Infrared propagation modeling beneath marine stratus clouds

Airborne measurements of aerosol size distributions are used to determine t he vertical profiles of infrared (IR) extinction and absorption coefficient s and asymmetry factors in eight different maritime stratus cloud regimes d uring unstable boundary layer conditions where the sea temperature was grea ter than the ambient air temperature. The average values of these parameter s are determined relative to the level where the air temperature change wit h elevation was near a moist adiabatic lapse rare. A model to determine the effects of aerosols on IR propagation beneath these types of clouds is pre sented in terms of multiplying arrays compatible with the input format of t he transmittance/radiance computer code MODTRAN. The model is used in a mod ified version of MODTRAN to test its utility in system performance predicti ons beneath these types of clouds. The maximum detection range (MDR) of a s urface ship by an airborne forward looking infrared (FLIR) system was deter mined to be a factor of 3 in better agreement with the observed MDR than th at determined using the MODTRAN ICLD3 stratus model with the Navy Aerosol M odel (NAM) beneath the cloud. The predictions were found to be insensitive to the wave slope model used in the zero-range sea radiance calculations. T his is shown to be the result of a compensating effect between increased se a emissions and decreased cloud reflections for the larger wave slope varia nces associated with unstable boundary layer conditions as compared to thos e for stable or neutral conditions.