Abstract
Abstract. Clouds in the atmosphere play an important role in reflection, absorption and transmission of solar radiation and thus affect trace gas retrievals. The main goal of this paper is to examine the sensitivity of stratospheric and lower mesospheric ozone retrievals from limb-scattered radiance measurements to clouds using the SCIATRAN radiative transfer model and retrieval package. The retrieval approach employed is optimal estimation, and the considered clouds are vertically and horizontally homogeneous. Assuming an aerosol-free atmosphere and Mie phase functions for cloud particles, we compute the relative error of ozone profile retrievals in a cloudy atmosphere if clouds are neglected in the retrieval. To access altitudes from the lower stratosphere up to the lower mesosphere, we combine the retrievals in the Chappuis and Hartley ozone absorption bands. We find significant cloud sensitivity of the limb ozone retrievals in the Chappuis bands at lower stratospheric altitudes. The relative error in the retrieved ozone concentrations gradually decreases with increasing altitude and becomes negligible above approximately 40 km. The parameters with the largest impact on the ozone retrievals are cloud optical thickness, ground albedo and solar zenith angle. Clouds with different geometrical thicknesses or different cloud altitudes have a similar impact on the ozone retrievals for a given cloud optical thickness value, if the clouds are outside the field of view of the instrument. The effective radius of water droplets has a small influence on the error, i.e., less than 0.5% at altitudes above the cloud top height. Furthermore, the impact of clouds on the ozone profile retrievals was found to have a rather small dependence on the solar azimuth angle (less than 1% for all possible azimuth angles). For the most frequent cloud types, the total error is below 6% above 15 km altitude, if clouds are completely neglected in the retrieval. Neglecting clouds in the ozone profile retrievals generally leads to a low bias for a low ground albedo and to a high bias for a high ground albedo, assuming that the ground albedo is well known.
Highlights
Clouds play an important role in the Earth’s atmosphere
Clouds with different geometrical thicknesses or different cloud altitudes have a similar impact on the ozone retrievals for a given cloud optical thickness value, if the clouds are outside the field of view of the instrument
As follows from the results presented among others by Liou (1973) and Kokhanovsky (2001) for the reflection function of clouds in the visible spectral range, the reflected solar radiation in a cloudy atmosphere decreases with increasing solar zenith angle (SZA)
Summary
Clouds play an important role in the Earth’s atmosphere. The tropospheric cloud coverage is about 50% at any given time and 7% of the total tropospheric volume is occupied by clouds (Lelieveld et al, 1989; Pruppacher and Jaenicke, 1995; Mace et al, 2007). Clouds interact with incoming solar radiation and long wavelength radiation emitted by the Earth, affecting the atmospheric energy budget and atmospheric photochemistry. Tropospheric clouds affect the scattering and penetration of solar photons in the atmosphere (e.g., Vanbauce et al, 2003; Rozanov and Kokhanovsky, 2004). The scattering process impacts trace gas retrievals from satellite instruments or ground-based measurements (e.g., Erle et al, 1995; Rozanov and Kokhanovsky, 2008, and references therein). Satellite observations of the scattered solar radiation in limb viewing geometry have become one of the standard techniques to measure stratospheric profiles of ozone and other minor constituents (e.g., McPeters et al, 2000; von Savigny et al, 2003; Haley et al, 2004; Rault, 2004). The limb-scatter observation geometry is characterized by a complex radiative transfer, because the multiple scattering or diffuse radiation contribution to the observed limb radiances
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