||One of the objectives of the GEWEX program is to foster the development of relevant research and observing projects. Determination of the global three-dimensional distribution of cloud parameters from satellite observations was recognized at an early stage by the GEWEX Scientific Steering Committee as an urgent need not met by existing or planned sensors of the international space agencies. In 1992 the concept of a millimeter-wave cloud radar was presented and endorsed, first at the GEWEX Scientific Steering Group meeting and then later that year at two special workshops on GEWEX dedicated space missions in Jouy-en-Josas, France (June 1992) and Tokyo, Japan (November 1992).
A simple nadir-pointing radar is thought to provide a suitable sample of the large-scale cloud information on a monthly-mean time scale and on a spatial scale of approximately 250-500 km, which is adequate for the large-scale cloud variability of significance to climate. A radar that provides vertical cloud profiles with a resolution of 500 m is adequate for preserving radiative surface and atmospheric budgets to better than 5-10 Wm**-2. A preliminary engineering feasibility study indicates that a 94 GHz spaceborne, nadir-pointing radar can be designed to meet the science requirements.
Several ground based 35 and 94 GHz instruments have been collecting data (Penn State and the NOAA/ETL Radar and Meteorology and Oceanography Division in Boulder) and a limited number of aircraft flights have been made (University of Wyoming). Aircraft instrument development programs are underway at NASA and JPL, at the GKSS Research Institute in Germany, and in Japan. These activities will provide the basis for furthering the concept of a GEWEX spaceborne cloud radar system.