Infrared Propagation Modeling beneath Marine Stratus Clouds

Abstract

Airborne measurements of aerosol size distributions are used to determine the vertical profiles of infrared (IR) extinction and absorption coefficients and asymmetry factors in eight different maritime stratus cloud regimes during unstable boundary layer conditions where the sea temperature was greater than the ambient air temperature. The average values of these parameters are determined relative to the level where the air temperature change with elevation was near a moist adiabatic lapse rate. A model to determine the effects of aerosols on IR propagation beneath these types of clouds is presented in terms of multiplying arrays compatible with the input format of the transmittance/radiance computer code MODTRAN. The model is used in a modified version of MODTRAN to test its utility in system performance predictions beneath these types of clouds. The maximum detection range (MDR) of a surface ship by an airborne forward looking infrared (FLIR) system was determined to be a factor of 3 in better agreement with the observed MDR than that determined using the MODTRAN ICLD3 stratus model with the Navy Aerosol Model (NAM) beneath the cloud. The predictions were found to be insensitive to the wave slope model used in the zero-range sea radiance calculations. This is shown to be the result of a compensating effect between increased sea emissions and decreased cloud reflections for the larger wave slope variances associated with unstable boundary layer conditions as compared to those for stable or neutral conditions.