Converting Geostationary Narrowband Radiances to Fluxes for the CERES SYN1deg Product

The CERES mission was designed to measure changes to the net TOA flux on time scales relevant to climate change. To achieve this goal, CERES provides the SSF1deg, SYN1deg, and EBAF monthly-regional 1˚x1˚ level TOA flux level-3 products. The single satellite (Terra or Aqua) SSF1deg shortwave flux depends on one daytime measurement and assumes constant meteorology to model the diurnal changes in albedo. To more accurately describe regions with a prominent diurnal signal, the SYN1deg Edition4 dataset employs hourly geostationary (GEO) measurements. This is an improvement upon Edition 3, which utilized 3-hourly GEO measurements that required temporal interpolation. The EBAF product combines the temporal stability of the SSF1deg product with the diurnally complete SYN1deg product and removes the CERES instrument calibration bias by constraining the net flux balance to the ocean heat storage term.

This research performs an uncertainty analysis on each of the steps utilized to convert GEO radiances to fluxes. This will help pinpoint inadequate steps and to minimize potential errors that can propagate through the process. The GEO narrowband radiances are converted to fluxes using the following steps. The CERES project inter-calibrates the GEO radiances using MODIS as a reference and for temporal stability; GEO imagers were designed for weather forecasting and most do not have onboard calibration systems. Once the GEOs are calibrated the narrowband radiances must be converted to broadband CERES-like radiances. This is challenging, since each of the GEOs have their own spectral response functions. The converted radiances are then passed to an angular distribution model (ADM) to derive the fluxes. This step requires consistent scene type identification across GEO domains. GEO derived fluxes are then normalized to the CERES fluxes in order to maintain CERES calibration. This study will focus on the narrowband to broadband process and the ADM used in the SYN1deg product.