Molecular Line Absorption in a Scattering Atmosphere. Part II: Application to Remote Sensing in the O2 A band

Abstract

This paper explores the feasibility of using O2 A-band reflectance spectra in the retrieval of cloud optical and physical properties. Analyses demonstrate that these reflection spectra are sensitive to optical properties of clouds such as optical depth τc and phase function, vertical profile information including cloud-top pressure, pressure thickness, and the surface albedo. An estimation method is developed to demonstrate how well this information might be retrieved from synthetic spectra calculated by a line-by-line spectral multiple scattering model assuming realistic instrument parameters (spectral resolution, calibration accuracy, and signal-to-noise properties). The quality of the retrievals is expressed in terms of two indices, one relating to total error and another that quantifies the extent of reliance of the retrieval on the measurement, or conversely on other a priori information. Sources of total error include instrument-related errors, forward model errors including phase function errors, and errors in a priori data. The retrievals presented show the following: (i) The optical depth, surface albedo, cloud-top pressure, and cloud layer pressure thickness can be retrieved with an accuracy of approximately 5% for most cases of low cloud except when these clouds are optically thin and over bright surfaces. The spectra also contain information about the pressure thickness of the low-level cloud and this information also can be retrieved with an expected accuracy of less than 10% and with little reliance on any a priori data. (ii) Significantly larger errors result for retrievals for high clouds when no attempt is made to constrain the uncertainties associated with the unknown character of the scattering phase function. (iii) Retrieval of a limited amount of information about the phase function is possible under certain circumstances. It is possible to retrieve the asymmetry parameter sufficiently well to improve the accuracy of the forward model. This results in a shrinking of the errors in τc to less than 10% for τc > 0.1. (iv) The pressure information about scattering layers inherent in the A-band spectra is shown to provide a limited amount of vertical profiling capability (four to five layers of information at the most) provided the measurements are obtained with a spectral resolution of about 0.5 cm?1 and obtained with an accuracy of 2% or better. A specific example demonstrates the capability of not only detecting the presence of thin high cloud above lower brighter cloud but also the capability of estimating the optical depths of both clouds. (v) The advantage of additional information such as provided by active profilers (radar and lidar) is explored. The advantages of this additional profile information are quantitatively shown to improve not only the retrieval of vertical profiles of extinction but also the optical properties of individual cloud layers.