Abstract

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 lead to strong mantle echoes around the sides and tops of the clouds. When the environmental air is exceptionally dry, the S-band Bragg scattering signals are as strong as 10 dBZ at cloud boundaries, with weaker echoes in the cloud cores where hydrometeor scattering is also present. The Bragg signal at S-band is typically about 19 dB stronger than that at X-band, as expected from theory. However, there is in many cases an unexplained, Bragg-like return from the clouds at S-band that correlates with the X-band echo but is only about 10 dB stronger. The X-band 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 profiles can be used to estimate the adiabatic droplet concentration, given rough knowledge of the cloud-base height and temperature. The first precipitation shafts often occur before the cloud tops reach the 0°C level, are narrow, and probably consist of low concentrations of drops several millimeters in diameter.

Corresponding author address: Dr. Charles A. Knight, National Center for Atmospheric Research, P.O. Box 3000, Boulder, CO 80307-3000.

Email: knightc@ncar.ucar.edu

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