ON THE DETERMINATION OF THE OPTIMAL SCAN MODE SEQUENCE FOR THE TRMM CERES INSTRUMENT

Clouds and the Earth's Radiant Energy System (CERES) is a NASA spacebo rne measurement program for monitoring the radiation environment of th e earth-atmosphere system. The first CERES instrument is scheduled to be launched on board the Tropical Rainfall Measuring Mission (TRMM) sa tellite in late 1997. In addition to gathering traditional cross-track fixed azimuth measurements for calculating monthly mean radiation fie lds, this single CERES scanner instrument will also be required to col lect angular radiance data using a rotating azimuth configuration for developing new angular dependence models (ADMs). Since the TRMM single CERES instrument can only be run in either one of these two configura tions at any one time, it will need to be operated in a cyclical patte rn between these two scan modes to achieve the intended measurement go als. To minimize the errors in the derived monthly mean radiation fiel d due to missing cross-track scanner measurements during this satellit e mission, determination of the optimal scan mode sequence for the TRM M single CERES instrument is carried out. The Earth Radiation Budget E xperiment S-4 daily mean cross-truck scanner data product for April an d July 1985 and January 1986 is used with a simple temporal sampling s cheme to produce simulated daily mean cross-track scanner measurements under different TRMM CERES operational scan mode sequences. Error ana lysis is performed on the monthly mean radiation fields derived from t hese simulated datasets. It is found that the best monthly mean result occurred when the cross-track scanner is operated on a ''2 days on an d I day off'' mode. This scan mode sequence will effectively allow for 2 consecutive days of cross-track scanner data and 1 day of angular r adiance measurement for each 3-day period. The root-mean-square errors for the monthly mean all-sky (clear sky) longwave and shortwave radia tion field, due to missing cross-track scanner measurements for this p articular case, are expected to be less than 2.5 (0.5) and 5.0 (1.5) W m(-2), respectively.