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

The role of clouds as a major influence on the Earth´s radiation budget and therefore in the control of the global climate system is well-recognised. Global estimation of the short-wave (SW) components of the surface radiation budget (SRB) from satellite radiance measurements have shown some promise (Rieland and Stuhlmann, 1993). However, estimation of the downwelling long-wave (LW) component of the SRB and atmospheric heating profiles requires the use of active sounding methods to provide the necessary measurements of cloud base and cloud top and of the liquid- or ice-water path. A frequency of 94 GHz or thereabouts has been proposed for a spaceborne cloud radar (IGPO, 1994) primarily because of the sensitivity it provides, its relative freedom from atmospheric attenuation, and its compact antenna size. The primary aim of the paper is to assess the potential of such a radar for providing global measurements of the vertical distribution of cloud condensate with accuracy sufficient to enable the determination of the associated radiation budget effects to within some defined limits. The authors confine their attention to stratiform ice clouds. They also concentrate on the consideration of LW fluxes, since it is in this area that current space-based estimates of the SRB and flux profiles are most clearly deficient and for which the determination of vertical cloud profiles and, in particular, the determination of cloud base is expected to make a significant contribution