Interannual variations in satellite observed top-of-atmosphere longwave clear-sky radiation and consistency with diagnostics from global climate models

The fundamental role of water vapor in linking radiation, convection and dynamics has been widely studied and debated as a classical problem in climate dynamics. Water vapor feedback associated with climate change has been a subject of particularly intense debate since it is judged to be the most prominent feedback and the principal mechanism supporting claims of global warming scenarios.

The present study uses a combination of space-based observations, radiative transfer modeling, and global climate models to examine sensitivity of TOA longwave clear-sky fluxes (LWCS) to tropical SST anomalies. In particular we provide estimates of water vapor feedback associated with recent warm and cool phases of ENSO. Two long-term data sets based on the TOVS HIRS2 sensors are used. One of these, the Path-A LWCS product, is complete back to 1987. A second algorithm developed at the University of Maryland is statistically based and uses 4 HIRS2 channels to estimate LWCS. Both of these algorithms are compared with CERES LWCS estimates avaliable from January through August 1998. Additional calculations of water vapor feedback from a microwave-only algorithm based on the MSU, SSM/I and SSMT2 are compared to the TOVS/HIRS2 and CERES measurements.

Diagnostics from the CCM3 and the NCEP Global Spectral Models forced by observed SSTs are also produced. We evaluate the ability of these models to replicate the observed variations in LWCS and the contributions by water vapor, surface temperature, and lapse rate feedback.