14 research outputs found
Using ocean colour remote sensing products to estimate turbidity at the Wadden Sea time series station Spiekeroog
Time series measurements at the Wadden Sea time series station Spiekeroog (WSS) in the southern North Sea were used to empirically develop approaches for determining turbidity from ocean colour remote sensing products (OCPs). Turbidity was observed by a submerged optical sensor. Radiometric quantities were collected using hyperspectral radiometers. Surface reflected glint correction was applied to the radiometric quantities to compute remote sensing reflectance (RRS) and the RRS was converted into perceived colour of seawater matching the Forel-Ule colour Index (FUI) scale. The empirical approaches for determining turbidity from OCPs showed good least squares linear correlations and statistical significance (R2 > 0.7, p < 0.001). These OCP approaches had relatively low uncertainties in predicting turbidity with encouraging mean absolute percent difference less than 31 %. The problem of bio-fouling on submerged sensors and the potential application of OCPs to monitor or correct for sensor drifts was evaluated. A protocol is proposed for the acquisition and processing of hyperspectral radiometric measurements at this optically complex station. Use of the classic FUI as a time series indicator of surface seawater changes did show promising results. The application of these OCPs in operational monitoring changes in water quality was also explored with the aim to evaluate the potential use of the WSS datasets in calibration and validation of satellite ocean colour remote sensing of these very turbid coastal waters
Copernicus Marine Service ocean state report, issue 4
This is the final version. Available from Taylor & Francis via the DOI in this record.âŻFCT/MCTE
Temperature and Salinity Anomalies in the Sea Surface Microlayer of the South Pacific During Precipitation Events
AbstractWe present the results of salinity (ÎS) and temperature (ÎT) anomalies in the sea surface microlayer (SML) in relation to the underlying mixed bulk water (bulk). Several light to moderate rain events were recorded in the southern Pacific near Fiji using our remotely operated catamaran. Precipitation and evaporation drive freshwater fluxes across the sea surface (i.e., the SML) and are the most essential processes of the hydrologic cycle. However, measurements of the SML during precipitation are rare, but necessary to fully understand freshwater exchange at the airâsea interface. Here we show that freshwater can mix rapidly with the bulk water through windâinduced mixing, as ÎS and ÎT show a clear dependence on wind speed. At high wind speeds (5.1â11.6 m sâ1), anomalies approach zero (ÎS = â0.02 ± 0.49 g kgâ1, ÎT = â0.09 ± 0.46°C) but can reach ÎS = 1.00 ± 0.20 g kgâ1 and ÎT = â0.37 ± 0.09°C at lower wind speeds (0â2 m sâ1). We find shallow freshwater lenses and fronts, likely caused by past rainfall, with ÎS and ÎT of up to â1.11 g kgâ1 and 1.77°C, respectively. Our observations suggest that freshwater lenses can be very shallow (<1 m depth) and missed by conventional measurements. In addition, the temperature and salinity in the SML respond to freshwater fluxes instantaneously. It highlights the role of the SML in a mechanistic understanding of the fate of freshwater over the ocean and, therefore, the global hydrologic cycle.Plain Language Summary: Rain and evaporation are the most important processes in the global water cycle, causing either the supply to or the removal of freshwater from the upper ocean, thereby changing the salinity of the sea surface. Evaporation also removes heat and lowers the temperature on the ocean surface. We used the measurements of sea surface microlayer (SML) salinity and temperature as key indicators to study hydrologic cycle processes during our cruise with the RV Falkor in the South Pacific and found that freshwater mixes rapidly with the underlying bulk water during strong winds (5.1â11.6 m sâ1). We also detected shallow freshwater lenses and fronts, most likely caused by past rainfall, with ÎS and ÎT of up to â1.11 g kgâ1 and 1.77°C, respectively. Our observations suggest that freshwater lenses can occur at the sea surface and that the SML respond to freshwater fluxes instantaneously. It highlights the role of the SML for future studies of the global hydrologic cycle.Key Points: Small scale airâsea interactions (freshwater fluxes) during precipitation were investigated in the southern Pacific. Temperature and salinity anomalies occur with a high spatial variability.Measurements with remote controlled catamaran revealed shallow freshwater lenses, which were not detectable with ship based measurements.German Research Foundationhttps://doi.org/10.7284/908805https://www.rvdata.us/search/cruise/FK191120https://bec.icm.csic.es/https://smos-diss.eo.esa.int/https://doi.org/10.48670/moi-00165https://disc.gsfc.nasa.gov/datasets/GPM_3IMERGHH_06/summar
Acoustic and optical methods to infer water transparency at Time Series Station Spiekeroog, Wadden Sea
Water transparency is a primary indicator of optical water quality that is
driven by suspended particulate and dissolved material. A data set from the
