Laboratory for
Climatology and Remote Sensing

Abstract:

This Letter describes the adjustment of the Convective-Stratiform Technique (CST) to estimate 1991± 93 El NinÄ o rainfall in Ecuador and Peru by means of IR radiation data recorded by the Meteosat-3 VISSR sensor. The adjustment includes (1) a regionalization of a one-dimensional cloud model using local radiosonde data, (2) a ® eld-of-view correction of the VISSR-derived cloud top temperature to model requirements and (3) an adapted discrimination scheme between raining and non-raining clouds. Validation reveals a strong correlation between calculated and observed rainfall for all climatic regimes and rainfall intensities within the study area.

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.

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.