Shortwave and Longwave Top-of-Atmosphere Radiative Flux Estimation From the Clouds and the Earth’s Radiant Energy System Instrument (Invited Presentation)

One of the largest uncertainties in global climate models is the representation of how clouds and aerosols influence the Earth’s radiation budget at the surface, within the atmosphere and at the top of the atmosphere. Because of the uncertainty in cloud-aerosol-radiation interactions, model predictions of climate change vary widely from one model to the next. In order to understand the reason for these discrepancies and to identify key areas where climate models can be improved, global observations are needed. The two Clouds and the Earth’s Radiant Energy System (CERES) satellite instruments aboard the Terra spacecraft provide highly accurate shortwave (SW), longwave (LW) and window (WN) radiance measurements and top-of-atmosphere (TOA) radiative flux estimates globally at 20-km spatial resolution. These data, together with coincident cloud and aerosol properties inferred from the Moderate Resolution Imaging Spectrometer (MODIS), provide a consistent cloud-aerosol-radiation dataset for studying the critical role that clouds and aerosols play in modulating the radiative energy flow within the Earth-atmosphere system.

One of the challenges involved in producing radiation datasets from satellites is the need to convert the radiance measurements at a given sun-Earth-satellite configuration to outgoing reflected solar and emitted thermal top-of-atmosphere (TOA) radiative fluxes. To estimate TOA fluxes from measured CERES radiances, one must account for the angular dependence in the radiance field, which is a strong function of the physical and optical characteristics of the scene (e.g. surface type, cloud fraction, cloud/aerosol optical depth, cloud phase), as well as the illumination angle. Because the CERES instrument can rotate in azimuth as it scans in elevation, it acquires data over a wide range of angles. Consequently, one can construct angular distribution models (ADMs) for radiance-to-flux conversion from the CERES measurements. Furthermore, since CERES and MODIS are on the same spacecraft, the ADMs can be derived as a function of MODIS-based scene type parameters that have a strong influence on radiance anisotropy.

This presentation provides a brief overview of the methodology and validation results for a new set of global CERES ADMs developed from two years of CERES measurements on the Terra spacecraft. We provide a summary of the uncertainties in both instantaneous and regional mean TOA radiative fluxes for clear and all-sky conditions over ocean, land, desert and snow surfaces. Further stratification of the uncertainties by cloud type will also be shown along with preliminary examples demonstrating how these data can be used to quantify the radiative effect of aerosols and different cloud types.