Effect of surface albedo variations on UV‐visible limb‐scattering measurements of the atmosphere

Abstract

In a new technique to measure stratospheric ozone profiles a near‐UV and visible spectrometer is used to detect backscattered solar light in limb‐viewing geometry. Several satellite‐based instruments using this technique will be launched in 2001–2002. A limb‐viewing instrument does not see Earth's surface or tropospheric clouds directly. However, indirect light reflected from the surface or low‐altitude clouds (which have an effect similar to a highly reflective surface) can make up tens of percents of limb radiance. Our simulations by a three‐dimensional Monte Carlo radiative transfer model show that reflection from an area at Earth's surface that extends over 1000 km along the instrument line of sight and 200 km across it contributes to the signal. The exact area depends on tangent altitude of the measurement, wavelength, surface elevation, and atmospheric conditions. Over this distance the reflectivity of the surface can vary by almost 100%, which should be taken into account in measurement data analysis. Normally, radiative transfer models that are used to analyze scattering measurements assume that the atmosphere and the surface are spherically symmetric. In such a model the surface albedo is a constant, and it is not possible to include a spatially varying surface into the model. We present an approximative method to average the reflectivity of Lambert surfaces, which can be used to estimate limb radiance over an inhomogeneous surface by a spherically symmetric model.