Atmos. Chem. Phys. Discuss., 2, 1403-1427, 2002
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This discussion paper has been under review for the journal Atmospheric Chemistry and Physics (ACP). Please refer to the corresponding final paper in ACP.
The potential of polarization measurements from space at mm and sub-mm wavelengths for determining cirrus cloud parameters
J. Miao1, K.-P. Johnsen2, S. Buehler1, and A. Kokhanovsky1
1Institute of Environmental Physics, University of Bremen, Germany
2GKSS Research Center Geesthacht GmbH, Geesthacht, Germany

Abstract. The millimeter and sub-millimeter waves have been attracting a lot of attention recently in the cloud remote sensing community. This is largely because of its potential use in measuring cirrus cloud parameters with airborne or space-borne radiometers. In this study, we examine the possibility of using polarization measurements in this frequency range to retrieve microphysical parameters of cloud ice particles. By using a simple radiative transfer model, the polarization differences of the brightness temperatures measured at the vertical and horizontal polarization channels are calculated at the following seven frequencies: 90, 157, 220, 340, 463, 683, and 874 GHz. The cirrus clouds are modeled with nearly spherical particles, circular cylinder, and circular plate, as well as with mixtures of these types. We found that the polarization difference shows a unique ``resonance'' feature with the change of ice particle characteristic size: it has a strong response only in a certain range of ice particle size, beyond that range it approaches zero. The size range where this resonance happens depends to a large extent on particle shape and aspect ratio, but to a much less extent on particle orientation. This resonance feature remains even when the particles are mixed with different shapes, although the magnitude and the position of the resonance peak may change, depending on how ice particles are mixed. Oriented particles generally show larger polarization difference than randomly oriented ones, and plates have larger polarization difference than cylinders. However, particle orientations have a significantly stronger influence on the polarization difference than particle shapes (cylinder or plate). This makes it difficult to distinguish particle shapes using millimeter and sub-millimeter radiometric measurements, if there is no information available on particle orientations. However, if the state of particle shape mixture can be predetermined by other approaches, polarization measurements can help to determine ice particle characteristic size and orientation. This information, in turn, will benefit the retrieval of cloud ice water path.

Citation: Miao, J., Johnsen, K.-P., Buehler, S., and Kokhanovsky, A.: The potential of polarization measurements from space at mm and sub-mm wavelengths for determining cirrus cloud parameters, Atmos. Chem. Phys. Discuss., 2, 1403-1427, doi:10.5194/acpd-2-1403-2002, 2002.
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