Two Frontier Topics In Remote Sensing Of Ocean Color: Oceanic Air Bubbles And Vertically Nonuiform Profiles

At present, the retrieval algorithms and biogeochemical applications of remote sensing of ocean color are typically based on the assumption that the ocean is vertically homogeneous. Moreover, the contribution of oceanic air bubbles that are primarily resulted from breaking waves to scattering is not taken into consideration. In fact, significant vertical variations in the concentration of constituents including air bubbles are often observed in the surface ocean, especially in coastal waters. These two frontier topics were further investigated in this paper. Based on the analysis of observing data, the contribution of air bubbles to remote-sensing reflectance was examined, as well as the influence of air bubbles on atmospheric correction. Moreover, numerical simulations of radiative transfer were made to investigate the effects of nonuniform vertical profiles of suspended paniculate matter, and air bubbles on remote-sensing reflectance of coastal waters. The specific conclusions of this paper are listed as follows:1. On the basis of observations of ocean optics parameters and sea surface wind speed, as well as numerical simulations of radiative transfer, it was proved that the changes in remote-sensing reflectance under different wind speeds in an optically stable open ocean were resulted from oceanic air bubbles. By fitting 73 simultaneous data of remote-sensing reflectance and wind speed, a statistical relationship between remote-sensing reflectance and wind speed was developed, as well as a relationship between remote-sensing reflectance and air bubble concentration. There were 23 match-up data of remote-sensing reflectance from MODIS/Aqua and from in-situ observations. The errors between the MODIS/Aqua and in-situ remote-sensing reflectance due to air bubbles were associated with wind speed, and a statistical relationship was established to obtain a correction equation for satellite remote-sensing reflectance.2. By validating the 23 MODIS/Aqua remote-sensing reflectance, the effects of air bubbles on atmospheric correction and its mechanism was investigated. The errors of atmospheric correction increased with an injection of air bubbles and with an increase of air bubble concentrations. Even in the open ocean, the dark-black pixel (DP) assumption is violated due to air bubbles. As a result, the radiance resulting from aerosol contributions are overestimated, the selected aerosol models might be wrong, and in turn the remote-sensing reflectance in the visible spectral ranges is underestimated. 3. Based on numerical simulations of radiative transfer, the effects of nonuniform vertical profiles of SPM and associated IOPs on remote-sensing reflectance were investigated. The vertical structure was assumed to consist of two homogeneous layers with different concentrations of suspended particulate matter (SPM). The IOPs of colored dissolved organic matter (CODM) were assumed to covary with SPM. The simulations for the two-layer ocean were compared with simulations for a reference homogeneous ocean. The SPM vertical structure was parameterized to examine the sensitivity of remote-sensing reflectance to SPM profile. In the red-NIR spectral region a relationship between remote-sensing reflectance and the parameters describing SPM profile was developed for the first time.4. Based on numerical simulations of radiative transfer and Mie theory, the effects of vertically stratified air bubbles on remote-sensing reflectance of coastal waters were examined. The background ocean considerd were two cases:uniform and nonuniform. The air bubble size distribution was determined from literatures, which is a function of depth and wind speed. The difference in remote-sensing reflectance produced by air bubbles depends on the wind speed, the thickness of the upper layer, the attenuation coefficient of the background ocean, as well as light wavelength. If the seawater is turbid, the contribution of oceanic air bubbles on remote-sensing reflectance can be neglected.