Robust Satellite Techniques (RST) For Saharan Dust Monitoring

Saharan dust has been particularly studied for its relevant climatological implications and for damage mitigation purposes. Due to dust-clouds optical properties - which are very similar to those exhibited by low/thin meteorological clouds, as well as by clouds fringes - their identification during the day is still the main problem which is made more intricate because of the fairly high visible reflectance of Saharan background. In this paper the problem of identifying Saharan dust clouds, distinguishing them from small, low or thin, meteorological clouds, is faced by combining spatial and spectral signatures in the visible and thermal infrared AVHRR remotely sensed radiances. Spatial structure analysis, performed on AVHRR (Advanced Very High Resolution Radiometer) imagery, for different typologies of meteorological and dust clouds over Saharan desert background, permitted to explain why discrimination strategies based on sounders, like METEOSAT-IR, having spatial resolution lower than 5 km, become effective only at larger scale (with better results beyond 60 km). In the main time a more marked spatial signature, at a scale shorter than 5 km, has been recognized which suggests that an early detection of sandstorm sources could be possible, in daytime, by using sounders having at least 1 km spatial resolution in the visible spectral range. A new algorithm is proposed which mainly relies on a preliminary variogram analysis of the peculiar texture exhibited, in the daytime, by low height dust clouds and their shadows over the desert background. Reference fields computed following the general RAT (Robust AVHRR Technique) approach - now named RST) - have been used in order to automatically identify llocal (i.e. depending on the location and on the time of observation) thresholds to be applied in the detection phases both to visible and thermal infrared radiances. Global coverage, low-cost and high repetition rate offered by NOAA-AVHRR, EOS-MODIS and MSG-SEVIRI satellite packages, have been considered for possible implementations in an operational monitoring context and their performance evaluated even by comparison with traditional (single image, fixed threshold) techniques. Higher sensitivity, very low false alarm rate, easy exportability to different geographic areas as well as different satellite sensors, are the main advantage exhibited by the proposed method after tests performed on several events occurred in Northern Africa and Middle East.