Environmental Impact on Directed Energy, Volume 1 (Directed Energy and Cloud Shadows)

Author(s):

Gail Vaucher, Michael Lee, and Andrew Gatzke

Abstract:

Atmospheric cloud shadows have a positive and negative effect on “seeing” (image degradation due to atmospheric turbulence) and on coherent light propagation, such as that of a laser. To minimize visual or laser distortion, minimal density variations along an open-air propagation pathway are required. Unfortunately, the natural state of the atmosphere includes an abundance of density variations, which are characterized as optical turbulence via an index of refraction structure function (Cn 2 ) parameter. The highly turbulent atmosphere is largely fueled by solar radiation heating the soil, which creates an unstable environment. When a cloud shadow moves over an optical path, direct sunlight abruptly becomes diffuse. The sharp surface heat reduction inhibits vertical mixing, lowering Cn 2 . This condition persists as long as the shadowed environment suppresses the buoyancy/thermal eddies. At the shadow’s edge, Cn 2 sharply rises, due to the localized large thermal gradient (density variations). If a cloud shadow persists, favorable seeing continues until the heat released by the ground becomes trapped between the ground and cloud cover, such as in an overcast scenario. In this report, cross-discipline concepts (atmospheric, directed energy) are presented, followed by two representative cloud shadow cases (partly cloudy and overcast). The report concludes with a summary of an investigative modeling project that used machine learning techniques in constructing a 3-D cloud model from which cloud shadow mapping is anticipated.