Numerical simulations show that underwater radiances, irradiances, and reflectances are sensitive to the shape of the scattering phase function at intermediate and large scattering angles, although the exact shape of the phase function in the backscatter directions (for a given backscatter fraction) is not critical if errors of the order of 10% are acceptable. We present an algorithm for generating depth- and wavelength-dependent Fournier-Forand phase functions having any desired backscatter fraction. Modeling of a comprehensive data set of measured inherent optical properties and radiometric variables shows that use of phase functions with the correct backscatter fraction and overall shape is crucial to achieve model-data closure.
A spectrum-matching and look-up-table (LUT) methodology has been developed and evaluated to extract environmental information from remotely sensed hyperspectral imagery. The LUT methodology works as follows. First, a database of remote-sensing reflectance ͑R rs ͒ spectra corresponding to various water depths, bottom reflectance spectra, and water-column inherent optical properties (IOPs) is constructed using a special version of the HydroLight radiative transfer numerical model. Second, the measured R rs spectrum for a particular image pixel is compared with each spectrum in the database, and the closest match to the image spectrum is found using a least-squares minimization. The environmental conditions in nature are then assumed to be the same as the input conditions that generated the closest matching HydroLight-generated database spectrum. The LUT methodology has been evaluated by application to an Ocean Portable Hyperspectral Imaging Low-Light Spectrometer image acquired near Lee Stocking Island, Bahamas, on 17 May 2000. The LUT-retrieved bottom depths were on average within 5% and 0.5 m of independently obtained acoustic depths. The LUT-retrieved bottom classification was in qualitative agreement with diver and video spot classification of bottom types, and the LUT-retrieved IOPs were consistent with IOPs measured at nearby times and locations.
If the benthic boundary in optically shallow waters is spatially inhomogeneous or sloping, the underwater light field is inherently three-dimensional (3D). Numerical simulations of 3D underwater radiances were made for environmental conditions observed in shallow Bahamian waters. The simulations show that if the pattern of bottom reflectance for an inhomogeneous but level bottom has a spatial scale much smaller than the bottom area seen by a radiometer, the inhomogeneous bottom can be replaced by a homogeneous bottom whose reflectance is the areaweighted average of the actual bottom reflectances. For large-scale patterns of bottom reflectance, the 3D light fields near the edges of bottom patches of different reflectances can be predicted from analytical models incorporating the sensor geometry and one-dimensional (1D) light fields computed for homogeneous bottoms, with errors of order 10% when compared to the exact 3D solutions. The same holds true for uniformly sloping bottoms, whose 3D light fields can be modeled in terms of the 1D light fields computed for level bottoms, with errors of less than 10% for bottom slopes of 20Њ or less.
Ocean ecosystem models require accurate calculations of both hydrodynamics and biology; those calculations in turn require accurate calculation of in-water irradiance. Ecosystem models now achieve great accuracy in their hydrodynamical predictions, and the biological modules are becoming correspondingly sophisticated. The optical calculations are however often oversimplified, to the possible detriment of the physical and biological predictions. We used a recently developed, extremely fast radiative transfer code, EcoLight-S, to study differences in ecosystem and thermal development in an idealized upwellingdownwelling system when simple versus accurate irradiance calculations are used. The use of accurate irradiances gave up to 57% differences in chlorophyll concentrations after two weeks of simulated time, compared to predictions based on irradiances obtained using a simple exponential attenuation formula. Accurate irradiance calculations increased sea surface temperatures and decreased temperatures at depth, leading to increased stratification. Use of EcoLight-S couples the physical and biological calculations so that biology feeds back to physics, and vice versa. EcoLight-S outputs ancillary quantities such as remote sensing reflectance and in-water spectral irradiance, which can be used to validate ecosystem predictions using remotely sensed ocean color imagery or optical measurements from buoys or gliders, without the need to convert such measurements to chlorophyll values. After optimization, the ecosystem model total run times with EcoLight-S were less than 20% more than for the analytical irradiance models. We also found that the use of 24 h average irradiances gave factor-of-two differences in chlorophyll concentrations compared to the use of a diel irradiance pattern with the same 24 h average value.
Abstract. Coupled physical-biological-optical ocean ecosystem models often use sophisticated treatments of their physical and biological components, while oversimplifying the optical component to the possible detriment of the ecosystem predictions. To bring optical computations up to the standard required by recent ecosystem models, we developed a computationally fast numerical model, named EcoLight, that solves the azimuthally averaged radiative transfer equation to obtain accurate in-water spectral irradiances for use in calculations of photosynthesis and photo-oxidation. To evaluate its computational features, we incorporated EcoLight into an idealized physical-biological model for open-ocean Case 1 waters and compared ten-year simulations for a simple analytical irradiance model vs. EcoLight numerical calculations. After optimization, the EcoLight run times are less than 30% more than for the analytical irradiance model. Moreover, EcoLight is suitable for use in Case 2 and optically shallow waters, for which no analytical irradiance models exist. EcoLight also computes ancillary quantities such as the remote-sensing reflectance, which can be useful for ecosystem validation.
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