Solar zenith and viewing geometry-dependent errors in satelliteretrieved cloud optical. thickness: Marine stratocumulus case

摘要

The error in the domain-averaged cloud optical thickness retrieved from satellite-based imagers is investigated using a cloud field generated by a cloud model and a 3Dradiative transfer model. The objective of this study is to identify the optimal geometry forthe optical thickness retrieval and quantify the error. The cloud field used in the simulationis a relatively uniform (retrieved shape parameter of a gamma distribution averaged overall simulated viewing and solar zenith angles is 18) and nearly isotropic stratocumulusfield. The retrieved cloud cover with a 1-km pixel resolution is 100%. The domain-averaged optical thickness error is separated into two terms, the error caused by anassumption of a horizontally uniform cloud over a 1-km pixel (internal variability)and error caused by neglecting the horizontal flux through the boundary of subpixels,(external variability). For the cloud field used in this study, the external variability termincreases with solar zenith angle and the sign changes from negative to positive while theinternal variability term is generally negative and becomes more negative as the solarzenith angle increases. At a small solar zenith angle, therefore, both terms are negative, butthe error partially cancels at a large solar zenith angle. When the solar zenith angle isless than 30°, both terms are small; the error in the viewing zenith angle and domain-averaged cloud optical thickness derived from the relative azimuth angle Smaller' than150 is less than 10%. However, if the optical thickness is derived from nadir view only foroverhead sun, the domain-averaged optical thickness is underestimated by more than10%. When the solar zenith angle increases to 60°, the internal variability term exceeds10%, especially viewed from the forward direction, but the domain and viewing zenithangle averaged optical thickness error can be less than 10% in the backward direction.When the solar zenith angle is 70°, both terms are greater than 10%. The shape parameterof a gamma distribution derived from retrieved optical thicknesses increases with theviewing zenith angle but decreases with solar zenith angle. On the basis of this simulationand Terra Moderate Resolution Imaging Spectroradiometer (MODIS) viewing geometryand solar zenith angle at the sampling time over the northeastern Pacific, the error inthe domain-averaged retrieved optical thickness of uniform stratocumulus overnortheastern Pacific is less than 10% in March and September.