Characterization and evaluation of AIRS-based estimates of the deuterium content of water vapor

Abstract

Abstract. Single-pixel tropospheric retrievals of HDO and H2O concentrations are retrieved from Atmospheric Infrared Sounder (AIRS) radiances using the optimal estimation algorithm developed for the Aura Tropospheric Emission Spectrometer (TES) project. We evaluate the error characteristics and vertical sensitivity of AIRS measurements corresponding to 5 d of TES data (or five global surveys) during the Northern Hemisphere summers between 2006 and 2010 (∼600 co-located comparisons per day). We find that the retrieval characteristics of the AIRS deuterium content measurements have similar vertical resolution in the middle troposphere as TES but with slightly less sensitivity in the lowermost troposphere, with a typical degrees of freedom (DOFS) in the tropics of 1.5. The calculated measurement uncertainty is ∼30 ‰ (parts per thousand relative to the deuterium composition of ocean water) for a tropospheric average between 750 and 350 hPa, the altitude region where AIRS is most sensitive, compared to ∼15 ‰ for the TES data. Comparison with the TES data also indicates that the uncertainty of a single target AIRS HDO ∕ H2O measurement is ∼30 ‰. Comparison of AIRS and TES data between 30∘ S and 50∘ N indicates that the AIRS data are biased low by ∼-2.6 ‰ with a latitudinal variation of ∼7.8 ‰. This latitudinal variation is consistent with the accuracy of TES data compared to in situ measurements, suggesting that both AIRS and TES have similar accuracy.

Comparison of optimal estimation HDO∕H<sub>2</sub>O retrievals from AIRS with ORACLES measurements

“…The single-footprint AIRS HDO profile data used in this work were produced using the retrieval algorithm, named the MUlti-SpEctra, MUlti-SpEcies, MUlti-Sensors (MUSES) algorithm (D. Fu et al, , 2016Fu et al, , 2018Fu et al, , 2019Worden et al, 2019). The MUSES algorithm can use radiances from multiple instruments, including AIRS and other instruments (CrIS, TES, OMI, OMPS, TROPOMI, and MLS), to quantify geophysical observables that affect the corresponding radiance.…”

“…The MUSES algorithm can use radiances from multiple instruments, including AIRS and other instruments (CrIS, TES, OMI, OMPS, TROPOMI, and MLS), to quantify geophysical observables that affect the corresponding radiance. The AIRS single-footprint HDO profile retrievals have been described by Worden et al (2019), and have heritage from the TES algorithm (Worden et al, 2004(Worden et al, , 2011(Worden et al, , 2012Bowman et al, 2006Bowman et al, , 2002. The Optimal Spectral Sampling (OSS) fast radiative transfer model (Moncet et al, 2008(Moncet et al, , 2015 for single-footprint AIRS measurements has been integrated into the MUSES algorithm, in support of the operational data production towards the multi-decadal record of global HDO profiles.…”

“…The Supplement discusses the sensitivity of the retrievals to the choice of the forward model. The retrieval uses the optimal estimation (OE) method to quantify atmospheric HDO and H 2 O (Worden et al, , 2012(Worden et al, , 2019. For both AIRS and TES retrievals, height discrimination of the HDO/H 2 O ratio in the troposphere is provided by spectral resolution of pressurebroadened and the temperature-broadened absorption features of their corresponding lines (Beer et al, 2002).…”

“…For both AIRS and TES retrievals, height discrimination of the HDO/H 2 O ratio in the troposphere is provided by spectral resolution of pressurebroadened and the temperature-broadened absorption features of their corresponding lines (Beer et al, 2002). The algorithms and spectral microwindows are described by Worden et al (2019). Chemical species CH 4 , CO, HDO, and H 2 O are jointly retrieved along with atmospheric temperature, surface temperature, land emissivity and clouds (Worden et al, 2012).…”

“…In the last decade, satellite retrievals of tropospheric water vapor isotopic composition (HDO and H 2 O) have been developed, including Envisat/SCIAMACHY (Scanning Imaging Absorption Spectrometer for Atmospheric Chartography) (Frankenberg et al, 2009), IASI (Infrared Atmospheric Sounding Interferometer) aboard the MetOp satellites (Herbin et al, 2009;Schneider and Hase, 2011;Lacour et al, 2012) and TES (the Tropospheric Emission Spectrometer) on the Aura spacecraft . More recently, Worden et al (2019) developed HDO retrievals from the Aqua Atmospheric InfraRed Sounder (AIRS) single-footprint Level 1B radiance data. These AIRS retrievals are the subject of the present comparison with aircraft data.…”