Observations of the Clear-Sky Spectral Longwave Feedback at Surface
Temperatures Between 210K and 310K
Abstract
The longwave feedback λ characterizes how Earth’s outgoing longwave
radiation changes with surface temperature Ts, making it an important
quantity to estimate Earth’s climate sensitivity. Compared to the
traditionally studied λ, its spectrally resolved counterpart λν offers
deeper insights into the underlying physical processes. Both λ and λν
are known to vary with Ts, but this Ts dependence has so far only been
investigated using models. Here, we derive the clear-sky spectral
longwave feedback λν for surface temperatures Ts between 210K and 310K
based on observations of the AIRS instrument onboard the Aqua satellite.
We disentangle the radiative signatures of the atmospheric general
circulation by simulating λν based on a single-column model with
different degrees of idealization. We find that at low Ts, the observed
λν is dominated by the surface response and sensitive to biases in
Earth’s skin temperature. At higher Ts, changes in atmospheric
temperature and humidity, as well as their vertical distribution, play
an important role in shaping λν . These changes impact both the
absorption of surface emission in the atmospheric window and the
atmospheric emission in the water vapor and CO2 absorption bands. Our
results demonstrate that we can fully understand the observed λν at a
wide range of Ts using a simple conceptual model of Earth’s atmosphere.
This understanding can be used to better constrain changes in R and T
with warming in Earth’s climate using satellite observations, as well as
for paleoclimate and exoplanet studies.