Carla Pietrapertosa

Carolina Filizzola

Nicola Pergola

Rossana Paciello

Valerio Tramutoli

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DETECTION OF SAHARAN DUST BY SPATIAL/SPECTRAL SIGNATURES 
IN VIS-TIR SATELLITE RADIANCES  
Valerio Tramutoli(1,2), Carolina Filizzola(2), Rossana Paciello(2), Nicola Pergola(2,1), Carla Pietrapertosa(2)
(1) University of Basilicata, Department of Engineering and Physics of the Environment, via dell’Ateneo Lucano 10, 85100 Potenza, 
Italy, valerio.tramutoli@unibas.it
(2) National Research Council, Institute of Methodologies for Environmental Analysis (IMAA),  c/da S.Loja- 85050 Tito Scalo (PZ),
Italy, filizzola@imaa.it; paciello@imaa.cnr.it; pergola@imaa.cnr.it; pietrapertosa@imaa.cnr.it 
1. INTRODUCTION 
 Several techniques have been proposed until now in order to characterize dust clouds from satellite observations.  They are 
more or less successful depending on observation conditions (day/night, land/sea, etc.) and specific aerosol properties (mainly size 
distribution and complex refractive index). In the visible part of solar spectrum dust clouds appear clearly brighter over dark and 
uniform backgrounds so it is generally easier to describe them over surfaces having low/known visible reflectance, as sea and inland
water basins or large vegetated areas [1, 2, 3, 4, 5]. 
Over poorly vegetated land areas, spatial heterogeneity of optical surface properties, higher values of visible reflectances, have 
brought to consider thermal infrared sounding more appropriate not only for night-time observations. 
In fact, during the day, dust haze attenuates both the upward Earth radiative flux  and  the incoming solar radiation producing an 
apparent local cooling effect which can be observed by thermal infrared soundings in the 10-12 m atmospheric window [5, 6].
During the night only the first term remains producing a local warming effect which can be observed by comparison with reference
fields built for the same area in clear-sky conditions [5, 6]. 
In alternative, blurring effects on visible imagery have been considered in order to characterize dust haze [5, 7] in presence of well 
contrasting surface features (as river or mountains) and lineaments. 
In order to overcome problems related to unknown, highly spatially variable, surface radiative properties, usually a low day-to-day 
(year-to-year) variability is assumed and reference fields in clear-sky (i.e. no dust, no meteorological clouds) conditions are built to 
which satellite observations at hand can be compared to quantify atmospheric turbidity variations  due to the presence of tropospheric 
aerosols (e.g.[8]). 
Legrand et al. ([3, 9]) using IR METEOSAT observations, proposed to discriminate dust from meteorological clouds on the base of
their differences in horizontal structure and spatial homogeneity. 
Compared with a reference IR field (built from similar observations in clear-sky conditions) they found that dust clouds show lower
spatial variability than do water/ice clouds when image portions of about 100 100 km2 [9] or 70 70 km2 [3] are considered. 
Nevertheless the definition of suitable thresholds (in terms of spatial signal variability) proved to be not easy in the case of midday 
observations over naturally contrasted soils [3].  
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. For damage mitigation purposes an early detection, as close as possible to dust sources over
deserts and semi-desert regions, would be required which is made difficult by the relatively similar mineral composition (and spectral
signatures) of  airborne and surface dust [10].   
2. METHODS AND RESULTS 
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, as 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, [11]) approach - now named RST (Robust Satellite Technique, [12]) – have 
been used in order to automatically identify llocal (i.e. depending on the location and on the time of observation, [11]) 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 satellites, together with spectral and spatial resolution offered by 
