Modelling Ocean Colour

摘要

Remote sensing algorithms for retrieving estimates of oceanic constituent concentrations, such as chlorophyll concentration, require as input measurements of spectral water-leaving radiance. This measurement is typically obtained from space-based sensors such as SeaWiFS or MODIS. Some polar orbiting sensors have tilt and scan capabilities such that sensor view angles can at times exceed 70 degrees from the zenith. Inherent in algorithms applied to such remotely sensed measurements are assumptions on how the off-nadir radiance varies in intensity compared to the total upwelling irradiance. Typical ocean reflectance equations incorporate two factors, f (in R = fb_b/a) and Q (where Q = E_u/L_u), which relate the water-leaving radiance in 3 dimensions to the incident (sky and Sun) irradiance. A Monte Carlo ocean optical model was developed and used to investigate the three dimensional nature of water leaving radiance for typical open ocean optical conditions. Equations for predicting f were developed as well as a data base of Q factors for different solar and viewing geometries as functions of b, bb and a. Results derived from the Monte Carlo model were then used to develop a more in-depth study of f based on HYDROLIGHT. Improved equations for predicting f have been developed and are compared to predictions of f from HYDROLIGHT.3 Examples of how these improved estimates of f and Q may be applied to a chlorophyll concentration algorithm for open ocean waters will be presented.