Chapter 4 IN-WATER INSTRUMENTATION AND PLATFORMS FOR OCEAN COLOR REMOTE SENSING APPLICATIONS

Remote sensing of reflected sunlight from the upper ocean is a tremendous tool for studying biological, chemical, geological, and physical processes over a broad range of time and space scales. Global biogeochemical phenomena spanning seasonal (e.g., spring bloom), multi-year (e.g., the El Niño Southern Oscillation), to decadal (e.g., climatic variability) time scales can be resolved by an orbiting satellite imager. Reflected light in the visible domain (wavelengths of ~ 400 to 700 nm) is particularly useful in the study of upper ocean processes, as many important biogeochemical components of seawater absorb and scatter light effectively in this spectral range (the term “ocean color” specifically relates to the spectral character of this water-leaving visible light). These dissolved and particulate seawater components play key roles in the cycling of carbon in the ocean and serve as indicators of ecosystem health. In-water measurements help elucidate the link between these components and the remotely sensed signal. Down-looking, passive remote sensors in air and space measure sunlight that is reflected upward into the sensor; in addition to the atmospherically scattered photons, a portion of the measured radiance results from photons that have exited the ocean and passed back through the atmosphere to the sensor in orbit. This portion is termed spectral upwelled water-leaving radiance, Lu (W m -2 nm sr) and primarily consists of light scattered in the backward direction off the particles and molecules of seawater (for a complete discussion, refer to Zaneveld et al., Chapter 1). Sunlight incident at the ocean surface is represented as spectral downwelling surface irradiance, Ed (W m -2 nm), and the so-called remote sensing reflectance, Rrs, is derived from Lu/Ed, with Lu strictly defined in the nadir direction (normal to the plane of the ocean surface). Although Lu consists of primarily backscattered light, equally important in terms of its information content is the component of incident sunlight missing in the upwelled light. This is light that has been absorbed (or filtered) by the constituents of seawater in the upper ocean. The dependence of Rrs on these optical processes of backscattering and absorption just below the ocean surface (represented as 0) can be simply written (Morel and Prieur, 1977):