Exploring Clear-Sky Longwave Radiative Closure: A downwelling case study

Far-infrared (FIR) radiation plays a fundamental role in regulating the Earth’s greenhouse effect, particularly in cold and dry regions where a large fraction of the outgoing longwave radiation is emitted at FIR wavelengths. Measurements across this range are limited and previous studies have found it challenging to achieve radiative closure between different instruments across the mid-far infrared. In January and February 2025, ESA funded a campaign in the Gault Nature reserve (Canada) in support of the FORUM (Far-infrared Outgoing Radiation Understanding and Monitoring) Earth explorer mission. One of the campaign aims was investigate the consistencies between atmospheric states modelled and measured at the surface. This study exploits measurements taken by the Far INfrarEd Spectrometer for Surface Emissivity (FINESSE) instrument, a ground-based Fourier transform spectrometer, which was based at Gault.  

First, clear sky scenes were selected through local HALO Doppler lidar measurements and a bi-spectral method using the FINESSE radiances. The high-resolution downwelling spectra are compared to radiative transfer simulations (LBLRTM) run with atmospheric profiles from a local radiosonde (IMET4) and the 5th ECMWF atmospheric reanalysis (ERA5). These residuals were averaged over time and the different sources of uncertainty, including spectroscopic uncertainty, were combined.  

Preliminary results from this study indicate that simulations driven by ERA5 profiles generally show improved agreement with the observed radiances compared to those using IMET4 radiosonde inputs. This suggests that ERA5 more accurately captures the vertical structure of temperature and humidity during the campaign period. The radiosonde was launched very close to FINESSE, however there were strong winds and the sondes tended to fly east of FINESSE. The impact of the movement on the radiance residuals has been characterised.

The largest discrepancies in both cases are seen in the 400-600 cm-1 region and in the 𝜈2CO2 band wings between around 580 - 620 cm-1 and 710-750 cm-1. These are regions which are particularly sensitive to the water vapour and temperature profiles. Within these areas, there are sections where the residuals extend beyond the estimated combined uncertainties. The radiosonde residuals in the 400-600 cm-1 region indicate a possible dry bias. We conducted Sensitivity experiments exploring how variations in humidity within different atmospheric layers influences the simulated radiances. Furthermore, the impact of recent revisions to H₂O line parameters on the radiance residuals is explored.