Uncertainty Quantification in the Infrared Surface Emissivity Model (ISEM)

Accurate modeling of surface emissivity is imperative for accurate radiative transfer simulation and forward modeling of satellite radiance observations. The Radiative Transfer for (A)TOVS (RTTOV) fast radiative transfer model uses the Infrared Surface Emissivity Model (ISEM) for the computation of sea surface emissivity in the infrared (IR) spectrum. However, the model does not incorporate the effect of surface-emitted surface reflected (SESR) radiation and dependence of wind speed in the emissivity calculation. This paper investigates the uncertainty in the ISEM model caused by ignoring the SESR radiation and wind speed effects in the 3 IR bands, 3.7, 11, and 12 μm. First, we develop a new model called Surface Emissivity Model in IR with SESR (SEMIS) that takes the SESR radiation and wind speed effects into account. The uncertainty in the ISEM model is then quantified by comparing the ISEM emissivity against SEMIS derived emissivity. The comparison results suggest that two models are in excellent agreement below ~60° emission angle, implying no notable uncertainty in the ISEM model at smaller angles. Nevertheless, uncertainty tends to significantly increase with increasing emission angle above ~60°, which is even more notable at high wind speed (~15 m/s). Two models are further compared against emissivity measurements from a radiometer. The ISEM model has produced large errors as opposed to the SEMIS.