Satellite validation of column-averaged methane on global scale: ground-based data from 15 FTIR stations versus last generation ENVISAT/SCIAMACHY retrievals

Previous work has shown that the precision of ground-based mid-infrared (MIR) FTIR
spectrometry is sufficient to detect day-to-day variability of columnar methane, while first
retrievals from ENVISAT/SCIAMACHY (channel 8) satellite measurements were impacted
by a significant time-dependent bias due to detector icing. This
prevented insight into true methane temporal variability at that time. The goal of our
updated study is to investigate the precision of the last generation (channel 6)
SCIAMACHY retrievals IMAP-DOAS v49 and WFM-DOAS v1.0 in comparison to retrievals
from ground-based MIR measurements of the European FTIR network. We first briefly
discuss the origin and magnitude of the natural variability of columnar methane.
Subsequently, our study investigates all factors which can be optimized to improve
precision of ground-based MIR-FTIR retrievals of columnar methane. This includes an
optimized Tikhonov-type regularization tuned in a way to minimize the diurnal variability of
retrieved columnar methane. We also discuss ways to select and average individual-pixel
satellite data in order to reflect true day-to-day variability and make them comparable to
ground-based data.