Observations and parameterizations of surfzone albedo

Abstract

Incident shortwave solar radiation entering the ocean depends on albedo α and plays an important role in the temperature variability and pathogen mortality of the nearshore region. As foam has an elevated albedo, open-ocean albedo parameterizations include whitecapping effects through a wind-based foam fraction. However, surfzone depth-limited wave breaking does not require wind. Surfzone albedo observations are very rare, the variability of surfzone albedo is not known, and parameterizations are not available. New, year-long upwelling and downwelling shortwave radiation observations were made from the Scripps Institution of Oceanography pier spanning the surfzone and inner-shelf. Surfzone albedo was elevated due to foam with mean observed albedo of α=0.15 and one-minute average albedo as high as α=0.45, far exceeding expected albedo (0.06) from standard parameterizations. Using a pier-mounted GoPro camera, an image-based albedo parameterization is developed that estimates the fractional foam area to derive albedo. This parameterization has high skill (r2=0.90) on time scales as short as a wave period (9 s). A second wave-model based parameterization for (hourly) averaged albedo is developed relating the non-dimensional roller energy dissipation to the mean foam fraction and thus albedo. The parameterization has good skill (r2=0.68) and resolves cross-shore albedo variations. These new parameterizations can be used where imagery is available or wave models are applicable, and can be used to constrain local heat budgets and pathogen mortality.