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

Far-IR usually refers to the portion of the electromagnetic spectrum with a wavelength longer than 15 microns. For terrestrial atmosphere, 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 and, thus, plays a critical role in the earth's radiation budget and related changes in response to future climate change. 

Due to the overwhelming role played by the water vapor absorption in the far-IR, the traditional wisdom that assumes blackbody surface and non-scattering cloud in the longwave radiation scheme are well justified for the tropics and mid-latitude. However, such approximations widely adopted by virtually all the climate models break 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, with the most prominent impact in the polar winter. Using the NCAR CESM and DoE E3SM models, we quantitatively show 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 the faithful simulation of polar climate. Climate models should include both of them, which are missing in virtually all the current models.

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. The last spectrally resolved measurements from space for 15 -25 microns were made a half-century ago in 1970 -1971. 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 (16.7-29 microns). We show here some initial results for relevant retrieval algorithm developments and expected uncertainties for the surface spectral emissivity and cloud properties retrieved from such far-IR measurements.