LOWTRAN modeling of near-horizon infrared sky radiances in the presence of clouds

A set of infrared (8 to 12 micrometer) sky radiances and meteorological (radiosonde) measurements are used to test the utility of the LOWTRAN 6 radiance algorithm to predict infrared sky radiances close to the horizon when clouds are present. The effects of clouds on the calculated radiances at the optical horizon are shown to be minimal but must be properly included at slightly higher elevations. Although the measurements pertain to good surface visibility conditions, small contributions by aerosols must be included in the LOWTRAN 6 calculations in order to match the measured radiances. This points out the possibility of inferring an appropriate (but not unique) vertical aerosol size distribution model from remotely sensed sky radiances. A deficiency in the radiance algorithm at a zenith angle of 90 degrees was noted, and its sensitivity to the number of lower layers used in the atmospheric model is discussed.