Ozone profiles from clear sky thermal infrared measurements of the Infrared Atmospheric Sounding Interferometer: A retrieval approach accounting for thin cirrus

Abstract

[1] In this work we discuss the retrieval of ozone profiles from Infrared Atmospheric Sounding Interferometer (IASI) thermal infrared measurements in the presence of thin cirrus. An algorithm is presented which accounts for optically thin cirrus, described by an absorption layer at fixed altitude, where an effective cirrus optical depth is determined from IASI measurements at the 15 μm CO2 absorption band. Subsequently, the cirrus characteristics are used for the ozone profile retrievals at the 9.6 μm O3 absorption band. To retrieve ozone we employ a Tikhonov regularization scheme in combination with the L curve approach. A sensitivity study shows that neglecting thin cirrus in the ozone profile retrieval leads to errors of >20% in the troposphere for an infrared optical depth of <0.1 (τ550nm < 0.05), while errors are around 5% when accounting for thin cirrus in the proposed manner. Uncertainties on cirrus height are mostly compensated by the retrieval of the effective cloud parameter. The findings are confirmed when we apply the retrieval scheme to IASI measurements which are filtered according to water clouds and optically thick cirrus. The ozone profiles are validated with 193 collocated ozonesonde profiles from nine stations and show good improvement at all altitudes. For the different stations the differences in the mean profiles between IASI ozone profiles and sonde profiles at 5 km altitude improve from (−10)%–(+80)% when cirrus is not accounted for in the retrieval to (−10)%–(+35)% when employing the new retrieval scheme. At the same time the root-mean-square differences between IASI ozone profiles and sonde profiles improve from 30%–100% to 20%–50%.