The measured relationship between ice water content and cloud radar reflectivity in tropical convective clouds

Abstract

In this paper, unprecedented bulk measurements of ice water content (IWC) up to approximately 5 g m-3 and 95-GHz radar reflectivities Z95 are used to analyze the statistical relationship between these two quantities and its variability. The unique aspect of this study is that these IWC-Z95 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-Z95 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 Z95 > 16 dBZ, as a result of the distortion of the IWC-Z95 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-Z95 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.