The potential of polarization measurements from space at mm and sub-mm wavelengths for determining cirrus cloud parameters

[摘要] The millimeter and sub-millimeter waves have been attracting a lot ofattention recently in the cloud remote sensing community. This is largely because of their potential use in measuring cirrus cloud parameters withairborne or space-borne radiometers. In this study, we examine the possibility of using polarization measurements in this frequency range to getinformation on the microphysical properties of cirrus clouds. By using a simple radiative transfer model, we calculated the brightness temperaturedifferences at the vertical and horizontal polarization channels for the following seven frequencies: 90, 157, 220, 340, 463, 683, and874 GHz. The ice crystals in 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 signal shows a unique"resonance'' feature with the change of ice particle characteristic size: ithas a strong response only in a certain range of ice particle size, beyondthat range it approaches zero. The size range where this resonance happensdepends to a large extent on particle shape and aspect ratio, but to a muchless extent on particle orientation. This resonance feature appears even whenice clouds are composed of a mixture of ice crystals in different shapes, although the magnitude and the position of the resonance peak may change,depending on how the mixture is made. Oriented particles generally show larger polarization difference than randomly oriented ones, and plates havelarger polarization difference than cylinders. However, the state of particleorientation has a significantly stronger effect on the polarization difference than the particle shape (cylinder or plate). This makes itdifficult to distinguish particle shapes using millimeter and sub-millimeterradiometric measurements, if there is no information available on particleorientations. However, if the state of particle shape mixture can be predetermined by other approaches, polarization measurements can help todetermine ice particle characteristic size and orientation. This information,in turn, will benefit our retrieval of the ice water path of cirrus clouds.

[学科分类] 大气科学