Assessment of Saharan dust absorption in the visible from SeaWiFS imagery

We have examined forty SeaWiFS images acquired during 1997 and 1998 off the west coast of Africa to develop theoretical models of Saharan dust radiative properties that could be used for atmospheric correction in this region, i.e., to predict the dust contribution to the measured reflectance in the visible from that measured in the near infrared (NIR). In contrast to nonabsorptive or weakly absorbing aerosols, the dust reflectance significantly decreases from the NIR to the blue because of the absorption of mineral particles in the visible. We chose simple but realistic vertical structures for the dust layer and examined the applicability of a set of aerosol size distributions and refractive indices with radiative properties computed from Mie theory. We found that 18 models (six aerosol size-refractive index distributions times three aerosol vertical distributions) were general enough to estimate the dust reflectance in the visible with an absolute RMS error of the order of 5%. We show that these dust models can be used within a “spectral matching algorithm” [Gordon et al., 1997] to effect atmospheric correction of ocean color imagery in dust-contaminated regions. We also found that our models all produce very similar top-of-atmosphere outgoing visible flux but that this flux may be significantly different from that predicted by other conventional dust models.

[1]  S. Hooker.  An overview of SeaWiFS and ocean color , 1992 .

[2]  André Morel,et al.  A multiple scattering algorithm for atmospheric correction of remotely sensed ocean colour (MERIS instrument): Principle and implementation for atmospheres carrying various aerosols including absorbing ones , 1999 .

[3]  I. Jankowiak,et al.  Long‐term daily monitoring of Saharan dust load over ocean using Meteosat ISCCP‐B2 data: 2. Accuracy of the method and validation using Sun photometer measurements , 1997 .

[4]  James W. Brown,et al.  A semianalytic radiance model of ocean color , 1988 .

[5]  E. Shettle,et al.  Optical and Radiative Properties of a Desert Aerosol Model , 1986 .

[6]  Eric P. Shettle,et al.  Atmospheric Aerosols: Global Climatology and Radiative Characteristics , 1991 .

[7]  S. H. Melfi,et al.  Validation of the Saharan dust plume conceptual model using lidar, meteosat, and ECMWF Data , 1999 .

[8]  Y. Balkanski,et al.  Modeling the mineralogy of atmospheric dust sources , 1999 .

[9]  G. Bergametti,et al.  Atmospheric input of trace metals to the western Mediterranean: uncertainties in modelling dry deposition from cascade impactor data , 1988 .

[10]  I. Sokolik,et al.  Complex refractive index of atmospheric dust aerosols , 1993 .

[11]  H. Fukushima,et al.  Asian dust aerosol: Optical effect on satellite ocean color signal and a scheme of its correction , 1997 .

[12]  Susan Walsh,et al.  Ocean color: Availability of the global data set , 1989 .

[13]  P. Formenti,et al.  Optical properties of Saharan dust during ACE 2 , 1998 .

[14]  H. Gordon,et al.  Remote sensing of ocean color and aerosol properties: resolving the issue of aerosol absorption. , 1997, Applied optics.

[15]  François Dulac,et al.  Long‐term daily monitoring of Saharan dust load over ocean using Meteosat ISCCP‐B2 data: 1. Methodology and preliminary results for 1983–1994 in the Mediterranean , 1997 .

[16]  Toby N. Carlson,et al.  Vertical and areal distribution of Saharan dust over the western equatorial north Atlantic Ocean , 1972 .

[17]  Menghua Wang,et al.  Retrieval of water-leaving radiance and aerosol optical thickness over the oceans with SeaWiFS: a preliminary algorithm. , 1994, Applied optics.

[18]  R. Feely,et al.  Atmospheric iron inputs and primary productivity: Phytoplankton responses in the North Pacific , 1991 .

[19]  Irina N. Sokolik,et al.  Direct radiative forcing by anthropogenic airborne mineral aerosols , 1996, Nature.

[20]  H. Gordon.  Atmospheric correction of ocean color imagery in the Earth Observing System era , 1997 .

[21]  M. Mishchenko,et al.  Modeling phase functions for dustlike tropospheric aerosols using a shape mixture of randomly oriented polydisperse spheroids , 1997 .

[22]  Larry L. Stowe,et al.  Characterization of tropospheric aerosols over the oceans with the NOAA advanced very high resolution radiometer optical thickness operational product , 1997 .

[23]  L. Schütz,et al.  LONG RANGE TRANSPORT OF DESERT DUST WITH SPECIAL EMPHASIS ON THE SAHARA * , 1980 .

[24]  O. Boucher,et al.  Uncertainties in assessing radiative forcing by mineral dust , 1998 .

[25]  Didier Tanré,et al.  Satellite Climatology of Saharan Dust Outbreaks: Method and Preliminary Results , 1992 .

[26]  E. Patterson.  Optical properties of the crustal aerosol - Relation to chemical and physical characteristics , 1981 .

[27]  François Dulac,et al.  Control of atmospheric export of dust from North Africa by the North Atlantic Oscillation , 1997, Nature.

[28]  François Dulac,et al.  An additional low layer transport of Sahelian and Saharan dust over the north-eastern Tropical Atlantic , 1995 .

[29]  Y. Kaufman,et al.  Passive remote sensing of tropospheric aerosol and atmospheric , 1997 .

[30]  W. Esaias,et al.  SeaWiFS technical report series. Volume 1: An overview of SeaWiFS and ocean color , 1992 .

[31]  J. Seinfeld,et al.  Radiative forcing by mineral dust aerosols : sensitivity to key variables , 1998 .