Use of ScaRaB Measurements for Validating a GOES-Based TOA Radiation Product

Abstract

Lack of calibrated radiation measurements at the top of the atmosphere (TOA) between major spaceborne radiation missions entails inference of the TOA radiation budget from operational weather sensors. The inferred data are subject to uncertainties due to calibration, narrow- to broadband conversion, etc. In this study, a surrogate TOA earth radiation budget product generated from GOES-7 (Geostationary Operational Environmental Satellite) imagery data for use in the U.S. Atmospheric Radiation Measurement (ARM) program was validated using measurements from the ScaRaB radiometer flown on board the METEOR-3/7 satellite. Comparisons were made between coincident and collocated shortwave and longwave radiative quantities derived from GOES and ScaRaB sensors over an ARM experimental locale in the South Great Plains of Oklahoma, during April and July 1994. The comparisons are proven to be instrumental in validating the calibration and narrow- to broadband conversion used to obtain broadband radiative quantities from GOES digital counts. Calibrations for both visible and infrared window channels have small uncertainties, whereas narrow- to broadband conversion of shortwave measurements contains large systematic errors. The caveat stems from use of a quadratic conversion equation instead of a linear one, as was found from ScaRaB narrow- and broadband measurements. The ensuing errors in the estimates of broadband albedo depend on scene brightness, underestimation for bright scenes, and overestimation for dark scenes. As a result, the magnitude of the TOA cloud radiative forcing is underestimated by about 14 W m?2 or 7.5% on a daytime mean basis. After correcting this error, the ratio of cloud radiative forcing (a measure of the impact of clouds on atmospheric absorption) derived from ARM measurements turns out to be 1.07, which is in even closer agreement with radiative transfer models than found from previous studies using original GOES products.