Simulation of CERES and GERB TOA products over the Valencia Anchor Station for GERB validation purposes

Abstract

The Geostationary Earth Radiation Budget (GERB) instrument on board Meteosat-8 and -9 provides accurate measurements of shortwave and longwave broadband radiances and fluxes at the Top of the Atmosphere (TOA) with very high temporal resolution of 15 minutes. The aim of the study presented here is to validate radiances and fluxes at the TOA measured by GERB during the IV GERB Ground Validation Campaign at the Valencia Anchor Station (VAS) area (31stJuly – 6thAugust 2006). In the study, GERB enhanced spatial resolution data (GERB High Resolution) is used, where the resolution of the computed fluxes is improved through the combination of well-calibrated GERB broadband data with SEVIRI narrow-band high-sampling-rate data. The resolution enhancement process transforms accurate fluxes computed at GERB footprint original resolution (nominally 50 km x 50 km at nadir) to a 3*3 SEVIRI footprint resolution (nominally 9 km x 9 km at nadir). Clouds and the Earth’s Radiant Energy System (CERES) Terra FM2 data is also used from dedicated PAPS (Programmable Azimuth Plane Scanning) observations over the study area during the campaign. The validation capabilities of the Valencia Anchor Station have previously been assessed by successfully reproducing CERES TOA radiances and fluxes with the occasion of different ground validation campaigns. The methodology consists in performing radiative transfer simulations of CERES and GERB TOA radiances and fluxes from independent ground measurements of surface and atmospheric parameters (such as derived precipitable water vapour content from CIMEL sunphotometer and GPS (Global Positioning System) instruments, radiosoundings from the Spanish stations of Madrid and Murcia, aerosol optical thickness also from CIMEL, broadband albedo and temperature over shrubs, bare soil and vineyards in the study area) in conjunction with other satellite products such as TOMS (Total Ozone Mapping Spectrometer) ozone, CERES/SARB emissivity and MODIS BRDF (Bidirectional Reflectance Distribution Function). The latter allows us to analyze the contribution of each land use to the anisotropy of the shortwave radiation field and constitutes a good improvement of the methodology that had been tested for the case of CERES in previous campaigns. The comparison between simulations and CERES calibrated and validated data provides a good indicator of the reliability of the methodology to be applied as a validation tool for GERB. The results presented here show the comparison between simulated and measured CERES and GERB radiances under clear sky conditions for the campaign dates.