Project: Development of new solar radiation and primary production products from MODIS ocean-color measurements

The present MODIS standard products for the oceans include the above-surface photosynthetic available radiation (PAR(0+)) and the diffuse attenuation coefficient at 490 nm (Kd(490)). They are valuable for many studies, but are not adequate for the estimation of solar radiation in the upper water column. This is mainly because: 1) the transmittance of light from above surface to below surface is not a constant, but varies with location and time; 2) Kd(490), a single wavelength product, cannot be used to calculate the vertical attenuation of PAR (a broad band product); and 3) PAR(0+) represents daily total radiation, while Kd(490) (and the evaluation Kd(PAR) product) represents an instantaneous attenuation. Because the light attenuation coefficient also varies diurnally, a combination of daily PAR(0+) and instantaneous Kd(PAR) will result in significant (can be a factor of 2 or 3, as it is depth dependent) errors in the estimated PAR at depth.

Separately, the primary production (PP) values of the global oceans estimated from the traditional chlorophyll-based approaches contain significant uncertainties, which make the current satellite-derived water-column PP unreliable for the assessment of spatial and temporal variations. Mainly the uncertainty arises from the large spatial and temporal variation of chlorophyll-specific absorption coefficient (a*ph), which is required, explicitly or implicitly, for the retrieval of chlorophyll-a concentration from water color and for the estimation of biomass-normalized photosynthetic efficiency. It has long been argued that it is necessary to take an approach that estimates PP without the engagement of a*ph.

To overcome the limitations listed above, and more importantly to maximize the value offered by MODIS Terra/Aqua for ocean physical and biogeochemical studies, we will develop and refine new algorithms for the generation of these new products from MODIS ocean-color measurements: 1) sub-surface, daily, usable solar radiation (USR), 2) diffuse attenuation coefficient of USR (K(USR)), and 3) daily, water-column primary production (PPeu). USR is defined as the solar radiation in the spectral window of 400-560 nm, a window that penetrates most in the global oceans and is most significant for phytoplankton photosynthesis. Because USR and K(USR) products are consistent both spectrally and temporally, it will then be straightforward, and accurate, to estimate solar radiation at depths when the values of K(USR) and subsurface USR are known. Further, the proposed PPeu estimation will be based on the 'first principles of phytoplankton photophysiology and light penetration' [Behrenfeld 1998], i.e., it will use directly the absorption coefficient of phytoplankton for the estimation of absorbed photons, thus by-passing the engagement of a*ph when using chlorophyll concentration. The resulted PP products will then significantly improve the spatial and temporal characterizations of PPeu in the global oceans.

Completion of this project will not only result in innovative products directly applicable to studies of the ocean's heat budget and photosynthesis in the upper water column, but will also be important for investigating photo-oxidation and phytoplankton phenology. The project will further enhance our capabilities in meeting NASA Strategic Sub-goal 3A 'to study Earth from space to advance scientific understanding and meet societal needs'.