Monte Carlo Radiative Transfer Simulations on the Influence of Surface Waves on Underwater Light Fields

A Monte Carlo model has been developed for calculating the penetration of light into the ocean. For monochromatic light (490 nm) the spatial exact allocation of the scattered parts of underwater light is described in terms of the variation of particle content and the angle of light incidence. Based on this model, it is possible to generate complex spatiotemporal fluctuating light fields according to every possible shape of the water surface. By means of single gravity waves the focusing effect and its importance of radiance supply for water depths up to 100 m is discussed

Solar radiative transfer into the ocean: A study on underwater light fluctuations due to surface waves

The thesis is about the solar radiative transfer into the ocean. Particular emphasis is placed on underwater light fluctuations due to focusing surface waves. The study includes measurements at high sea and computational simulations of the light propa-gation in seawater and wave-induced radiative variability. Surface waves of different sizes have an optical lensing effect; they cause focusing of light beams at various depths. The underwater propagation of light depends on the scattering and absorption properties of seawater. Strongest light fluctuations appear near the surface at water depths of 0.5 to 10 m. At 1 m depth, radiative enhancements with a factor of 40 compared to the mean light level can be achieved. These short-term extreme values refer to the downwelling irradiance. The reason for the most intense irradiance peaks are surface waves with lengths of 20 cm to 5 m. In theory, light flashes with a radiative intensification of the factor 1.5 can appear down to 80 m of water depth. The range of possible irradiance peaks is discussed with respect to all relevant ocean waves. Even 200 m long swell waves can originate small irradiance changes below the 90 m depth level. In natural sea states waves of different sizes are superposed. Their respective lensing effect controls the subsurface light regime. The mechanisms of those interactions are analyzed. Local wind, which is primarily associated with ultra-gravity waves, strongly affects light fluctuations within the near-surface region down to 10 m depth. The most intense radiative peaks result from moderate wind conditions with velocities of 2 to 7 m s-1. Below 10 m depth, the temporal and spatial light variability is driven by super-posed fully developed gravity waves of the corresponding sea state. Comparable strong variations arise from 1.5 m high waves. Even in 100 m depth slight wave-induced light field variance was found

On the influence of wind and waves on underwater irradiance fluctuations

The influence of various wind and wave conditions on the variability of downwelling irradiance Ed (490 nm) in water is subject of this study. The work is based on a two-dimensional Monte Carlo radiative transfer model with high spatial resolution. The model assumes conditions that are ideal for wave focusing, thus simulation results reveal the upper limit for light fluctuations. Local wind primarily determines the steepness of capillary-gravity waves which in turn dominate the irradiance variability near the surface. Down to 3 m depth, maximum irradiance peaks that exceed the mean irradiance Ed by a factor of more than 7 can be observed at low wind speeds up to 5 m s−1. The strength of irradiance fluctuations can be even amplified under the influence of higher ultra-gravity waves; thereby peaks can exceed 11 Ed. Sea states influence the light field much deeper; gravity waves can cause considerable irradiance variability even at 100 m depth. The simulation results show that under realistic conditions 50% radiative enhancements compared to the mean can still occur at 30 m depth. At greater depths, the underwater light variability depends on the wave steepness of the characteristic wave of a sea state; steeper waves cause stronger light fluctuations

Modeling of wave-induced irradiance variability in the upper ocean mixed layer

A Monte Carlo based radiative transfer model has been developed for calculating the availability of solar radiation within the top 100 m of the ocean. The model is optimized for simulations of spatial high resolution downwelling irradiance Ed fluctuations that arise from the lensing effect of waves at the water surface. In a first step the accuracy of simulation results have been verified by measurements of the oceanic underwater light field and through intercomparison with an established radiative transfer model. Secondly the potential depth-impact of nonlinear shaped single waves, from capillary to swell waves, is assessed by considering the most favorable conditions for light focusing, i.e. monochromatic light at 490 nm, very clear oceanic water with a low chlorophyll a content of 0.1 mg m−3 and high sun elevation. Finally light fields below irregular wave profiles accounting for realistic sea states were simulated. Our simulations suggest that under open ocean conditions light flashes with 50 % irradiance enhancements can appear down to 35 m depth, and light variability in the range of ±10 % compared to the mean Ed is still possible in 100 m depth

Der solare Strahlungstransport in den Ozean: Unterwasser-Lichtfluktuationen aufgrund von Oberflächenwellen

