Satellite Haze Detection on July July 16-18,1999. Rudolf B. Husar CAPITA, Washington University October 1999. The Haze Episode of July 16-18,1999.
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Rudolf B. Husar
CAPITA, Washington University
At noon on July 16, the ocean near the northeast coastline was free of clouds. However, a haze pall was observed from Delaware to Maine.
The image is ‘truecolor’, synthesized from the red, green and blue channels of the SeaWiFS 8 channel sensor. Scattering due to air is removed to highlight the haze.
At blue (0.412) wavelength, the haze reflectance is higher while the land surface reflectance is lower then at other wavelengths and virtually the entire reflectance is due to haze or clouds. Air scattering (not included) would add further reflectance in the blue. The surface features are obscured by haze and barely discernable. Hence, the blue wavelength is well suited for aerosol detection over land but surface detection is difficult .
At green wavelength (0.555um) over land, the haze is reduced and the vegetation reflectance is increased. Haze and vegetation have comparable reflectance. The surface features are obscured by haze but discernable. Due to the low reflectance over, haze reflectance dominates. Hence, the green not well suited for haze detection over land but appropriate for haze detection the ocean and the detection of surface features.
At red wavelength (0.412) over land, dark vegetation is distinctly different from brighter yellow-gray soil. Over vegetation haze and plant have similar reflectance. The surface features, particularly water (darkest, R0<0.01), vegetation (dark, R0<0.04), and soil (brighter, R0<0.30) are are easily distinguishable. Due low water reflectance, haze reflectance dominates. Hence, the red is suitable for haze detection over dark vegetation and the ocean as well as for surface detection.
In the near IR (0.865) over land, the surface reflectance is high (R0>0.30) over both vegetation and soil with only minor difference in brightens. Water on the other hand is completely dark (R0<0.01). Land and water are clearly distinguishable. The excess reflectance due to haze is barely perceptible. Due low water reflectance, haze reflectance dominates. Hence, the near IR is suitable for haze detection over water and land-water differentiation.
The haze and cloud-free image is derived from 29 days between July 15 and August 15, 1999 by retaining the lowest reflectance through the period. Clearly, some cloud and haze residues are still retained.
Reconstructed Haze-Free SurfaceDerived from removing the haze over vegetation and dark oceanThe imperfection of reconstructed surface is a measure of the retrieved haze
Within the thicker haze defined here as mist, the slope of the spectral AOT is low (b<0.5) compared to the lighter haze (b>1.0). This indicates larger particles (D~0.6 um) in the mist.
The aerosol optical thickness, AOT is ~ 0.3 (blue) over the much ocean surrounding Cape Cod. However, just to the the south, patches of thicker haze are seen, with AOT > 0.5.
The blue areas of low Angstrom exponent (larger particle size) indicate sharply delineated patches of mist near Nantucket and Martha’s Vineyard Islands.