Optical closure in marine waters from in situ inherent optical property measurements

Ina Lefering,Charles Trees,Rüdiger Röttgers,Fethi Bengil,Alex Nimmo-Smith,Jill Schwarz,David Bowers,David Mckee

doi:10.1364/oe.24.014036

Abstract

Optical closure using radiative transfer simulations can be used to determine the consistency of in situ measurements of inherent optical properties (IOPs) and radiometry. Three scattering corrections are applied to in situ absorption and attenuation profile data for a range of coastal and oceanic waters, but are found to have only very limited impact on subsequent closure attempts for these stations. Best-fit regressions on log-transformed measured and modelled downwards irradiance, Ed, and upwards radiance, Lu, profiles have median slopes between 0.92 - 1.24, revealing a tendency to underestimate Ed and Lu with depth. This is only partly explained by non-inclusion of fluorescence emission from CDOM and chlorophyll in the simulations. There are several stations where multiple volume scattering function related data processing steps perform poorly which suggests the potential existence of unresolved features in the modelling of the angular distribution of scattered photons. General optical closure therefore remains problematic, even though there are many cases in the data set where the match between measured and modelled radiometric data is within 25% RMS%E. These results are significant for applications that rely on optical closure e.g. assimilating ocean colour data into coupled physical-ecosystem models.

Highlights

Ocean colour remote sensing (OCRS) has transformed our ability to observe complex interactions between physical and biogeochemical processes in surface waters of the ocean [1,2]
The impact of the three different AC-9 correction approaches on absorption and attenuation data is presented for two example West Coast of Scotland (WCS) stations
The vast majority of stations in our data set showed little sensitivity to the choice of AC-9 scattering correction method. Reasons for this include: (a) the data set only included a small number of highly turbid stations where the effect of new corrections are expected to be most significant, (b) the greatest effect of scattering corrections on AC-9 data is for attenuation values which have little impact on optical closure [53], and (c) such absorption differences as were observed occurred in the red-NIR which is dominated by water absorption for optical closure purposes

Summary

Introduction

Ocean colour remote sensing (OCRS) has transformed our ability to observe complex interactions between physical and biogeochemical processes in surface waters of the ocean [1,2] It has provided new insights into the spatial and temporal variability of algal blooms, sediment transport and a host of other features that were previously only poorly observed [3,4]. It has to be realised that this approach is effectively an exercise in optical closure i.e.there is an implicit assumption that retrieved parameters and radiometric data are mutually consistent. This leads to the important question: How close to optical closure can we reasonably expect to get with currently available technology?

Methods

Results

Discussion

Conclusion