Cloud sensitivity studies for stratospheric and lower mesospheric ozone profile retrievals from measurements of limb-scattered solar radiation

摘要

Clouds in the atmosphere play an important role in reflection,absorption and transmission of solar radiation and thus affect tracegas retrievals. The main goal of this paper is to examine thesensitivity of stratospheric and lower mesospheric ozone retrievalsfrom limb-scattered radiance measurements to clouds using theSCIATRAN radiative transfer model and retrieval package. Theretrieval approach employed is optimal estimation, and theconsidered clouds are vertically and horizontally homogeneous.Assuming an aerosol-free atmosphere and Mie phase functions forcloud particles, we compute the relative error of ozone profileretrievals in a cloudy atmosphere if clouds are neglected in theretrieval. To access altitudes from the lower stratosphere up to thelower mesosphere, we combine the retrievals in the Chappuis andHartley ozone absorption bands. We find significant cloudsensitivity of the limb ozone retrievals in the Chappuis bands atlower stratospheric altitudes. The relative error in the retrievedozone concentrations gradually decreases with increasing altitudeand becomes negligible above approximately 40 km. The parameterswith the largest impact on the ozone retrievals are cloud opticalthickness, ground albedo and solar zenith angle. Clouds withdifferent geometrical thicknesses or different cloud altitudes havea similar impact on the ozone retrievals for a given cloud opticalthickness value, if the clouds are outside the field of view of theinstrument. The effective radius of water droplets has a smallinfluence on the error, i.e., less than 0.5% at altitudes above thecloud top height. Furthermore, the impact of clouds on the ozoneprofile retrievals was found to have a rather small dependence onthe solar azimuth angle (less than 1% for all possible azimuthangles). For the most frequent cloud types, the total error is below6% above 15 km altitude, if clouds are completely neglected in theretrieval. Neglecting clouds in the ozone profile retrievalsgenerally leads to a low bias for a low ground albedo and to a highbias for a high ground albedo, assuming that the ground albedo iswell known.