Abstract
Abstract. A new satellite remote sensing method is described whereby the sensitivity of
thermal infrared wave resonance absorption to small ice crystals is exploited
to estimate cirrus cloud ice-particle number concentration N, effective
diameter De and ice water content IWC. This method uses
co-located observations from the Infrared Imaging Radiometer (IIR) and from
the CALIOP (Cloud and Aerosol Lidar with Orthogonal Polarization) lidar
aboard the CALIPSO (Cloud-Aerosol Lidar and Infrared Pathfinder Satellite
Observation) polar orbiting satellite, employing IIR channels at 10.6 and
12.05 µm. Using particle size distributions measured over many
flights of the TC4 (Tropical Composition, Cloud and Climate Coupling) and the
mid-latitude SPARTICUS (Small Particles in Cirrus) field campaigns, we show
for the first time that N∕IWC is tightly related to
βeff; the ratio of effective absorption optical depths at
12.05 and 10.6 µm. Relationships developed from in situ aircraft
measurements are applied to βeff derived from IIR
measurements to retrieve N. This satellite remote sensing method is
constrained by measurements of βeff from the IIR and is by
essence sensitive to the smallest ice crystals. Retrieval uncertainties are
discussed, including uncertainties related to in situ measurement of small
ice crystals (D<15 µm), which are studied through comparisons
with IIR βeff. The method is applied here to single-layered
semi-transparent clouds having a visible optical depth between about 0.3 and
3, where cloud base temperature is ≤235 K. CALIPSO data taken over
2 years have been analyzed for the years 2008 and 2013, with the dependence
of cirrus cloud N and De on altitude, temperature, latitude,
season (winter vs. summer) and topography (land vs. ocean) described. The
results for the mid-latitudes show a considerable dependence on season. In
the high latitudes, N tends to be highest and De smallest,
whereas the opposite is true for the tropics. The frequency of occurrence of
these relatively thick cirrus clouds exhibited a strong seasonal dependence
in the high latitudes, with the occurrence frequency during Arctic winter
being at least twice that of any other season. Processes that could
potentially explain some of these micro- and macroscopic cloud phenomena are
discussed.
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