Validation of the CERES/TRMM ERBE-like monthly mean clear-sky longwave dataset and the effects of the 1998 ENSO event

The Clouds and the Earth's Radiant Energy System (CERES) is a new National Aeronautics and Space Administration space-borne measurement project for mo nitoring the radiation environment of the earth-atmosphere system. The firs t CERES instrument was launched into space on board the Tropical Rainfall M easuring Mission (TRMM) satellite on 27 November 1997. The purpose of this paper is 1) to describe the initial validation of the new CERES/TRMM Earth Radiation Budget Experiment (ERBE)-like monthly mean clear-sky longwave (CL W) dataset and 2) to demonstrate the scientific benefit of this new dataset through a data application study on the 1998 El Nino-Southern Oscillation (ENSO) episode. The initial validation of the CERES CLW data is carried out based on comparisons with both historical ERBE observations and radiative transfer simulations. While the observed CERES CLWs are initially larger th an the historical ERBE record during the first part of the 1998 ENSO event, these differences are diminished by the end of the ENSO event in July 1998 . These unique ENSO-related CLW radiation signatures are captured well by t he radiative transfer model simulations. These results demonstrate that the new CERES CLW fluxes are theoretically consistent with the underlying phys ics of the atmosphere. A CERES data application study is performed to exami ne the relationship between the CERES CLW anomaly and changes in sea surfac e temperature (SST) and atmospheric column precipitable water content (PWC) during the January 1998 ENSO event. While the changes in the SST pattern a re basically uncorrelated with changes in the CLW field, a negative correla tion is found between the PWC anomaly and the changes in the CLW radiation field. These observed features point to 1) the significant role of the wate r vapor field in modulating the tropical outgoing CLW radiation field durin g the 1998 ENSO event and 2) the important effects of water vapor absorptio n in decoupling the top of the atmosphere tropical outgoing CLW radiation f rom the surface upward CLW field.