Aerosol Optical Depth over Oceans: High Space- and Time-Resolution Retrieval and Error Budget from Satellite Radiometry

Abstract

A method to retrieve aerosol vertical optical depth at 0.64 ?m from satellite observations of cloud-free scenes over oceans with high spatial resolution (?1°) and instantaneous temporal resolution is described and evaluated. The observed radiance is treated as the linear sum of contributions to path radiance by different scattering processes in the atmosphere?ocean system. This treatment allows examination of errors in the retrieved vertical aerosol optical depth contributed by each process and approximation. Random error in retrieved aerosol optical depth is typically 0.03. The systematic error due to absolute calibration uncertainty in the measured radiance is 0.01. The largest errors and biases are due to radiative transfer approximations (+22%) and assumptions regarding aerosol microphysical and optical properties (?20%). The latter errors, which are due to the optical properties (e.g., phase function), vary systematically with latitude and season because of the variation of the mean observing geometry. This method is applied to Advanced Very High Resolution Radiometer global area coverage data, and example maps of aerosol optical depth are presented for specific dates in July and October 1986. The aerosol optical depth derived from the satellite data is suitable for examining large aerosol signatures by instantaneous comparison of the amplitude and location of aerosol plumes with model predictions based on meteorological conditions at and preceding the time of observation.