The thesis is about the solar radiative transfer into the ocean. Particular emphasis is placed on underwater light fluctuations due to focusing surface waves. The study includes measurements at high sea and computational simulations of the light propagation in seawater and wave-induced radiative variability. Surface waves of different sizes have an optical lensing effect; they cause focusing of light beams at various depths. The underwater propagation of light depends on the scattering and absorption properties of seawater. Strongest light fluctuations appear near the surface at water depths of 0.5 to 10 m. At 1 m depth, radiative enhancements with a factor of 40 compared to the mean light level can be achieved. These short-term extreme values refer to the downwelling irradiance. The reason for the most intense irradiance peaks are surface waves with lengths of 20 cm to 5 m. In theory, light flashes with a radiative intensification of the factor 1.5 can appear down to 80 m of water depth. The range of possible irradiance peaks is discussed with respect to all relevant ocean waves. Even 200 m long swell waves can originate small irradiance changes below the 90 m depth level. In natural sea states waves of different sizes are superposed. Their respective lensing effect controls the subsurface light regime. The mechanisms of those interactions are analyzed. Local wind, which is primarily associated with ultra-gravity waves, strongly affects light fluctuations within the near-surface region down to 10 m depth. The most intense radiative peaks result from moderate wind conditions with velocities of 2 to 7 m/s. Below 10 m depth, the temporal and spatial light variability is driven by superposed fully developed gravity waves of the corresponding sea state. Comparable strong variations arise from 1.5 m high waves. Even in 100 m depth slight wave-induced light field variance was found.In dieser Arbeit geht es um die Sonneneinstrahlung in den Ozean und insbesondere um Schwankungen des Strahlungsangebots aufgrund von fokussierenden Wellen auf der Wasseroberfläche. Die Untersuchungen umfassen sowohl Messungen auf See, als auch Computer-Simulationen der Unterwasser-Lichtausbreitung und der wellenbedingten Strahlungsvariabilität. Verschieden große Wellen wirken als optische Linsen und verursachen damit eine Bündelung von Sonnenstrahlen in unterschiedlichen Tiefen. Die Ausbreitung des Lichts hängt maßgeblich von den Streu- und Absorptionseigenschaften des Wassers ab. Die stärksten Lichtschwankungen treten in Wassertiefen von etwa 0,5 bis 10 m auf und können das Strahlungsniveau in der Tiefe um mehr als das 40-fache übersteigen. Solche kurzzeitigen Extremwerte, bezogen auf die abwärtsgerichtete Strahlungsflussdichte, werden durch Wellen von 20 cm bis 5 m Länge hervorgerufen. Theoretisch können Lichtblitze mit einer 1,5-fachen Strahlungserhöhung in bis zu 80 m Wassertiefe auftreten. Die Bandbreite der möglichen Strahlungserhöhungen ist für alle relevanten Wellen im Ozean erörtert; sogar 200 m lange Dünungswellen können das Lichtangebot in größeren Tiefen (> 90 m) beeinflussen. In einem natürlichen Seegang sind Wellen verschiedener Größe überlagert, die durch ihre jeweilige Linsenwirkung das Unterwasserlichtregime beeinflussen. Die Mechanismen der gegen¬seitigen Verstärkungen und Abschwächungen von Strahlungswerten werden genau analysiert. Der momentane Wind über einem Seegebiet und die damit verbundenen kleineren Ultra-Schwerewellen haben bis etwa 10 m Tiefe starken Einfluss auf die Unterwasser-Lichtfluktuationen. Die größten Schwankungen treten bei mäßigen Windverhältnissen von 2 bis 7 m/s auf. Unterhalb von 10 m werden die Fluktuationen zeitlich und räumlich von überlagerten voll ausgereiften Schwerewellen des entsprechenden Seegangs bestimmt. Die stärksten Lichtschwankungen werden hier von etwa 1,5 m hohen Wellen hervorgerufen. Noch in 100 m Wassertiefe können leichte seegangsbedingte Strahlungsschwankungen nachgewiesen werden

Experimental Characterization of Single-Color Power LEDs Used as Photodetectors

Semiconductor-based light emitting diodes can be used for photon emission as well as for detection of photons. In this paper, we present a fair comparison between off-the-shelf power Light emitting diodes (LEDs) and a silicon photodetector with respect to their spectral, temporal, and spatial properties. The examined LED series features unexpected good sensitivity and distinct optical bandpass characteristic suitable for daylight filtering or color selectivity. Primary application is short range optical underwater communication, but results are generally applicable

Effects and Constraints of Optical Filtering on Ambient Light Suppression in LED-Based Underwater Communications

Optical communication promises to be a high-rate supplement for acoustic communication in short-range underwater applications. In the photic zone of oceanic and coastal waters, underwater optical communication systems are exposed by remaining sunlight. This ambient light generates additional noise in photodetectors, thus degrading system performance. This effect can be diminished by the use of optical filters. This paper investigates light field characteristics of different water types and potential interactions with optical underwater communication. A colored glass and different thin film bandpass filters are examined as filter/detector combinations under varying light and water conditions, and their physical constraints are depicted. This is underlined by various spectral measurements as well as optical signal-to-noise ratio calculations. The importance of matching the characteristics of the light emitting diode (LED) light source, the photodetector, and the filter on the ambient conditions using wider angle of incidents is emphasized

Application of Sentinel-2 MSI in Arctic Research: Evaluating the Performance of Atmospheric Correction Approaches Over Arctic Sea Ice

