The retrieval of cloud optical depth and particle radius is relatively straightforward for water clouds over low-albedo surfaces
during the daylight hours. Under these conditions the nearly orthogonal relationship between optical depth and effective radius
at one absorbing and one non-absorbing wavelength (e.g., the AVHRR 0.6 and 3.7 micron channels) can be exploited. However,
high latitudes are characterized by highly-reflective surfaces and many months without solar radiation. The bright surfaces
give rise to multiple solutions for thin clouds, and the long polar night precludes the use of shortwave channels. Thermal
bands are therefore used to resolve the ambiguity when there are multiple solutions in the shortwave retrievals and to provide
a solution when there is no solar radiation. But thermal methods must account for another unknown: the cloud height. Investigations
using modeled radiances for a range of cloud heights show that two pairs of three thermal channels at 3.7, 11, and 12
microns can be used to obtain a unique solution. Unfortunately, validation data are not yet available so we assess the accuracy
of these methods using estimates of surface radiative fluxes.
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