Characterization of time-varying regimes in remote sensing time series: application to the forecasting of satellite-derived suspended matter concentrations

International audienceSatellite data, with their spatial and temporal coverage, are particularly well suited for the analysis and characterization of space-time-varying relationships between geophysical processes. We investigate here the forecasting of a geophysical variable using both satellite observations and model outputs. As example we study the daily concentration of mineral suspended particulate matters estimated from satellite-derived datasets, in coastal waters adjacent to the French Gironde River mouth. We forecast this high resolution dataset using environmental data (wave height, wind strength and direction, tides and river outflow) and four multi-latent-regime models: homogeneous and non-homogeneous Markov-switching models, with and without an autoregressive term, i.e. the suspended matter concentration observed the day before. We clearly show, using a validation dataset, significant improvements with multi-regime models compared to a classical multi-regression and a state-of-the-art non-linear model (Support Vector Regression (SVR) model). The best results are reported for a mixture of 3 regimes for autoregressive model using non-homogeneous transitions. With the autoregressive models, we obtain at day+1 forecasting performances of 93% of the explained variance for the mixture model compared to 83% for a standard linear model and 85% using a SVR. These improvements are even more important for the non-autoregressive models. These results stress the potential of the identification of geophysical regimes to improve the forecasting or the inversion. We also show that for short periods of lack of observations (typically lesser than 15 days), non-homogeneous transition probabilities and estimated autoregressive term, the observation of the previous day not being available, help to enhance forecasting performances

Glider and satellite monitoring of the variability of the suspended particle distribution and size in the Rhône ROFI

An experiment was carried out in the Gulf of Lions (NW Mediterranean) in February 2014 to assess the temporal and spatial variability of the distribution and size of suspended particulate matter (SPM) in the Rhône Region of Freshwater Influence (ROFI). A set of observations from an autonomous underwater glider, satellite ocean color data, and meteorological and hydrological time-series data highlighted the high variability of the Rhône River surface turbid plume and presence of a bottom nepheloid layer (BNL) that depended on wind and river discharge conditions. While continental winds pushed the surface plume offshore, marine winds pressed the plume at the coast and favored the sedimentation of as well as nourishment of the BNL. Moderate storm events favored breakage of the plume stratification and along-shelf transport of Rhône River particles. The spectral slopes of glider and satellite-derived light backscattering coefficients, γ, were used as a proxies of the SPM size distribution. The results clearly showed that the change of the SPM size in the nepheloid layers was induced by the flocculation of fine sediments, which became finer seaward throughout the ROFI, as well as the effect of rough weather in the breakup of flocs

Glider and satellite monitoring of the variability of the suspended particle distribution and size in the Rhône ROFI

An experiment was carried out in the Gulf of Lions (NW Mediterranean) in February 2014 to assess the temporal and spatial variability of the distribution and size of suspended particulate matter (SPM) in the Rhône Region of Freshwater Influence (ROFI). A set of observations from an autonomous underwater glider, satellite ocean color data, and meteorological and hydrological time-series data highlighted the high variability of the Rhône River surface turbid plume and presence of a bottom nepheloid layer (BNL) that depended on wind and river discharge conditions. While continental winds pushed the surface plume offshore, marine winds pressed the plume at the coast and favored the sedimentation of as well as nourishment of the BNL. Moderate storm events favored breakage of the plume stratification and along-shelf transport of Rhône River particles. The spectral slopes of glider and satellite-derived light backscattering coefficients, γ, were used as a proxies of the SPM size distribution. The results clearly showed that the change of the SPM size in the nepheloid layers was induced by the flocculation of fine sediments, which became finer seaward throughout the ROFI, as well as the effect of rough weather in the breakup of flocs

Saturation of water reflectance in extremely turbid media based on field measurements, satellite data and bio-optical modelling

