Effects of intermittent entrainment of air bubbles by breaking wind waves on ocean reflectance and underwater light field

Abstract

Abstract. Light-scattering properties of air bubbles suspended in water and observational evidence of bubble entrainment by breaking wind waves indicate that bubble clouds may influence ocean reflectance and in-water light field characteristics within the surface layer. We estimate potential changes in remote sensing reflectance and in-water light field associated with a bubble entrainment event observed at a wind speed of 10 rn s -•. Our approach combines acoustic measurements of bubble concentration as a function of time and depth and radiative transfer simulations of the light field within and leaving a water body. We show that the remote sensing reflectance can increase significantly (more than twofold) due to bubble entrainment, and these large variations occur over time periods on the order of minutes or less. The bubble clouds have a spectral effect on ocean reflectance such that the water patch containing bubbles will appear greener or more yellowish than the surrounding waters with no bubbles. These results are relevant to measurements of remote sensing reflectance made from just above the water surface with downlooking radiance meters whose spot size at the surface is of O(0.1-1) m. The light field characteristics within the bubble layer are also significantly affected, with most pronounced effects seen in the profiles of upwelling irradiance and upwelling radiance. Therefore the bubble entrainment can be a source of error in the estimation of reflectance from extrapolation of underwater measurements at a depth up to and across the surface. While our radiative transfer simulations were made using a model with a plane-parallel geometry, future efforts to examine the effects of spatial distribution of bubble clouds and their properties on ocean reflectance from spatially integrating [O(10-1000) m] satellite measurements will have to be based on a three-dimensional approach.

On the influence of wind and waves on underwater irradiance fluctuations

“…In this case it has to be assumed that in reality the lensing efficiency is considerably reduced because of nonlinear hydrodynamic interactions at the surface and air bubbles that are induced by breaking waves (e.g. Stramski and Tegowski, 2001). Thus, it has to be assumed that at high wind speed our model overestimates the strength of irradiance variability down the water column.…”

“…More relevant are air bubbles in the upper water layer as they cause enhanced reflectance (in clear water the reflectance has a strong dependence on the light spectrum). But in contrast to the upwelling light, bubbles induce only very small enhancement in downwelling irradiance within the top several tens of centimetres just beneath the surface and below that layer, E d is reduced compared to bubble-free water (Stramski and Tegowski, 2001). For this reason we must consider that air bubbles in water impair the effectiveness of wave lensing, and thus damp the intensity of the described fluctuations, in particular at increasing wind speed.…”

Quantification of Typhoon-Induced Phytoplankton Blooms Using Satellite Multi-Sensor Data

“…Strong typhoon winds may induce massive entrainment of air bubbles into the water column through the wave breaking; these bubbles have a spectral effect on ocean reflectance, and therefore, the bubble entrainment may affect the estimation accuracy of surface Chl-a concentration from remote sensing observations [40]. In situ measurements from a Bio-Argo float with fluorescence method to detect the Chl-a concentration located at the central Bay of Bengal indicate that there was substantial Chl-a increase (to as high as 4.5 mg·m −3 ) induced by a tropical cyclone, Hudhud, and the Bio-Argo observations on the Chl-a profiles suggest that the nutrient inject is a major reason for the Chl-a increase under the storm condition, leading to the chlorophyll bloom [41].…”

A theoretical study of the effect of subsurface oceanic bubbles on the enhanced aerosol optical depth band over the southern oceans as detected from MODIS and MISR

“…For example, Zhang et al (1998) found that organic coatings on bubbles will significantly enhance the backscattering and proposed that bubbles could be the strongest contributor to the light coming out of ocean. Stramski and Tegowski (2001) illustrated the temporal variation of the light field caused by bubble injection under a wave breaking event (wind speed = 10 m s −1 ) recorded by an acoustic backscatter in coastal waters and found a > 2-fold increase in reflectance (400-700 nm) over time periods on the order of several minutes or less. Clearly, ocean surface reflectance patterns can be altered by immersed bubbles under windy conditions, which may further affect satellite aerosol retrievals from passive sensors.…”

“…Within a pixel of satellite observation, whitecaps are sporadic and scattered whereas bubbles in water form a more or less uniform layer that could exist over regions that are free from whitecap contamination (e.g., Monahan and Lu, 1990). While whitecaps serve as a diffuse reflector, reflecting solar radiation directly at the surface (Frouin et al, 1996;Whitlock et al, 1982), bubbles interact with light below the surface, enhancing water-leaving radiance (Stramski and Tegowski, 2001;Terrill et al, 2001;Zhang et al, 1998). Studies have shown that the contributions of bubbles to water-leaving radiance are rather significant.…”