DETERMINATION OF FOG OPTICAL-THICKNESS OVER NORTHERN FRANCE USING AVHRR IMAGERY

This paper reports on a study that was performed as part of a larger French program to investigate radiation fog development. It concentrates on the derivation of fog optical thicknesses from satellite observations. The technique, which employs Advanced Very High Resolution Radiometer (AVHRR) measurements from channels 1 and 2, is applied to three satellite images corresponding to a fog event that occurred over Northern France on February 8, 1989. The paper first investigates the sensitivity of the retrieved optical thickness to the various parameters affecting the inversion method, namely the effects of ozone, water vapor, aerosols, droplet size, surface albedo as well as the calibration of the AVHRR sensor. These effects are simulated using a radiative transfer code, based upon the Discrete Ordinate Method, that calculates the radiances reflected by a vertically inhomogeneous scattering atmosphere. The optical thicknesses obtained by the code, through inversion of the top of atmosphere reflectances, are then compared to independent in situ pyranometer measurements. It is found that accurate calibration of the AVHRR is critical to the inversion technique. Consequently new calibration coefficients were computed using in situ pyranometer measurements. These coefficients were found to be very close (< 2%) to those derived from a collection of NOAA 11 satellite data. Moreover, the very good agreement between the calibration coefficients derived using two successive images, corresponding to different viewing angles, seems to indicate that using a plane-parallel model is justified for inversion of NOAA 11 data over a fog layer. Independent of the calibration errors, the accuracy in computing the optical thicknesses is estimated to be about 10-20% using channel 1 data and 15-30% using channel 2 data, in the case of dense fog.