International audienceEvidence of water reflectance saturation in extremely turbid media is highlighted based on both field measurements and satellite data corrected for atmospheric effects. This saturation is obvious in visible spectral bands, i.e., in the blue, green and even red spectral regions when the concentration of suspended particulate matter (SPM) reaches then exceeds 100 to 1000 g.m −3. The validity of several bio-optical semi-analytical models is assessed in the case of highly turbid waters, based on comparisons with outputs of the Hydrolight radiative transfer model. The most suitable models allow to reproduce the observed saturation and, by inversion, to retrieve information on the SPM mass-specific inherent optical properties

Scattering error corrections for in situ absorption and attenuation measurements

Monte Carlo simulations are used to establish a weighting function that describes the collection of angular scattering for the WETLabs AC-9 reflecting tube absorption meter. The equivalent weighting function for the AC-9 attenuation sensor is found to be well approximated by a binary step function with photons scattered between zero and the collection half-width angle contributing to the scattering error and photons scattered at larger angles making zero contribution. A new scattering error correction procedure is developed that accounts for scattering collection artifacts in both absorption and attenuation measurements. The new correction method does not assume zero absorption in the near infrared (NIR), does not assume a wavelength independent scattering phase function, but does require simultaneous measurements of spectrally matched particulate backscattering. The new method is based on an iterative approach that assumes that the scattering phase function can be adequately modeled from estimates of particulate backscattering ratio and Fournier-Forand phase functions. It is applied to sets of in situ data representative of clear ocean water, moderately turbid coastal water and highly turbid coastal water. Initial results suggest significantly higher levels of attenuation and absorption than those obtained using previously published scattering error correction procedures. Scattering signals from each correction procedure have similar magnitudes but significant differences in spectral distribution are observed

The Arctic Nearshore Turbidity Algorithm (ANTA) - A multi sensor turbidity algorithm for Arctic nearshore environments

The Arctic is greatly impacted by climate change. The increase in air temperature drives the thawing of permafrost and an increase in coastal erosion and river discharge. This leads to a greater input of sediment and organic matter into coastal waters, which substantially impacts the ecosystems by reducing light transmission through the water column and altering the biogeochemistry, but also the subsistence economy of local people, and changes in climate because of the transformation of organic matter into greenhouse gases. Yet, the quantification of suspended sediment in Arctic coastal and nearshore waters remains unsatisfactory due to the absence of dedicated algorithms to resolve the high loads occurring in the close vicinity of the shoreline. In this study we present the Arctic Nearshore Turbidity Algorithm (ANTA), the first reflectance-turbidity relationship specifically targeted towards Arctic nearshore waters that is tuned with in-situ measurements from the nearshore waters of Herschel Island Qikiqtaruk in the western Canadian Arctic. A semi-empirical model was calibrated for several relevant sensors in ocean color remote sensing, including MODIS, Sentinel 3 (OLCI), Landsat 8 (OLI), and Sentinel 2 (MSI), as well as the older Landsat sensors TM and ETM+. The ANTA performed better with Landsat 8 than with Sentinel 2 and Sentinel 3. The application of the ANTA to Sentinel 2 imagery that matches in-situ turbidity samples taken in Adventfjorden, Svalbard, shows transferability to nearshore areas beyond Herschel Island Qikiqtaruk

High-frequency turbulence and suspended sediment concentration measurements in the Garonne River tidal bore

The study details new sediment concentration measurements associated with some turbulence characterisation conducted at high-frequency in the undular tidal bore of the Garonne River (France). Acoustic Doppler velocimetry was used, and the suspended sediment concentration was deduced from the acoustic backscatter intensity. The field data set demonstrated some unique flow features of the tidal bore including some large and rapid turbulent velocity fluctuations during and after the bore passage. Some unusually high suspended sediment concentration was observed about 100 s after the tidal bore front lasting for more than 10 min. It is thought that the tidal bore passage scoured the bed and convected upwards the bed material, reaching the free-surface after the bore passage. Behind the tidal bore, the net sediment flux magnitude was 30 times larger than the ebb tide net flux and directed upstream. A striking feature of the data set was the intense mixing and suspended sediment motion during the tidal bore and following flood tide. This feature has been rarely documented

