16b.1 Coherent Particle Scatter in Developing Cumulus Clouds

Two mechanisms important for the interpretation of radar observations of the atmosphere, are: coherent scattering from refractive index fluctuations of the clear-air and incoherent scattering from particles. These scattering mechanisms have very different wavelength dependencies and their contributions to the radar return can be separated by using two radars with different wavelengths. A difference of 0 dB or 19 dB in the radar reflectivities is expected when both returns are dominated by incoherent scattering or coherent scattering, respectively. Recent observations of small developing cumulus clouds (Knight and Miller 1998) show a puzzling correlation: the S-band and X-band reflectivities cluster around a line, but with a difference of about 10 dB. In this paper, an explanation will be proposed in terms of coherent scattering from the droplets. The spatial fluctuations in liquid water content that cause the coherent scattering could be due to mixing with environmental air. It will be argued that fluctuations of water vapor content inside clouds can be much smaller than those in liquid water content, contrary to common belief. The paper is organized as follows. Firstly, some theory on coherent and incoherent scattering is reviewed. The measurements will be discussed next and a new explanation is presented. A discussion follows in section 5.