Concerning Causes of “Anomalous Gray Shades” in DMSP Visible Imagery

Abstract

Visible, Very High Resolution (VHR) and Light Fine (LF) data from the Defense Meteorological Satellite Program (DMSP) frequently exhibit areas of light-toned gray shades termed ?anomalous gray shades? which appear most notable over the ocean but also over land areas, where contrast is suitable. These gray shades are distinctly different in appearance from visible, highly reflective cloud formations, and they are often particularly well delineated in DMSP VHR or LF data and either appear weakly or do not appear at all in near-simultaneous data from other satellite systems. This paper identifies low-level aerosols, such as haze, diffuse cloud particles or light fog, as one of the major causes of anomalous gray shades and shows examples with available documentation. Since the DMSP VHR and LF sensor response curves are drastically different from those of other U.S. operational meteorological satellite systems, it was assumed that wavelength dependence on absorption and scattering effects was involved in the enhanced capability of the DMSP sensors to render anomalous gray shades visible. A simple radiative transfer model is adopted to test the effect of haze on emergent radiation intensity as a function of wavelength and as measured by a meteorological satellite. These results are compared to previously obtained experimental measurements of particulate matter optical thickness as a function of wavelength under varying atmospheric conditions ranging from foggy or hazy to essentially clear. The results confirm that near-infrared wavelengths, except in narrow band regions where appreciable water vapor absorption occurs, provide much greater contrast between hazy and clear conditions that do shorter wavelengths in the visible portion of the spectrum. Since the DMSP VHR and LF sensors have their peak response in the near infrared, this appears to be an important factor in accounting for the sensitivity of these sensors in delineating hazy areas more adequately than other sensors from other systems which yield a peak response at shorter wavelengths.