Abstract:
Two schemes for the determination of fog horizontal
visibility using AVHRR data both, day and night, are
presented. Ground truth has been performed by the
comparison of calculated visibilities and simultaneous
transmissometer measurements. The daytime approach is
based on the calculation of fog albedo from AVHRR #1 and
the relation of fog optical depth and geometrical thickness to
spectral extinction and horizontal visibility. The mean
deviation between observed and calculated visibilities is
about 56 m. The nighttime approach uses the relation
between fog and air temperature derived from AVHRR # 4
and 5 and a simple model for the approximation of the fog
droplet distribution. The resulting fog extinction is directly
related to horizontal visibility. Because fog optical depth is
not available at night, the calculation of horizontal visibility
using the nighttime approach is only valid if fog extinction is
nearly homogeneous over the whole fog layer. Mean
deviations between observed and calculated visibilities
therefore slightly increase to about 61 m.
