Version 4 CALIPSO Imaging Infrared Radiometer ice and liquid water cloud microphysical properties – Part I: The retrieval algorithms
Authors: Garnier Anne , Pelon Jacques , Pascal Nicolas , Vaughan Mark A. , Dubuisson Philippe , Yang Ping , Mitchell David L. 
Published: May 2021

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