What Did We Learn from Co-located CALIPSO Lidar and Thermal Infrared Passive Observations?
Résumé
Cirrus clouds are of particular importance for understanding climate change due to their extensive
impacts on the Earth’s radiation budget. Cirrus cloud optical depths are retrieved in the visible from
CALIPSO’s Cloud and Aerosol Lidar with Orthogonal Polarization (CALIOP) lidar and in the
thermal infrared (IR) from the Imaging Infrared Radiometer. Thermal IR retrievals are only weakly
sensitive to scattering and to microphysical assumptions required for conversion from the IR to the
visible spectrum. IIR retrievals thus provide a critically important reference to assess and improve
CALIOP retrievals, which need lidar ratios to convert backscatter to extinction and must be corrected
to account for multiple scattering effects. MODIS thermal IR observations have likewise been used
to resolve cirrus optical depth biases between CALIOP and MODIS visible/near IR retrievals.
The three IIR channels at 8.65, 10.6 and 12.05 μm were chosen specifically for retrievals of cirrus
microphysical properties. The size information is contained in the 12.05-to-10.6 and 12.05-to-8.65
ratios of effective absorption optical depths, called, respectively, the beff12/10 and beff12/08 micro-
physical indices. These indices are very sensitive to the presence of small particles. By taking
advantage of CALIOP’s layer detection sensitivity, IIR provides ice crystal effective diameters (De)
in thin cold cirrus with optical depths smaller than 0.3 over oceans, thus augmenting existing
retrievals from MODIS and the CloudSat radar. By applying a novel technique that uses in situ
measurements acquired during various field campaigns to relate the beff12/10 index to other relevant
microphysical quantities, ice crystal number concentrations can be retrieved from IIR measurements,
thereby offering new insight into cirrus cloud formation through heterogeneous and/or homogeneous
nucleation. Synergies with CALIOP are further reinforced by using the CALIOP extinction profiles
to infer the fraction of the cloud layer to which IIR is sensitive.
Cloud thermodynamical phase has a strong impact on the Earth’s radiation budget. IIR retrievals
show close agreement with the CALIOP ice/water phase classification scheme, which relies on the
so-called Hu diagram relating layer-integrated depolarization ratio and layer-integrated attenuated
backscatter at 532 nm. The beff12/10 index is unambiguously larger for CALIOP-identified water
clouds, where droplet diameters are typically smaller than the characteristic De of cirrus ice crystals.
Consequently, IIR standard retrievals are now extended to De retrievals in water clouds of moderate
optical depth. To ensure sufficient radiative contrast with respect to the surface, these retrievals are
primarily applied to supercooled water clouds. This additional IIR beff12/10 and De information,
along with the CALIOP 1064/532 backscatter color ratio, can now augment the quantities used in the
Hu diagram, thus enabling accurate identification of mixed phase clouds. For aerosol analyses,
collocated CALIOP and IIR observations also contribute to a better understanding of the longwave
direct radiative effect of dust with, in particular, retrievals of dust IR optical depth