Comparison of Optimal Estimation HDO/H2O Retrievals from AIRS with ORACLES measurements

9 In this paper we evaluate new retrievals of the deuterium content of water vapor from the 20 Aqua Atmospheric InfraRed Sounder (AIRS) with aircraft measurements of HDO and 21 H2O from the ObseRvations of Aerosols above Clouds and their intEractionS 22 (ORACLES) field mission. Single footprint AIRS radiances are processed with an 23 optimal estimation algorithm that provides vertical profiles of the HDO/H2O ratio, 24 characterized uncertainties, and instrument operators (i.e., averaging kernel matrix). 25 These retrievals are compared to vertical profiles of the HDO/H2O ratio from the Oregon 26 State University Water Isotope Spectrometer for Precipitation and Entrainment Research 27 (WISPER) on the ORACLES NASA P-3B Orion aircraft. Measurements were taken over 28 the Southeast Atlantic Ocean from 31 August to 25 September 2016. HDO/H2O is 29 commonly reported in dD notation, which is the fractional deviation of the HDO/H2O 30 ratio from the standard reference ratio. For collocated measurements, the satellite 31 instrument operator (averaging kernels and a priori constraint) is applied to the aircraft 32 profile measurements. We find that AIRS dD bias relative to the aircraft is well within 33 the estimated measurement uncertainty. In the lower troposphere, 1000 to 800 hPa, AIRS 34 dD bias is -6.6‰ and the Root Mean Square (RMS) deviation is 20.9‰, consistent with 35 the calculated uncertainty of 19.1‰. In the mid-troposphere, 800 to 500 hPa, AIRS dD 36 bias is -6.8‰ and RMS 44.9‰, comparable to the calculated uncertainty of 25.8‰.