Measurements from the Clouds and Earth Radiant Energy System (CERES) scanner aboard the Tropical Rainfall Measurement Mission TRMM are compared with measurements from the wide field-of-view (WFOV) radiometers aboard the Earth Radiation Budget Spacecraft ERBS on a quasi-instantaneous basis. The CERES radiances are numerically integrated to compute the longwave and shortwave fluxes which are measured by the WFOV radiometers. The difference in direction of the radiation between that measured by the scanning radiometer and that measured by the WFOV radiometer is taken into account by the use of bidirectional reflectance distribution functions for the shortwave radiation and limb-darkening functions for the longwave radiation.
The ERBS has an altitude of 620 km and the TRMM is much lower at 350 km. As a consequence, the swath width measured by the CERES scanner is narrower than the field of view of the WFOV radiometers aboard the ERBS. It is thus necessary to extrapolate the scanner results to cover the WFOV field of view, adding to the random scatter of the results.
At night, the longwave radiation measurements come from total radiation channels of both instruments. The mean longwave flux at the ERBS altitude at night for the orbit intersection points is found to be 207 W-m**-2 for the WFOV and the CERES result is 1.1 W-m**-2 higher, so that the total channels of the two instruments agree to better than 1 percent. The mean shortwave flux measured by the shortwave WFOV radiometer is 141 W-m**-2 and the CERES result is 3.4 W-m**-2 higher, for an agreement better than 2.5 percent. The daytime longwave results depend on the total and shortwave channels for each instrument. The mean daytime longwave flux at the ERBS spacecraft is measured by the WFOV to be 223 W-m**-2 and the result computed from CERES radiances is 3.9 W-m**-2 higher, or 1.7 percent