A case study on the determination of fog optical depth and liquid water path using AVHRR data and relations to fog liquid water content and horizontal visibility

Abstract

An improved direct method is presented for the calculation of fog optical depth and liquid water path from fog top albedo using AVHRR channel 1. Fog top albedo is determined by means of a simple radiative transfer model under special consideration of radiometric corrections. By the conversion of channel I digital counts into a reflectance factor, a correction has to be performed, due to the degradation of channel 1 radiometer with time. By omitting atmospheric and anisotropy corrections, modal uncertainties in the calculations of fog top albedo in the order of ±25 per cent and ± 5 per cent were obtained. Assuming a homogeneous droplet distribution within fog layers at scan lime, a horizontal visibility was calculated from fog optical depth and fog geometrical thickness, whereas fog liquid water content was derived from liquid water path and fog thickness. Ground truth from high resolution transmiss-ometer observations showed an encouraging accuracy with a mean deviation between the observed and calculated visibility of about 56 m and a mean error in the determination of fog liquid water content of about 0·025 gm⁻³.