Methane Retrievals in the Thermal and Short-Wave Infrared from IASI

Abstract:

RAL has developed an optimal estimation scheme to retrieve global height-resolved information on methane from IASI measurements in the thermal infrared (7.9 micron) band. The use of IASI thermal infrared observations produces methane data at relatively high spatial resolution, both day and night, over land and ocean. The retrieval scheme extracts two independent pieces of information on the profile, with sensitivity extending into the lower troposphere. Column-averaged mixing ratios are derived from the retrieved profile with a precision of 20-40 ppbv. The retrieval scheme has been extensively validated and shown to perform well in comparison with column-averages from both ground-based observations (TCCON) and from the satellite short-wave infrared sounder GOSAT; a paper is currently in preparation (R.Siddans et al., ‘Global height-resolved methane retrievals from IASI’).

A limitation of thermal infrared retrievals is that sensitivity to the boundary layer is low compared to short-wave infrared sounders. However, IASI also measures methane lines in the 3.7 micron spectral range, where there is significant solar contribution in the daytime over land. Utilising this spectral region therefore has the potential to increase IASI sensitivity to methane in the boundary layer, although the 3.7 micron band has yet to be exploited due to the complexity of modelling both solar and thermal contributions to the measurements and the relatively high noise level of IASI in this spectral region.

A continuous series of observations are planned by IASI on MetOp-A, -B and –C, followed by IASI-NG on MetOp-SG, covering the period from 2007-2040. Here we present 7 years of global methane retrievals from IASI MetOp-A thermal infrared data, spanning the period 2007-2013, along with validation and model comparison results. We also present early findings from an investigation into the potential of adding information on near-surface methane by exploiting the 3.7 micron band.