The profile of upwelling 11‐μm radiance through the atmospheric boundary layer overlying the ocean

Abstract

The gradient of the upwelling 11‐μm radiance from the ocean surface through the atmospheric boundary layer has been measured with a high‐precision radiometer to test a radiative transfer model prediction of the outgoing sea surface radiance. The measured values of radiance agree with standard line‐by‐line radiative transfer calculations of the outgoing flux to within a few percent, suggesting that present empirical expressions for the water vapor continuum absorption are accurate, at least over the range of atmospheric conditions studied here. Using a pressure squared Weighting function to describe the mean value of the partial pressure of water vapor, the mean absorption coefficient for the spectral range 900–980 cm −1 is determined for the first kilometer vertical path length. For a water vapor mean partial pressure of 0.020 atm and a mean temperature of 20°C the, coefficient is 2.556×10−22 mol−1 cm2 atm−1. When the contribution from line absorption is removed, the coefficient decreases by 15%. Infrared data acquired from a satellite sensor (advanced very high resolution radiometer) are compared with numerical computations of the brightness temperature as seen from space for the measured atmospheric conditions; the observed and modeled values are found to agree to within 0.5°C. The emissivity of the ocean surface is shown to be less than unity by about 1%, a result that is qualified by the possibility of anomalous absorption effects very near the ocean surface. Simple radiative transfer model simulations show that the atmospheric component dominates the total outgoing radiation to space for all but the driest conditions and for average geometries of observation; such predictions are confirmed by the experimental results described here.