Comparison of optimal estimation HDO/H2O retrievals from AIRS with ORACLES measurements

摘要

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