Methane Cross-Validation Between Spaceborne Solar Occultation Observations from ACE-FTS, Spaceborne Nadir Sounding from Gosat, and Ground-Based Solar Absorption Measurements, at a High Arctic Site.

Abstract:

We present cross-validation of remote sensing observations of methane profiles in the Canadian High Arctic. Methane is the third most important greenhouse gas on Earth, and second only to carbon dioxide in its contribution to anthropogenic global warming. Accurate and precise observations of methane are essential to understand quantitatively its role in the climate system and in global change. The Arctic is a particular region of concern, as melting permafrost and disappearing sea ice might lead to accelerated release of methane into the atmosphere. Global observations require spaceborne instruments, in particular in the Arctic, where surface measurements are sparse and expensive to perform. Satellite-based remote sensing is an underconstrained problem, and specific validation under Arctic circumstances is required. Here, we show a cross-validation between two spaceborne instruments and ground-based measurements, all Fourier Transform Spectrometers (FTS). We consider the Canadian SCISAT ACE-FTS, a solar occultation spectrometer operating since 2004, and the Japanese GOSAT TANSO-FTS, a nadir-pointing FTS operating at solar and terrestrial infrared wavelengths, since 2009. The ground-based instrument is a Bruker Fourier Transform Infrared (FTIR) spectrometer, measuring mid-infrared solar absorption spectra at the Polar Environmental and Atmospheric Research Laboratory (PEARL) at Eureka, Nunavut (80°N, 86°W) since 2006. Measurements are collocated considering temporal, spatial, and geophysical criteria and regridded to a common vertical grid. We perform smoothing on the higher-resolution instrument results to account for different vertical resolutions. Then, profiles of differences for each pair of instruments are examined. Any bias between instruments, or any accuracy that is worse than expected, needs to be understood prior to using the data. The results of the study will serve as a guideline on how to use the vertically resolved methane products from ACE and GOSAT within the High Arctic region.