A two-layer model of whitecap spectral reflectance

Whitecaps influence ocean color remote sensing from space, as evidenced in theoretical and experimental studies. They enhance water reflectance, and, due their spectral optical properties, affect the extrapolation to the visible of the aerosol signal in the near infrared in atmospheric correction (AC) schemes. Without proper correction, the retrieved water reflectance may be impacted by unacceptable errors, even in the presence of a small amount of whitecaps. Whitecap reflectance is highly variable due to a multitude of factors, and the parameterizations commonly used in AC schemes (essentially based on wind speed) are inaccurate. In view of this, a two-layer model of whitecap reflectance is proposed, with large bubbles separated by a thin layer of fluid at the surface (surface foam layer, SFL) and bubbles injected during the wave breaking process below (underwater bubble layer, UBL). The SFL reflectance is characterized by its value at 400 nm, ρ_SFL0, and known spectral dependence. The UBL reflectance is parameterized as a function of geometric thickness, H_UBL, and optical thickness, τ_UBL. The three model parameters are adjusted in order to reproduce whitecap reflectance data in the range 400-1650 nm collected at the Scripps Institution of oceanography Pier. A good agreement with the radiometric measurements was obtained using a ρ_SFL0 of 0.11, a H_UBL of 15 cm, and a τ_UBL of 3. A spectral-matching algorithm is envisaged to retrieve the model parameters from top-of-atmosphere reflectance measurements in the near to shortwave infrared, with potential application to the future NASA PACE mission.