Development of a diffuse reflectance probe for in situ measurement of inherent optical properties in sea ice

Detailed characterization of the spatially and temporally varying inherent optical properties (IOPs) of sea ice is necessary to better predict energy and mass balances, as well as ice-associated primary production. Here we present the development of an active optical probe to measure IOPs of a small volume of sea ice (dm3) in situ and non-destructively. The probe is derived from the diffuse reflectance method used to measure the IOPs of human tissues. The instrument emits light into the ice by the use of an optical fibre. Backscattered light is measured at multiple distances away from the source using several receiving fibres. Comparison to a Monte Carlo simulated lookup table allows, in theory, retrieval of the absorption coefficient, the reduced scattering coefficient and a phase function similarity parameter γ, introduced by Bevilacqua and Depeursinge (1999). γ depends on the two first moments of the Legendre polynomials, allowing the analysis of the backscattered light not satisfying the diffusion regime. The depth reached into the medium by detected photons was estimated using Monte Carlo simulations: the maximum depth reached by 95 % of the detected photons was between 40±2 and 270±20 mm depending on the source–detector distance and on the ice scattering properties. The magnitude of the instrument validation error on the reduced scattering coefficient ranged from 0.07 % for the most scattering medium to 35 % for the less scattering medium over the 2 orders of magnitude we validated. Fixing the absorption coefficient and γ, which proved difficult to measure, vertical profiles of the reduced scattering coefficient were obtained with decimetre resolution on first-year Arctic interior sea ice on Baffin Island in early spring 2019. We measured values of up to 7.1 m−1 for the uppermost layer of interior ice and down to 0.15±0.05 m−1 for the bottommost layer. These values are in the range of polar interior sea ice measurements published by other authors. The inversion of the reduced scattering coefficient at this scale was strongly dependent on the value of γ, highlighting the need to define the higher moments of the phase function. This newly developed probe provides a fast and reliable means for measurement of scattering in sea ice.

Interpretation of Spectral LiDAR Backscattering off the Florida Coast

A multispectral backscattering LiDAR (Light detection and range) system (hereafter Oculus) was integrated into a wave glider and used to estimate the scattering order (i.e., single vs multiple collisions) of LIDAR backscattering, the water inherent optical properties (IOPs), the biogeo-chemical characteristics of particulate scatterers (i.e., relative size, composition) and their motion) on shelf waters of South East Florida. Oculus has a dual-wavelength configuration (473 and 532 nm) and two detection geometries (off- and on-axis). Characteristics of scatterers were investigated based on two complementary LiDAR-derived proxies (the Structural Dissimilarity Index and the spectral slope of LiDAR backscattering). In March 2017, field measurements showed a covariation between direct and diffuse backscattering contributions during morning hours and away from shore. LiDAR attenuation coefficients explained up to 57% of IOPs variability. The analysis of LiDAR-derived proxies suggested higher turbidity and larger particulates near the coast

Identificaction of phytoplankton blooms in the Gerlache Strait, West Antarctic Peninsula

In order to identify phytoplankton blooms, the inherent optical properties index (IOPIndex) was used in surface water samples and the maximum chlorophyll-a (MPC) that were taken in the Third (January 2017) and in the Fourth Expedition (January 2018) from Colombia to Antarctica in the Gerlache Strait (EG). The IOPIndex is calculated from the standardized spatial anomalies of the phytoplankton, detritus and colored dissolved organic matter (CDOM) absorption coefficients using wavelength 443. In addition, the phytoplankton size index was calculated using the Blue / Red ratio and this related to the IOPIndex to determine the size structure of the phytoplankton responsible for the blooms. In 2017, four stations were identified under bloom conditions, where two were superficial blooms and two subsurface bloom; to three in conditions of decrease of the bloom and to twelve in conditions of non-bloom, with a structure of diverse size where populations of the micro, nano and the picophytoplankton predominated. In 2018, two stations were identified in bloom conditions, being one superficial bloom and the other subsurface bloom; to two in conditions of decrease in bloom and nine in conditions of non-bloom, the size structure that dominated the blooms was microphytoplankton. With the IOPIndex, subsurface bloom conditions were observed in both expeditions, also proving their effectiveness for blooms generated by natural or seasonal conditions at different depths of the water column.

