Remote Sensing of Ice Cloud Microphysical Properties From Thermal Infrared Radiometry During the CIRCLE Experiment

Previous studies have reported that the two channels at 10.6 and 12 μm allow a determination of the optical thickness and effective size of ice cloud, for particle size less than 100 μm. In the context of the CALIPSO mission, a sensitivity study based on radiative transfer calculations has shown that the shape of non spherical ice particles and to a lesser extent, the size distribution, have noticeable effects on top-of- atmosphere brightness temperature. Using the split window technique (with channels at 10.6 and 12 μm) and assuming cloud structure knowledge, the influence of the cloud model leads to a theoretical accuracy of about 30 to 50 % on the retrieved effective size of the particle for small (< 20 μm) or large particle (> 20 μm), respectively. The accuracy on the retrieved optical thickness is rather good, of the order of +/- 10 %. This would however lead to an accuracy ranging from 10 to 50 % on the derived Ice Water Path. From a theoretical point of view, the use of the third channel (8.7 μm) should allow better constraining the cloud model, improving the determination of cloud properties. In this study, a method for ice cloud characterization from thermal infrared imagery is applied and evaluated. Infrared Imaging Radiometer (IIR) measurements at 8.7, 10.6 and 12 μm are used for reference scenes acquired during the CIRCLE experiment (summer 2007 over North / West of Europe). Indeed, validation flights have been performed with airborne measurements simultaneous with A-train ones. Especially, infrared radiances have been collected with the radiometer CLIMAT, which has similar infrared channel characteristics to IIR. Such a case study is now available including microphysics measurements as well as a detailed set of atmospheric data. Additional information about cloud structure is also available from airborne and space lidar (LNA and CALIOP) and radar (RASTA and CLOUDSAT) measurements. This database is of great interest to better constrain radiative transfer calculations. Comparisons between optical and microphysical properties of ice particles deduced from IIR or in situ measurements are presented. Contribution of the 8.7 μm channel to the cloud characterization is discussed.