operational Time Series Station Spiekeroog located at a tidal inlet of the
Wadden Sea was used to perform (i) an inter-comparison of observations
related to water transparency, (ii) correlation tests among these measured
parameters, and (iii) to explore the utility of both acoustic and optical
tools in monitoring water transparency. An Acoustic Doppler Current Profiler
was used to derive the backscatter signal in the water column. Optical
observations were collected using above-water hyperspectral radiometers and a
submerged turbidity metre. Bio-fouling on the turbidity sensors optical
windows resulted in measurement drift and abnormal values during quality
control steps. We observed significant correlations between turbidity
collected by the submerged metre and that derived from above-water radiometer
observations. Turbidity from these sensors was also associated with the
backscatter signal derived from the acoustic measurements. These findings
suggest that both optical and acoustic measurements can be reasonable proxies
of water transparency with the potential to mitigate gaps and increase data
quality in long-time observation of marine environments
Synoptic observations of sediment transport and exchange mechanisms in the turbid Ems Estuary: the EDoM campaign
An extensive field campaign, the Ems-Dollard Measurements (EDoM), was executed in the Ems Estuary,
bordering the Netherlands and Germany, aimed at better understanding the
mechanisms that drive the exchange of water and sediments between a relatively exposed outer estuary and a hyper-turbid tidal river. More specifically, the reasons for the large up-estuary sediment accumulation rates and the role of the tidal river on the turbidity in the outer estuary were insufficiently understood. The campaign was designed to unravel the hydrodynamic and sedimentary exchange mechanisms, comprising two hydrographic surveys during contrasting environmental conditions using eight concurrently operating ships and 10 moorings measuring for at least one springâneap tidal cycle. All survey locations were equipped with sensors measuring flow velocity, salinity, and turbidity (and with stationary ship surveys taking water samples), while some of the survey ships also measured turbulence and
sediment settling properties. These observations have provided important new insights into horizontal sediment fluxes and density-driven exchange flows, both laterally and longitudinally. An integral analysis of these
observations suggests that large-scale residual transport is surprisingly
similar during periods of high and low discharge, with higher river
discharge resulting in both higher seaward-directed fluxes near the surface
and landward-directed fluxes near the bed. Sediment exchange seems to be
strongly influenced by a previously undocumented lateral circulation cell
driving residual transport. Vertical density-driven flows in the outer
estuary are influenced by variations in river discharge, with a near-bed
landward flow being most pronounced in the days following a period with
elevated river discharge. The study site is more turbid during winter
conditions, when the estuarine turbidity maximum (ETM) is pushed seaward by river
flow, resulting in a more pronounced impact of suspended sediments on
hydrodynamics. All data collected during the EDoM campaign, but also
standard monitoring data (waves, water levels, discharge, turbidity, and
salinity) collected by Dutch and German authorities are made publicly
available at 4TU Centre for Research Data (https://doi.org/10.4121/c.6056564.v3; van Maren et al., 2022).</p
The Milan Campaign: Studying diel light effects on the airâsea interface
The sea surface microlayer (SML) at the airâsea interface is <1 mm thick, but it is physically, chemically, and biologically distinct from the underlying water and the atmosphere above. Wind-driven turbulence and solar radiation are important drivers of SML physical and biogeochemical properties. Given that the SML is involved in all airâsea exchanges of mass and energy, its response to solar radiation, especially in relation to how it regulates the airâsea exchange of climate-relevant gases and aerosols, is surprisingly poorly characterized. MILAN (Sea Surface Microlayer at Night) was an international, multidisciplinary campaign designed to specifically address this issue. In spring 2017, we deployed diverse sampling platforms (research vessels, radio-controlled catamaran, free-drifting buoy) to study full diel cycles in the coastal North Sea SML and in underlying water, and installed a land-based aerosol sampler. We also carried out concurrent ex situ experiments using several microsensors, a laboratory gas exchange tank, a solar simulator, and a sea spray simulation chamber. In this paper we outline the diversity of approaches employed and some initial results obtained during MILAN. Our observations of diel SML variability show, for example, an influence of (i) changing solar radiation on the quantity and quality of organic material and (ii) diel changes in wind intensity primarily forcing airâsea CO2 exchange. Thus, MILAN underlines the value and the need of multidiciplinary campaigns for integrating SML complexity into the context of airâsea interaction