Multispectral remote sensing may be a powerful tool for areal retrieval of biogeophysical parameters in the Arctic sea ice. The MultiSpectral Instrument on board the Sentinel-2 (S-2) satellites of the European Space Agency offers new possibilities for Arctic research; S-2A and S-2B provide 13 spectral bands between 443 and 2,202 nm and spatial resolutions between 10 and 60 m, which may enable the monitoring of large areas of Arctic sea ice. For an accurate retrieval of parameters such as surface albedo, the elimination of atmospheric influences in the data is essential. We therefore provide an evaluation of five currently available atmospheric correction processors for S-2 (ACOLITE, ATCOR, iCOR, Polymer, and Sen2Cor). We evaluate the results of the different processors using in situ spectral measurements of ice and snow and open water gathered north of Svalbard during RV Polarstern cruise PS106.1 in summer 2017. We used spectral shapes to assess performance for ice and snow surfaces. For open water, we additionally evaluated intensities. ACOLITE, ATCOR, and iCOR performed well over sea ice and Polymer generated the best results over open water. ATCOR, iCOR and Sen2Cor failed in the image-based retrieval of atmospheric parameters (aerosol optical thickness, water vapor). ACOLITE estimated AOT within the uncertainty range of AERONET measurements. Parameterization based on external data, therefore, was necessary to obtain reliable results. To illustrate consequences of processor selection on secondary products we computed average surface reflectance of six bands and normalized difference melt index (NDMI) on an image subset. Medians of average reflectance and NDMI range from 0.80–0.97 to 0.12–0.18 while medians for TOA are 0.75 and 0.06, respectively

Phytoplankton Group Identification Using Simulated and In-situ Hyperspectral Remote Sensing Reflectance

Given that the commonly used parameter obtained directly from hyperspectral earth observation sensors is the remote sensing reflectance (Rrs), we focused on identification of dominant phytoplankton groups by using Rrs spectra directly. Based on five standard absorption spectra representing five different phytoplankton spectral groups, a simulated database of Rrs (C2X database, compiled within the ESA SEOM C2X Project) that includes 105 different water optical conditions was built with HydroLight. In our previous study we have proposed an identification approach to determine phytoplankton groups with the use of simulated C2X data, and the skill of the identification were also tested by investigating how and to what extend water optical constituents (Chl, NAP, and CDOM) impact the accuracy of this identification (Xi et al. 2017). To furthermore test whether the approach is applicable in various natural waters, we have collected a large set of in situ data from waters with different optical types, including coastal waters such as the German Bight and British coastal waters, and inland waters such as Elbe River and several lakes in Germany. Both in situ Rrs and absorption spectra (ap) are used to identify the dominating phytoplankton group in these waters. Identification results from both approaches are compared, and the identification performance of the Rrs-based approach can therefore be evaluated for natural water applications

Optical remote sensing (Sentinel-3 OLCI) used to monitor dissolved organic carbon in the Lena River, Russia

In the past decades the Arctic has experienced stronger temperature increases than any other region globally. Shifts in hydrological regimes and accelerated permafrost thawing have been observed and are likely to increase mobilization of organic carbon and its transport through rivers into the Arctic Ocean. In order to better quantify changes to the carbon cycle, Arctic rivers such as the Lena River in Siberia need to be monitored closely. Since 2018, a sampling program provides frequent in situ observations of dissolved organic carbon (DOC) and colored dissolved organic matter (CDOM) of the Lena River. Here, we utilize this ground truth dataset and aim to test the potential of frequent satellite observations to spatially and temporally complement and expand these observations. We explored all available overpasses (~3250) of the Ocean and Land Colour Instrument (OLCI) on Sentinel-3 within the ice-free periods (May – October) for four years (2018 to 2021) to develop a new retrieval scheme to derive concentrations of DOC. OLCI observations with a spatial resolution of ~300 m were corrected for atmospheric effects using the Polymer algorithm. The results of this study show that using this new retrieval, remotely sensed DOC concentrations agree well with in situ DOC concentrations (MAPD=10.89%, RMSE=1.55 mg L−1, r²=0.92, n=489). The high revisit frequency and wide swath of OLCI allow it to capture the entire range of DOC concentrations and their seasonal variability. Estimated satellite-derived DOC export fluxes integrated over the ice-free periods of 2018 to 2021 show a high interannual variability and agree well with flux estimates from in situ data (RMSD=0.186 Tg C, MAPD=4.05%). In addition, 10-day OLCI composites covering the entire Lena River catchment revealed increasing DOC concentration and local sources of DOC along the Lena from south to north. We conclude that moderate resolution satellite imagers such as OLCI are very capable of observing DOC concentrations in large/wide rivers such as the Lena River despite the relatively coarse spatial resolution. The global coverage of remote sensing offers the expansion to more rivers in order to improve our understanding of the land-ocean carbon fluxes in a changing climate