Atmospheric composition and thermodynamic retrievals from the ARIES airborne FTS system ? Part 1: Technical aspects and simulated capability

In this study we present an assessment of the retrieval capability of the Airborne Research Interferome- ter Evaluation System (ARIES): an airborne remote-sensing Fourier transform spectrometer (FTS) operated on the UK Facility for Airborne Atmospheric Measurement (FAAM) aircraft. Simulated maximum a posteriori retrievals of partial column trace gas concentrations, and thermodynamic verti- cal profiles throughout the troposphere and planetary bound- ary layer have been performed here for simulated infrared spectra representative of the ARIES system operating in the nadir-viewing geometry. We also describe the operational and technical aspects of the pre-processing necessary for rou- tine retrieval from the FAAM platform and the selection and construction of a priori information. As exemplars of the ca- pability of the ARIES retrieval system, simulated retrievals of temperature, water vapour (H2O), carbon monoxide (CO), ozone (O3), and methane (CH4), and their corresponding sources of error and potential vertical sensitivity, are dis- cussed for ARIES scenes across typical global environments. The maximum Degrees of Freedom for Signal (DOFS) for the retrievals, assuming a flight altitude of 7 km, were 3.99, 2.97, 0.85, 0.96, and 1.45 for temperature, H2O, CO, O3, and CH4, respectively, for the a priori constraints specified. Re- trievals of temperature display significant vertical sensitivity (DOFS in the range 2.6 to 4.0 across the altitude range) as well as excellent simulated accuracy, with the vertical sensi- tivity for H2O also extending to lower altitudes (DOFS rang- ing from 1.6 to 3.0). It was found that the maximum sensitiv- ity for CO, O3, and CH4 was approximately 1-2 km below