A Geophysical Cross-Calibration Approach for Broadband Channels: Application to the ScaRaB Experiment

Abstract

For the determination of the earth radiation budget (ERB) from space, the reflected solar flux and the emitted longwave (LW: 4?100 ?m) fluxes are estimated using radiance measurements made in shortwave (SW: 0.2?4.5 ?m) and ?total? (TW: 0.2?100 ?m) channels. An accurate (1%) calibration and cross-calibration of these two channels is required for the determination of the ERB and for daylight determination of the LW radiance based on differences between the TW and the SW radiances. This paper presents an approach to calibrate the SW channel by using a cross-calibration between the SW channel and the SW part of the TW channel. This approach is applied and validated using data from the Scanner for Radiation Budget (ScaRaB) experiment. The principle of this cross-calibration is to estimate the LW radiance from the infrared window (IR: 10.5?12.5 ?m) radiance measurements over deep convective cloudiness in the Tropics. This estimate makes it possible to subtract the LW signal from the TW radiance measurement during daylight and thus to compare directly the SW radiances measured by the SW and the TW channels. An IR channel is already implemented on ScaRaB and on the Cloud and the Earth?s Radiant Energy System (CERES) instruments. Using ScaRaB data, it is shown that it is possible to estimate the LW radiance over deep convective cloudiness in the Tropics from IR radiance measurement with accuracy better than 1 W m?2 sr?1. The method applied to ScaRaB measurements gives calibration and cross-calibration parameters with accuracy better than 1%.