Towards observing changes in the diurnal cycle of top-of-atmosphere radiation using Level-3 CERES Data Products

Improved observations of the diurnal cycle of top-of-atmosphere (TOA) radiation are necessary to better understand the Earth's climate processes and provide a critical resource for the evaluation of climate models. This study seeks to examine whether or not there are detectible changes in the diurnal cycle of TOA radiation in the Clouds and the Earth's Radiant Energy System (CERES) record using the CERES gridded monthly hourly TOA fluxes. Two approaches for determining TOA fluxes are considered. The first (termed non-GEO) interpolates the CERES observations using the assumption of constant meteorological conditions. The second (GEO) uses 3-hourly radiance and cloud property data from 5 geostationary imagers (covering 60°S-60°N) to more accurately model the variability between CERES observation times. As the calibration in both datasets is tied to CERES, the difference between the two sets of TOA fluxes should mainly be due to diurnal changes in clouds and meteorology. In practice, however, one must also account for uncertainties in the observations, particularly the geostationary data. Monthly averages of TOA fluxes corresponding to different hours of the day for both GEO and nonGEO approaches are examined. We analyze trends in the difference between the two as a function of time-of-day at global and regional scales, and look for possible variations in the diurnal cycle associated with changes in the ENSO index (e.g., in convective and stratus regions).