Validation of satellite water products based on HYPERNETS in situ data using a Match-up Database (MDB) file structure

A Match-up Database (MDB) file structure and tools were developed to ease the validation analysis of satellite water products and to improve the exchange and processing of match-up data from different sites, missions and atmospheric correction processors. In situ remote sensing reflectance (Rrs) measurements were available from the HYPSTAR® (HYperspectral Pointable System for Terrestrial and Aquatic Radiometry), a new automated hyperspectral radiometer. An MDB file is a NetCDF file containing all the potential match-ups between satellite and in situ data on a specific site and within a given time window. These files are generated and manipulated with three modules developed in Python to implement the validation protocols: extract satellite data, associate each extract with co-located in situ radiometry data, and then perform the validation analysis. This work provides details on the implementation of the open-source MDB file structure and tools. The approach is demonstrated by a multi-site matchup comparison based on satellite data from the Sentinel-2 MSI and Sentinel-3 OLCI sensors, and HYPSTAR® data acquired over six water sites with diverse optical regimes from February 2021 to March 2023.The analysis of Sentinel-3 OLCI matchups across the six sites shows consistency with previous comparisons based on AERONET-OC data over extended reflectance range. We evaluated Sentinel-2 MSI reflectance data corrected with two atmospheric correction processors (ACOLITE and C2RCC) over four sites with clear to highly turbid waters. Results showed that the performance of the processors depends on the optical regime of the sites. Overall, we proved the suitability of the open-source MDB-based approach to implement validation protocols and generate automated matchup analyses for different missions, processors and sites

WATERHYPERNET: a prototype network of automated in situ measurements of hyperspectral water reflectance for satellite validation and water quality monitoring

This paper describes a prototype network of automated in situ measurements of hyperspectral water reflectance suitable for satellite validation and water quality monitoring. Radiometric validation of satellite-derived water reflectance is essential to ensure that only reliable data, e.g., for estimating water quality parameters such as chlorophyll a concentration, reach end-users. Analysis of the differences between satellite and in situ water reflectance measurements, particularly unmasked outliers, can provide recommendations on where satellite data processing algorithms need to be improved. In a massively multi-mission context, including Newspace constellations, hyperspectral missions and missions with broad spectral bands not designed for “water colour”, the advantage of hyperspectral over multispectral in situ measurements is clear. Two hyperspectral measurement systems, PANTHYR (based on the mature TRIOS/RAMSES radiometer) and HYPSTAR® (a newly designed radiometer), have been integrated here in the WATERHYPERNET network with SI-traceable calibration and characterisation. The systems have common data acquisition protocol, data processing and quality control. The choice of validation site and viewing geometry and installation considerations are described in detail. Three demonstration cases are described: 1. PANTHYR data from two sites are used to validate Sentinel-2/MSI (A&B); 2. HYPSTAR® data at six sites are used to validate Sentinel-3/OLCI (A&B); 3. PANTHYR and HYPSTAR® data in Belgian North Sea waters are used to monitor phytoplankton parameters, including Phaeocystis globosa, over two 5 month periods. Conclusion are drawn regarding the quality of Sentinel-2/MSI and Sentinel-3/OLCI data, including indications where improvements could be made. For example, a positive bias (mean difference) is found for ACOLITE_DSF processing of Sentinel-2 in clear waters (Acqua Alta) and clues are provided on how to improve this processing. The utility of these in situ measurements, even without accompanying hyperspectral satellite data, is demonstrated for phytoplankton monitoring. The future evolution of the WATERHYPERNET network is outlined, including geographical expansion, improvements to hardware reliability and to the measurement method (including uncertainty estimation) and plans for daily distribution of near real-time data