Comparison of Clear Sky OLR from AIRS and CERES Observations

Knowledge of earth's outgoing longwave radiation (OLR) is an essential part of global climate change studies. A number of radiative transfer models have been developed independently for different programs. Recognizing their differences and how they compare to measurements will give us greater confidence to extend their use in models. Study of the clear sky component is the first step. Two radiative transfer model calculations of clear sky outgoing longwave radiation (OLR) are compared to the Cloud and Earth's Radiant Energy System's (CERES) measurements. For both model calculations, retrievals NASA's Atmospheric Infrared Sounder (AIRS) of surface temperature, and profiles of temperature and water vapor are used. One model, Atmospheric and Environmental Research, Inc.'s (AER) rapid radiative transfer model (RRTM), was developed to be a radiatively consistent for numerical weather models and climate change studies. The model used for the AIRS version 5 Level 2 Standard Product clear-sky outgoing longwave radiation uses the retrieved state in a rapid radiative transfer algorithm documented in Mehta and Susskind (1999) and NASA Technical Report GSFC/CR-1999-208643. The AIRS Science team plans to use an improved version of the OLR radiative transfer code in the AIRS Version 6 OLR and clear sky calculations. The Clouds and the Earth's Radiant Energy System (CERES) instrument is onboard the EOS Aqua spacecraft (along with the AIRS instrument). CERES products include both solar-reflected and Earth- emitted radiation from the top of the atmosphere (TOA) to the Earth's surface. For this study we will compare the TOA clear sky OLR from the two models globally to the CERES OLR for November 16, 2002. We also validate AIRS and CERES OLR against measurements at the ARM SGP site at Aqua overpass times between the years 2003 and 2004. Atmospheric radiosondes at the SGP site are interpolated to AIRS overpass times so that clear sky RRTM flux calculations using derived AIRS surface temperatures and emissivities would be coincident. We are able to use the AIRS high spectral observations to constrain the atmosphere state assumptions. The temperature soundings are also appended with upper level atmospheric profiles from the ECMWF model output to study their impact on TOA net flux.