Ecosystem Predictions with Approximate vs. Exact Light Fields [report]

Curtis D. Mobley
2008 unpublished
LONG-TERM GOAL The overall goal of this work is to develop an extremely fast but accurate radiative transfer model, called EcoLight, for use in coupled physical-biological-optical ecosystem models, and then to demonstrate the ecological necessity and computational feasibility of including accurate light field predictions in coupled physical-biological-optical ecosystem models. OBJECTIVES Currently available ecosystem models often use fairly sophisticated treatments of the physics (e.g.,
more » ... n and upper-ocean thermodynamics and mixing) and biology (e.g., primary production, nutrient utilization, and grazing) but use grossly oversimplified treatments of the optics. The optics component of coupled ecosystem models is sometimes just a single equation parameterizing the scalar irradiance in terms of the chlorophyll concentration and a few parameters such as the solar zenith angle. Such simple models often fail even in Case 1 waters, and they can be wrong by orders of magnitude in Case 2 or optically shallow waters. The objective of this work was develop a radiative transfer model that can be used in coupled models to bring the optics component up to the level of accuracy and sophistication needed for ecosystem models that are being applied to any water body, including Case 2 and optically shallow waters. APPROACH The Hydrolight radiative transfer model (Mobley et al., 1993; Mobley 1994; provides an accurate solution of the radiative transfer equation (RTE) for any water body, given the absorption and scattering properties of the water body and boundary conditions such as incident sky radiance and bottom reflectance. However, the standard version of Hydrolight requires far too much computer time to make it suitable for use in ecosystem models where the light field must be computed at many grid points and at time intervals of order one hour. However, ecosystem models require only the scalar irradiance as a function of depth and wavelength, E o (z,λ), which makes it possible to solve an azimuthally averaged version of the radiative transfer equation (RTE), from which the irradiances can be obtained. Solving the azimuthally integrated RTE removes much of the computation load in Hydrolight, which solves for azimuthally dependent radiances. Report Documentation Page Form Approved OMB No. 0704-0188 Public reporting burden for the collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing this burden, to Washington Headquarters Services, Directorate for Information Operations and Reports, 1215 Jefferson Davis Highway, Suite 1204, Arlington VA 22202-4302. Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to a penalty for failing to comply with a collection of information if it does not display a currently valid OMB control number. Same as Report (SAR) 18. NUMBER OF PAGES 11 19a. NAME OF RESPONSIBLE PERSON a. REPORT unclassified b. ABSTRACT unclassified c. THIS PAGE unclassified Standard Form 298 (Rev. 8-98) Prescribed by ANSI Std Z39-18
doi:10.21236/ada519107 fatcat:rquftujrsvaefluihtx3hjor5i