The role of spaceborne millimeter-wave radar in the global monitoring of ice cloud

Abstract

The purpose of this paper is to assess the potential of spaceborne 94-GHz radar for providing useful measurements of the vertical distribution and water content of ice clouds on a global scale. Calculations of longwave (LW) fluxes for a number of model ice clouds are performed. These are used to determine the minimum cloud optical depth that will cause changes in the outgoing longwave radiation or flux divergence within a cloud layer greater than 10W m-2, and in surface downward LW flux greater than 5W m-2, compared to the clear-sky value. These optical depth values are used as the definition of a "radiatively significant' cloud. Different "thresholds of radiative significance' are calculated for each of the three radiation parameters and also for tropical and midlatitude cirrus clouds. A radar with a threshold of -30 dBZ should detect 99% (92%) of "radiatively significant' clouds in the midlatitudes (Tropics). This detection efficiency may be reduced significantly for tropical clouds at very low temperatures (-80°C) -from Authors