Development of a Diffuse Reflectance Probe for In Situ Measurement of Inherent Optical Properties in Sea Ice

Detailed characterization of the spatially and temporally varying inherent optical properties (IOPs) of sea ice is necessary to better predict energy and mass balances, as well as ice-associated primary production. Here we present the development of an active optical probe to measure IOPs of a small volume of sea ice (dm3) in situ and non-destructively. The probe is derived from the diffuse reflectance method used to measure the IOPs of human tissues. The instrument emits light into the ice by the use of optical fibre. Backscattered light is measured at multiple distances away from the source using several receiving fibres. Comparison to a Monte Carlo simulated lookup table allows to retrieve the absorption coefficient, the reduced scattering coefficient and a phase function similarity parameter γ, introduced by Bevilacqua and Depeursinge (1999), depending on the two first moments of the Legendre polynomials, allowing to analyze the backscattered light not satisfying the diffusion regime. Monte Carlo simulations showed that the depth cumulating 95% of the signal is between 40±2 mm and 270±20 mm depending on the source-detector distance and on the ice scattering properties. The magnitude of the instrument validation error on the reduced scattering coefficient ranged from 0.07% for the most scattering medium to 35% for the less scattering medium over the two orders of magnitude we validated. Vertical profiles of the reduced scattering coefficient were obtained with decimeter resolution on first-year Arctic interior sea ice on Baffin Island in early spring 2019. We measured values of up to 7.1 m−1 for the uppermost layer of interior ice and down to 0.15±0.05 m−1 for the bottommost layer. These values are in the range of polar interior sea ice measurements published by other authors. The inversion of the reduced scattering coefficient at this scale was strongly dependent of γ, highlighting the need to define the higher moments of the phase function. This novel developed probe provides a fast and reliable means for measurement of scattering into sea ice.

Application of Satellite Data for Retrieving the Light Absorption Characteristics in the Black Sea Waters

Purpose. he work is aimed at studying the effects of light absorption in the Black Sea waters with due regard for the variations of its individual components, and how they are manifested in the NASA archival results of calculating the chlorophyll a concentration obtained by processing satellite data using the universal operational method. Methods and Results. The NASA archival data of the MODIS and SeaWiFS satellite instruments, and the values of the light absorption components (determined by the method of Generalized ocean color inversion model for retrieving marine inherent optical properties (GIOP)) related to yellow substance and phytoplankton were analyzed. In order to avoid possible manifestations of various distortions in the results of determining the remote sensing reflectance of the sea and in the products resulted from application of the GIOP method, only the specially selected and sufficiently reliable test data from two areas located near the Crimea Southern Coast and south of the Danube estuary were used. Conclusions. In the considered examples with low content of chlorophyll a in the seawater, the yellow substance plays a predominant role in light absorption in the spectrum blue part, whereas if the chlorophyll a content is high, the phytoplankton contribution is dominant. The revealed relationship between the light absorption components related to yellow substance and phytoplankton significantly differs from that implicitly preset as a basis of the universal method (applied in NASA for the satellite data operational processing) for determining the chlorophyll a concentration. This, in its turn, is manifested in the fact that the data on the chlorophyll a concentration in the Black Sea stored in the NASA archive may be overestimated in case the chlorophyll a concentration is low, and underestimated – in case it is high.

Application of Satellite Data for Retrieving the Light Absorption Characteristics in the Black Sea Waters

Purpose. The work is aimed at studying the effects of light absorption in the Black Sea waters with due regard for the variations of its individual components, and how they are manifested in the NASA archival results of calculating the chlorophyll a concentration obtained by processing satellite data using the universal operational method. Methods and Results. The NASA archival data of the MODIS and SeaWiFS satellite instruments, and the values of the light absorption components (determined by the method of Generalized ocean color inversion model for retrieving marine inherent optical properties (GIOP)) related to yellow substance and phytoplankton were analyzed. In order to avoid possible manifestations of various distortions in the results of determining the remote sensing reflectances of the sea and in the products resulted from application of the GIOP method, only the specially selected and sufficiently reliable test data from two areas located near the Crimea Southern Coast and south of the Danube estuary were used. Conclusions. In the considered examples with low content of chlorophyll a in the seawater, the yellow substance plays a predominant role in light absorption in the spectrum blue part, whereas if the chlorophyll a content is high, the phytoplankton contribution is dominant. The revealed relationship between the light absorption components related to yellow substance and phytoplankton significantly differs from that implicitly preset as a basis of the universal method (applied in NASA for the satellite data operational processing) for determining the chlorophyll a concentration. This, in its turn, is manifested in the fact that the data on the chlorophyll a concentration in the Black Sea stored in the NASA archive may be overestimated in case the chlorophyll a concentration is low, and underestimated – in case it is high.