High-latitude surface-atmosphere radiative coupling in the far-IR: missing physics in climate models and opportunities in future observations

Abstract:

Far-IR usually refers to the portion of the electromagnetic spectrum with a wavelength longer than 15 microns. Water vapor pure rotational absorption is the most important gaseous absorption in the far -IR and ice clouds have a scattering peak around 25 microns. The far -IR consists of ~50% of the infrared energy emitted by our planet to space, playing a critical role in the earth's radiation budget and related changes in response to future climate change.

Two assumptions widely adopted by virtually all climate models are blackbody surface and non-scattering cloud in the longwave radiation scheme. A fact overlooked by the modeling community is that such traditional wisdom breaks down in the high-latitude due to small water vapor abundance. As a consequence, the surface-atmosphere longwave coupling is manifested in the high latitudes. Using DoE E3SM and NCAR CESM, we show here the statistically significant and seasonally dependent impact of such longwave coupling on simulated polar climate and surface energy budget. The effect of surface spectral emissivity and longwave scattering is linearly additive to each other, and the dominant contribution is from the far-IR region. Our results show that the longwave scattering and surface spectral emissivity are both necessities for a faithful simulation of polar climate. Climate models should include both of them as necessities.

Accurate and spectrally resolved measurements in the far -IR have been technically challenging. Though the outgoing mid -IR spectra have been routinely observed from space with high accuracy and dense sampling pattern, as of today, we still have had no global spectrally resolved far-IR measurements from space. Motivated by recent studies, both NASA and ESA have selected missions dedicated to the far-IR radiation measurements, namely PREFIRE by NASA and FORUM by ESA. Both missions will provide us with critically needed observations for characterizing the surface-atmosphere longwave coupling, primarily through retrieved surface spectral emissivity and cloud properties in the far-IR dirty window. We show here some initial results and our thoughts on how field campaigns in the Arctic and a myriad of models and data analysis activities can help the success of such far-IR missions.