Water vapor spectroscopy and thermodynamics constrain Earth's tropopause
temperature
Abstract
As Earth warms, the tropopause is expected to rise, but predictions of
its temperature change are less certain. One theory ties tropopause
temperature to outgoing longwave radiation (OLR), but this contradicts
simulations that exhibit a Fixed Tropopause Temperature (FiTT) even as
OLR increases. Another theory ties tropopause temperature to upper
tropospheric moisture, but is not precise enough to make quantitative
predictions. Here, we argue that tropopause temperature, defined by
where radiative cooling becomes negligible, is set by water vapor’s
maximum spectroscopic absorption and Clausius-Clapeyron scaling. This
“thermospectric constraint’ makes quantitative predictions for
tropopause temperature that are borne out in single column and general
circulation model experiments where the spectroscopy is modified and the
tropopause changes in response. This constraint underpins the FiTT
hypothesis, shows how tropopause temperature can decouple from OLR,
suggests a way to relate the temperatures of anvil clouds and the
tropopause, and shows how spectroscopy manifests in Earth’s general
circulation.