GISS Lunch Seminar
Speaker: Jing Feng (NOAA/GFDL)
Title: The Importance of Atmosphere on Outgoing Longwave Radiation and Longwave Feedback

Abstract:
The Earth's global radiation budget depends critically on the relationship between outgoing longwave radiation (OLR) and surface temperature (Ts). In a clear-sky condition, OLR tends to increase with Ts at 2 Wm-2K-1.While previous studies attributed this relationship to the rate at which surface thermal emission escapes to space, our findings reveal that the surface contribution only explains 60% of the observed OLR-Ts slope. In this talk, I will provide an overview of how the atmosphere contributes to the OLR-Ts slope and its importance for future climate.

First, it is found that the pressure-broadening effect on water vapor absorption lines changes the emission temperature of the atmosphere, contributing 0.4 Wm-2K-1 to the OLR-Ts slope, assuming constant relative humidity. Second, in warm regions, the surface contribution to the OLR-Ts slope diminishes rapidly with Ts due to increased infrared opacity of the atmosphere resulting from higher moisture content. As a result, the OLR in warm regions is controlled by atmospheric emission, which is sensitive to variations in temperature and humidity. In particular, we find that mid-tropospheric RH is important to the observed OLR on present-day Earth and explains a large fraction of inter-modal spreads in clear-sky OLR from AMIP simulations.

Moreover, our research reveals that the longwave feedback parameter in the tropics is as stable as in colder regions on Earth, owing to a stabilizing feedback process maintained by the atmospheric emission through the pressure-broadening effect and enhanced upper-tropospheric warming in the tropics. We conclude the atmospheric feedback process critically stabilizes Earth's climate and becomes more important as the climate warms.