Developments in ocean infrared emissivity/reflection modeling: comparisons against observations

The infrared (IR) spectral emissivity of the earth's surface is now recognized to be a key factor in radiative transfer forward modeling. The oceans, in particular, cover ~70% of the earth's surface, and a high degree of accuracy is generally necessary for sea surface skin temperature (SST) determination. A mere 0.5% departure from blackbody emission results in significant brightness temperature errors (~0.25 K).

Much progress has been made toward modeling the spectral IR emissivity of wind-roughened water surfaces. Existing emissivity models explicitly calculate the ensemble mean emissivity of the wavy surface for a given observer zenith angle and local wind speed. However, field observations of emissivity spectra obtained by the Marine Atmospheric Emitted Radiance Interferometer (M-AERI) suggest that emissivity models are deficient at larger view angles and wind speeds. In this work, we attempt to identify and explain the sources of error in these models using M-AERI data acquired at sea (e.g., during AEROSE 2004). Our results demonstrate that proper accounting for non-unity surface emissivity must ultimately include appropriate specification of the reflected IR radiation field, especially in window channels. Atmospheric IR surface reflectance becomes important for high accuracy applications (e.g., SST), that rely on window channel observations at zenith angles ≥ 45°. Lookup tables of ensemble mean effective incidence angle, rather than mean emissivity, are generated using different published mean square slope PDF models. The results roughly agree with recent findings by Hanafin and Minnett (2005). Lookup tables of ensemble mean local zenith incidence angle are also generated. This new approach to emissivity/reflection modeling will be refined and validated against M-AERI field data from several previous oceanographic cruises.