Satellite Estimates of Surface Shortwave Fluxes: Sensitivity to Sensor Calibration

Surface radiation budget (SRB) parameters are routinely produced at NOAA/NESDIS in support of the GEWEX Continental Scale International Project (GCIP) and the GEWEX Americas Prediction Project (GAPP). A reprocessing activity is in progress at the University of Maryland to address some of the shortcomings of the SRB algorithms and to provide a consistent, 5-year data set of SRB parameters for use in climate and hydrological studies. Several aspects of the NOAA/NESDIS procedures are under review during the reprocessing effort, including the vicarious calibration of the GOES-8 visible sensor. The calibration procedures used in the real-time processing at NOAA/NESDIS are outlined in Weinreb et al. (1997). In a study conducted at the NASA Langley Research Center by Minnis et al. (2003), research satellites carrying onboard calibration systems were used as references to develop new calibration coefficients for GOES-8. We investigated the sensitivity of the inferred parameters to sensor calibration by running the SRB model with both the NOAA and NASA calibrations and evaluating the results in a two-step process. Surface shortwave (SW) downward fluxes are highly dependent on cloud cover, so for each calibration method, we evaluated the sensitivity of cloud detection to calibration and compared the results with those from ISCCP and MODIS. Next, we evaluated the satellite-estimated SW downward fluxes obtained from the NOAA and NASA calibrations on daily and hourly time scales, using ground measurements from four sources: SURFRAD stations, the Arizona Meteorological Network (AZMET), the Illinois Water Survey Network and the Atmospheric Radiation Measurement (ARM) Southern Great Plains (SGP) site. Neither calibration method displayed a distinct advantage over the other in every situation, but our evaluation yielded an estimate of the size of the errors in fluxes that can be attributable to calibration. Detailed results of these experiments will be reported.