Fog Evolution in the Visible and Infrared Spectral Regions and Its Meaning in Optical Modeling

摘要

For the first time in the field of cloud physics, high-density fog drop-size data from the time near fog formation through the time near fog dissipation became available at 5-minute intervals from field measurements carried out at Meppen, Germany, on 3-4 March 1978. The fog was believed to have been brought about by frontal passage. Extinction coefficients in the 0.55 micrometers, 3.80 micrometers, and 10.6 micrometers spectral regions were calculated from the measured drop-size distributions and plotted as a function of time to depict optical evolution of the fog. Such spectral evolution in relation to fog microphysical evolution was examined in detail to attempt to formulate optical models or scaling laws for electro-optical sensors applications. A cursory comparison was made with the California advection and radiation-advection fogs observed at Fort Ord on 3 and 9 May 1978, respectively, because the available data portrayed only portions of their life cycles. Because of their low liquid water contents and high number concentration, the haze regimes in these fogs played a dominant role in 'messing up' spectral transmission in the 0.55 micrometers, and 3.80 micrometers regions, usually during the phase of fog formation. In the final analysis, the choice of a particular or a general model or scaling law should be dictated by the sensitivities and requirements of the electro-optical sensors to be deployed.