The Measured Relationship between Ice Water Content and Cloud Radar Reflectivity in Tropical Convective clouds.
Résumé
In this paper we use unprecedented bulk measurements of ice water content (IWC) up to approximately 5 gm-3 and 95 GHz radar reflectivities (Z95) 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 gm-3 are also included for the first time in such analysis, owing to improved bulk IWC probe technology and a flight program targeting high ice water content. Using a single IWC – 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> 15-16 dBZ, due to the distortion of the IWC – Z95 relationship by non-Rayleigh scattering effects. A non-linear relationship is proposed to reduce these errors down to 20% bias and 20-35% rms differences. This non-linear 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°C ± 5°C level – the cruise altitude of many commercial jet aircraft – IWCs greater than 1.5 gm-3 were found exclusively in convective profiles.
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