Version 4 CALIPSO Imaging Infrared Radiometer ice and liquid water cloud microphysical properties – Part I: The retrieval algorithms

Abstract. Following the release of version 4 Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) data products from and Infrared Pathfinder Satellite Observations (CALIPSO) mission, a new (version 4; V4) CALIPSO Imaging Radiometer (IIR) Level 2 has been developed. The IIR include cloud effective emissivities microphysical properties such as diameter ice or liquid water path estimates. Dedicated retrievals for clouds were added in V4, taking advantage high sensitivity retrieval technique to small particle sizes. This paper (Part I) describes improvements V4 algorithms compared those used 3 (V3) release, while results will be presented companion II) paper. algorithm modified improve accuracy very (close 0) large 1) emissivities. To reduce biases at that made evident V3, radiative transfer model compute clear-sky brightness temperatures over oceans updated tuned simulations using Modern-Era Retrospective analysis Research Applications (MERRA-2) match observations conditions. Furthermore, mask refined V3 by additional information now available CALIOP 5 km layer an input algorithm. After sea surface emissivity adjustments, observed computed differ less than ±0.2 K night three channels centered 08.65, 10.6, 12.05 µm, inter-channel are reduced several tens Kelvin 0.1 V4. We have also improved having refining determination temperature needed computation. initial estimate, namely centroid derived CALIOP, is corrected parameterized function difference between base top altitudes, absorption optical depth, multiple scattering correction factor. As shown Part II, this improvement reduces low depths seen increases number retrievals. use concept indices applied pairs 10.6 µm 08.65 µm. uses look-up tables (LUTs) built two habit models recent “TAMUice2016” database, single-hexagonal-column eight-element column aggregate model, which bulk synthesized gamma size distribution. Four sets diameters second approach reported Here, LUTs analytical functions relating index situ measurements tropical midlatitudes during Tropical Composition, Cloud, Climate Coupling (TC4) Small Particles Cirrus Science Operations Plan (SPARTICUS) field experiments.