The Measured Relationship between Ice Water Content and Cloud Radar Reflectivity in Tropical Convective Clouds

In this paper, unprecedented bulk measurements of ice water content (IWC) up to approximately 5 g m −3 and 95-GHz radar reflectivities Z 95 are used to analyze the statistical relationship between these two quantities and its variability. The unique aspect of this study is that these IWC– Z 95 relationships do not use assumptions on cloud microphysics or backscattering calculations. IWCs greater than 2 g m −3 are also included for the first time in such an analysis, owing to improved bulk IWC probe technology and a flight program targeting high ice water content. Using a single IW– Z 95 relationship allows for the retrieval of IWC from radar reflectivities with less than 30% bias and 40%–70% rms difference. These errors can be reduced further, down to 10%–20% bias over the whole IWC range, using the temperature variability of this relationship. IWC errors largely increase for Z 95 > 16 dB Z , as a result of the distortion of the IWC– Z 95 relationship by non-Rayleigh scattering effects. A nonlinear relationship is proposed to reduce these errors down to 20% bias and 20%–35% rms differences. This nonlinear relationship also outperforms the temperature-dependent IWC– Z 95 relationship for convective profiles. The joint frequency distribution of IWC and temperature within and around deep tropical convective cores shows that at the −50° ± 5°C level, the cruise altitude of many commercial jet aircraft, IWCs greater than 1.5 g m −3 were found exclusively in convective profiles.