Retrieval of cloud optical depth and particle effective radius at high latitudes using visible and thermal satellite data

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Abstract
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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