Fast and accurate irradiance calculations for ecosystem models

Abstract

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.

Capturing optically important constituents and properties in a marine biogeochemical and ecosystem model

“…We note that the radiative transfer component is a simplification from a full radiance model and, in particular, does not resolve the angular distribution of light or angular dependence of scattering. These assumptions have been shown to be small in terms of the needs for ecosystem models (Mobley et al, 2009). Though not a full radiative transfer model, our three-stream treatment does provide the relevant output for our objectives: the spectral light available for photosynthesis and an upwelling component that at the sea surface is similar to that seen by a satellite.…”

“…Mobley et al, 2009;Fujii et al, 2007;Gregg and Casey, 2007;Bisset et al, 1999). Such models include fully coupled radiative transfer, but differ in the levels of simplification for computational efficiency (e.g.…”

A spectrally-resolved light propagation model for aquatic systems: Steps toward parameterizing primary production

“…The code is parallelized 4 , and by increasing the available hardware to 5 cores, the computation time for the full 3D model area is reduced to 40-45% of the original calculation time. At this point the computational load is fully acceptable for the model to be used in SINMOD, but there are a number of further optimizations that can be done to reduce computation time, such as simplifying calculations for layers beyond which the light intensity is below a certain level, and reducing the spatial or temporal frequency of light computations (Mobley et al, 2009).…”

Ecosystem Predictions with Approximate vs. Exact Light Fields

“…All have contributed to this final report. This report is condensed from a draft journal article (Mobley et al, 2009), which is in preparation for submission to Biogeosciences Discussions.…”