Observing optically complex oceans in situ and from space

Satellite borne ocean colour sensors can provide data on the materials suspended in natural waters on spatial scales that are unmatched by any other sensor platform. Higher temporal resolution data can be obtained from optical sensors on moored observatories. The combination of both modes of data collection is important for providing data for models of ecosystems and climate change. However, there are currently serious limitations in the performance of interpretation algorithms for retrieving water quality parameters such as concentrations of chlorophyll. Standard algorithms have generally been optimised for clear oceanic waters and perform poorly in optically complex waters, where non-algal materials including inorganic particles (e.g. sediments and dusts), coloured dissolved organic materials (CDOM) and even bubbles can have a negative impact on performance. Another major problem is the fact that end-users of remote sensing data do not have easy access to information about the accuracy of remote sensing products. This project will resolve some of the uncertainties affecting interpretation of remote sensing optical signals through a combination of in situ optical measurements and radiative transfer simulations. Particular attention will be paid to developing improved quality in situ absorption measurements that are currently subject to errors in turbid waters. Methods will be developed for partitioning absorption and scattering coefficients (from in situ and remote sensing data) into components for different material constituents (algal and non-algal). The performance of remote sensing algorithms will be assessed for different water types using in situ data sets and a simulated data set over extended ranges of constituents obtained by radiative transfer modelling. The errors for remote sensing products will be determined and presented as colour-coded maps that will be made available to end-users. This work will result in improved algorithms for analysing remote sensing signals for coastal waters and will contribute to an improved understanding of relationships between biogeophysical processes and climate change factors in the marine environment. Improved interpretation algorithms and will result in better quality data products for monitoring agencies to assure compliance with legal obligations and policy makers to make informed decisions with.

The radiative transfer approach to interpretation of ocean colour remote sensing (OCRS) data is based upon quantitative measurement of inherent optical properties such as absorption, attenuation and backscattering coefficients along with associated constituent concentrations. These data are used to establish realistic simulations of underwater and water leaving light fields that can be used as a test-bed for algorithm development. The award of this Advanced Fellowship has led to significant progress in a number of key areas: •Advances in determining scattering properties of marine particles.•Significant improvement in measurement methodologies for in situ absorption and attenuation measurements.•New insight into the effect of multiple scattering on the angular distribution of scattered photons.•Further development of remote sensing algorithms for suspended sediment concentrations.•Analysis of links between physical and biogeochemical processes based on ocean colour observations.Each of these areas has resulted in publications published or submitted to leading journals. In addition there are several areas of on-going work that are close to publication readiness, including: development of a comprehensive set of specific inherent optical properties for the Ligurian Sea using state of the art IOP methods, development of a new spectral deconvolution method for OCRS data, based upon a well-established physical model, that is specifically designed to provide optimal estimates of constituent concentrations and associated uncertainties, analysis of OCRS algorithm performance using radiative transfer simulations and accounting for measurement uncertainty in both radiometric observations and measurement errors for validating constituents. The vast majority of initial objectives has therefore been achieved or will be within a short period from now. This work has benefitted from extensive collaborations with pre-existing and new partners including the NATO Underwater Research Centre (Italy), HZG (Germany), WETLabs Inc (USA), IOPAN (Poland), Argans Ltd (UK) and LOV (France). This international network has ensured that the work performed under the auspices of the Advanced Fellowship has been closely integrated with the wider marine optics and oceanographic community. Within the UK there has been on-going collaboration with NOC (Southampton and Liverpool) as well as the Universities of Plymouth and Bangor. OCRS data is increasingly been prepared for validation and assimilation into coupled physical-ecosystem models. There is also considerable interest from government agencies in the potential for using OCRS data for monitoring compliance with environmental legislation. During the course of the Fellowship I have provided advice to both the Scottish Government and also the European Space Agency on aspects of both OCRS data and in situ IOPs. The rigorous physical approach that has been developed here is closely aligned with user demand for quality assured products. My focus on determining product uncertainty has placed this research in the vanguard (with other leading groups) of a movement towards more accurate, quantitative OCRS and in situ IOP products. This is reflected in the growing international reputation of the Strathclyde Marine Optics and Remote Sensing Group and in continuing invitations to collaborate with other leading groups in the field.