Abstract:
The extension of radiative fog and low stratus, present in the early afternoon in the rhine graben, was mapped from a sample of 96 noaa-avhrr satellite pictures, selected from 1982 to 1993. Observed fog cover frequencies showed very different spatial distributions in terms of various weather types. This clearly indicates the influence of the mountains on the location of fog and residual cold layers. The height of the fog outline has been estimated for various areas of the rhine graben with the use of a digital elevation model. It was extremely varying day after day even if it was staying usually to 400 and 500 meters height. The main types of the fog expansion configuration have been classified with an algorithm of hierachical ascending classification with the combination of satellite pictures and standard climatological data. Each observed configuration answers to precise criterias of pressure and wind and forms a complementary tool for the forecast of the fog expansion in the beginning of the afternoon. The use of climatological data sets from instrumented towers has allowed a detailed investigation of the various thermodynamic and radiative processes occuring in some events for which a fog formation or dissipation was observed. During these events, different phases were identified and individualized in the southern rhine graben according to the criteria established by juisto and lala at albany in the u. S. A. The coming out and the duration of these phases depend both of the surrounding topography and of the evolution of the climatological conditions at meso-scale.
