Radar and in situ observations of small cumulus: physical interpretations of radar Bragg scatter

Radar observations of small cumulus clouds are compared to predictions of radar measurements based on in situ measurements and the theory of radar back‐scatter. At the wavelengths used, both Rayleigh and Bragg scatter can be important in small cumulus. A theoretical derivation of radar back‐scatter from small cumulus clouds, in which both terms are succinctly derived using a common mathematical model, is presented. For the earliest stages of cumulus clouds, the predictions of Bragg scatter, based on in situ measurements of water‐vapour fluctuations, are in close agreement with radar‐measured Bragg scatter. This suggests that the theory is adequate and our interpretations of the radar observations are correct. These interpretations include identifying regions where entrainment and mixing are ongoing, identifying adiabatic cores, and estimating the Kolmogorov microscale of turbulence. As the small cumulus develop and enter the early collision and coalescence stages, another source of Bragg scatter can become significant. It is argued, via observations, that the additional Bragg scatter comes from anomalous liquid‐water fluctuations. These liquid‐water fluctuations are called ‘anomalous’ because they exceed what would result if liquid water mixed as a passive scalar. The Bragg scatter caused by liquid water may reach effective values of 5‐10 dBZ, for a 3 cm‐wavelength (X‐band) radar, and thus confounds attempts to derive Rayleigh‐scatter values below this level using a dual‐wavelength (X‐ and S‐band) radar system. Copyright © 2007 Royal Meteorological Society