Microphysical Effects of Irradiating a Fog With a 10.6-μCO2Laser

Abstract

The microphysical effects due to illuminating a fog with a 10.6-μm CO2 laser are examined using a microphysical model that accounts for the sensible and latent heat transfers from the drop and the absorption of the laser energy by the liquid drops and by the water vapor. Since the laser radiation is in the “IR window region,” relatively little of the laser energy is absorbed by the water vapor. Thus, the drops absorb the laser energy and dissipate this energy by sensible and latent heat fluxes both directed away from the drops. Since the heating of the ambient air by the sensible heat flux increases the saturation vapor pressure (at the ambient temperature) faster than the vapor (latent heat) flux increases the ambient vapor density, the relative humidity decreases with time. Since the absorption of laser energy by the drops is proportional to their geometric cross section and the transfer of sensible and latent heat is proportional to the drop radii, larger drops become warmer than the smaller drops. The absorption cross section for drops radius smaller than 8 μm is proportional to their mass and, for this size range, the droplet spectrum narrows as the droplets evaporate, contrary to the normal situation in clouds. The time required to evaporate a given volume of fog and the resultant rise in the ambient temperature and relative humidity are examined as a function of the initial temperature of the fog, the liquid water content, the drop size, the ambient pressure, and the intensity of the laser radiation. A “multislice” model is used to examine the propagation of the clearing effect for three fog spectra observed at Otis Air Force Base.