AVHRR sounder, have been considered for possible implementations in an operational monitoring context.  
Looking to the results so far achieved by using AVHRR data (the case of dust-storms events that hit North Africa will be described
with some detail) it seems that the proposed technique, combining spatial and spectral signatures in the visible and thermal infrared 
bands, is actually able to distinguish between small, low or thin, meteorological clouds and dust clouds. The potential of the 
application of the proposed method to the new generation of geo-stationary satellite sounders (and particularly on  SEVIRI aboard
Metesosat Second Generation centered on African continent) with improved time-resolution and spectral capabilities will be moreover 
considered.  
3. REFERENCES 
[1] Y. J.  Kaufmann, and C. Sendra, “Satellite mapping of aerosol loading over vegetated areas”, In Aerosols and Climate, edited by 
P. V. Hobbs and M. P. McCormick (Hampton, Virginia: A. Deepak Publishing), pp. 51-67, 1988. 
[2] K. Masuda, T. Takashima, and C. R. N. Rao, ”Remote sensing of atmospheric aerosols over the oceans using multispectral 
radiances measured with the Advanced Very High Resolution Radiometer onboard the NOAA meteorological satellites”, In Aerosols 
and Climate, edited by P. V. Hobbs and M. P. McCormick (Hampton, Virginia: A. Deepak Publishing), pp. 38-49, 1988. 
[3] M. Legrand, J. J. Bertrand, M. Desbois, L. Meneger, and Y. Fouquart, “The Potential of Infrared Satellite data for the Retrieval of 
Saharan-Dust Optical Depth over Africa”, Journal of Applied Meteorology, vol. 28, pp. 309-318, 1989. 
[4] C. R. N. Rao, L. L. Stowe, E. P. McClain, and J. Sapper, “Remote sensing of aerosols over the oceans using AVHRR data. 
Theory, practice and applications”, International Journal of Remote Sensing, vol. 10, pp. 743-749, 1989. 
[5] B. Holben, E. Vermote, Y.J. Kaufman, D. Tanré, and V. Kalb, “Aerosol Retrieval over Land from AVHRR Data - Application for 
Atmospheric Correction”, IEEE Transactions on Geoscience and Remote Sensing, vol. 30, pp. 212-222, 1992. 
[6] M. Legrand, M. Desbois, and K. Vovor, “Satellite Detection of Saharan Dust: Optimized Imaging during Nightime”, Journal of 
Climate, vol. 1, pp. 256-264, 1988. 
[7] D. Tanre, C. Devaux, M. Herman, P. Y. Deschamps, “Estimation of Saharan aerosol optical thickness from blurring effects in 
Thematic Mapper data”, Journal of Geophysical Research (ISSN 0148-0227), vol. 93, Dec. 20,  p. 15955-15964. 
[8] M. Legrand, A. Plana-Fattori, and C. N'doumé, “Satellite detection of dust using the IR imagery of Meteosat 1. Infrared difference 
dust index”, J. Geophys. Res., vol. 106(D16), 18,251–18,274, 2001. 
[9] M. Legrand, J. J. Bertrand, and M. Desbois, “Dust clouds over West Africa: A characterisation by satellite data”, Annales
Geophysicae, vol. 3, pp. 777-784, 1985. 
[10] A.E. Wald, Y.J.Kaufman, D. Tanre, B.C. Gao, “Daytime and nighttime detection of mineral dust over desert using infrared 
spectral contrast”, JGR-Atmospheres 98JD01454, Vol. 103 , No. D24 , p. 32,307, 1998. 
[11] V. Tramutoli, “Robust AVHRR techniques (RAT) for environmental monitoring: theory and applications”, Earth surface remote 
sensing II.Proceedings of SPIE, vol. 3496, pp. 101-113, 1998.  
[12] V. Tramutoli Robust, “Satellite Techniques (RST) for Natural and Environmental Hazards Monitoring and Mitigation: Theory 
and Applications”, in Fourth International Workshop on the Analysis of Multitemporal Remote Sensing Images. 18-20 July, 2007, 
Louven, Belgium, doi: 10.1109/MULTITEMP.2007.4293057, 2007. 


DETECTION OF SAHARAN DUST BY SPATIAL/SPECTRAL SIGNATURES IN VIS-TIR SATELLITE RADIANCES

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DETECTION OF SAHARAN DUST BY SPATIAL/SPECTRAL SIGNATURES IN VIS-TIR SATELLITE RADIANCES

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