USE OF SCARAB MEASUREMENTS FOR VALIDATING A GOES-BASED TOA RADIATION PRODUCT

Lack of calibrated radiation measurements at the top of the atmosphere (TOA) between major spaceborne radiation missions entails inference o f the TOA radiation budget from operational weather sensors. The infer red data are subject to uncertainties due to calibration, narrow- to b roadband conversion, etc. In this study, a surrogate TOA earth radiati on budget product generated from GOES-7 (Geostationary Operational Env ironmental Satellite) imagery data for use in the U.S. Atmospheric Rad iation Measurement (ARM) program was validated using measurements from the ScaRaB radiometer flown on board the METEOR-3/7 satellite. Compar isons were made between coincident and collocated shortwave and longwa ve radiative quantities derived from GOES and ScaRaB sensors over an A RM experimental locale in the South Great Plains of Oklahoma, during A pril and July 1994. The comparisons are proven to be instrumental in v alidating the calibration and narrow- to broadband conversion used to obtain broadband radiative quantities from GOES digital counts. Calibr ations for both visible and infrared window channels have small uncert ainties, whereas narrow- to broadband conversion of shortwave measurem ents contains large systematic errors. The caveat stems from use of a quadratic conversion equation instead of a linear one, as was found fr om ScaRaB narrow- and broadband measurements. The ensuing errors in th e estimates of broadband albedo depend on scene brightness, underestim ation for bright scenes, and overestimation for dark scenes. As a resu lt, 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 im pact of clouds on atmospheric absorption) derived from ARM measurement s turns out to be 1.07, which is in even closer agreement with radiati ve transfer models than found from previous studies using original GOE S products.