Thomas Ohde

Validation of MERIS Level-2 products in the Baltic Sea, the Namibian coastal area and the Atlantic Ocean

Measurements of water‐leaving reflectance and optically active water constituents in the Baltic Sea, the Namibian coastal area and the Atlantic Ocean were performed to validate MERIS Level‐2 products. Two different validation methods were applied with the recommended match‐up analysis based on comparison of in situ measurements with MERIS Level‐2 products of simultaneous Envisat overpasses and the second analysis on comparison of natural variation ranges with derived MERIS product ranges. The MERIS water‐leaving reflectances correlate to in situ shapes and magnitudes in all investigated areas. The comparison shows mean deviations of MERIS reflectances between 23% and 26% for the current official instrument processing facility (IPF 4.07) but only 20% for the prototype version 7.0 up to now only available to the validation team. The comparison of water constituents shows the underestimation of MERIS ALGAL_2 index, YSBPA and TSM in turbid waters of the Baltic Sea of the current instrument processing facility (IPF 4.07). The mean MERIS ALGAL_1 index is close to in situ values but the mean MERIS ALGAL_2 index, the MERIS YSBPA and MERIS TSM products are still too low in Namibian coastal areas for the prototype version 7.0. The investigations demonstrate the quality improvement of all MERIS Level‐2 products from the current official instrument processing facility to the prototype version 7.0.

Ocean colour remote sensing relevant water constituents and optical properties of the Baltic Sea

Inherent and apparent optical properties have been investigated in coastal regions and in the open Baltic Sea for more than 20 years. The area of investigation included the open parts of the central and western Baltic Sea and the Pomeranian Bight, with the largest fresh water inflow into the western Baltic via the Oder river. The main focus of the research was the seasonal and regional variation in the concentration and composition of optically active water constituents and their inherent optical properties and the influence on the apparent optical properties like spectral reflectance and the vertical attenuation coefficient. This publication combines results of earlier studies and new investigations on the absorption of yellow substances and the volume‐scattering function. A volume‐scattering phase function, an important variable in modelling of radiative transfer in the water, was derived for the Baltic Sea. The best correlation was found in a two‐term Henyey–Greenstein function, in contrast to the Petzold phase function. The optical measurements were already used to modify reflectance models, to validate Coastal Zone Color Scanner (CZCS), MOS (Modular Optical Scanner) and Sea‐viewing Wide Field of view Sensor (SeaWiFS) data, and to derive ground data algorithms. They will be important for the future validation and adaptation of Medium Resolution Imaging Spectrometer (MERIS) data evaluation procedures on the conditions of the Baltic Sea.

Impacts of Saharan dust and clouds on photosynthetically available radiation in the area off Northwest Africa

ABSTRACT Radiation measurements in the area off Northwest Africa (research cruise, February 2008) and at Mindelo (Cape Verde Islands, May 2009) were used to investigate the impacts of Saharan dust and clouds on solar irradiance and on photosynthetically available radiation, to derive a relationship between dust aerosol optical depth (AOD) and photosynthetically available radiation and to determine the annual variations of photosynthetically available radiation. Three different kinds of atmospheric conditions were considered: cloudy skies, which decreased or increased the irradiance and dusty skies. The reduction by clouds was up to 67.2% at 400 nm and up to 84.4% at 700 nm. Enhancements of up to 21.9% at 400 nm and 34.0% at 700 nm were observed. The decrease by dust was up to 19.7% at 400 nm and up to 4.1% at 700 nm. Clouds decreased or increased the photosynthetically available radiation by up to 79.9% or up to 31.2%. The reduction by dust depended on the dust AOD and was between 3.6% and 12.3%. A linear relationship confirmed a decrease of photosynthetically available radiation of 1.2% by an increase of dust AOD of 0.1.

Interannual variability of sulphur plumes off the Namibian coast

Coastal sulphur plumes in the upper water layer are frequently observed off the Namibian coast. However, their temporal and spatial development, strength, size and impact on marine life differ at a wide range. This study compares the events of years 2007/2008 to the years 2004/2005 along the Namibian coast, including regional features on the basis of remote sensing satellite data, in situ measurements of a mooring and local observations. The remotely sensed derived intensity of coastal sulphur plumes of year 2008 was very weak compared to 2004 and especially to 2005, the year with the highest activity during the study period. In 2008, the overall maximum of sulphur plumes was observed in February, about 2 months earlier than in 2004 and 2005 when highest intensity was detected in April. Locally, differences in timing and strength of sulphur plumes occurred in the area of the Walvis Bay. There, the maximum intensity was observed in March 2004 and in April 2005, but in 2008 no clear maximum was found. The remotely sensed identified coastal sulphur plumes correlate with periods of low oxygen concentrations in bottom water and high fraction of South Atlantic central water (SACW) at a mooring 20 nautical miles off the central Namibian coast. An exception is the area in the vicinity of the Walvis Bay lagoon that seems to be driven by different local dynamics. The total variability of the sulphur plumes could not be explained by the observed oceanographic conditions alone. Additionally, microbiological and chemical processes in the sediment and the bottom water layer should be included in future investigations related to annual and interannual variability. Disastrous local events were observed in the year 2008 although the averaged intensity was low compared to the years 2004/2005. Therefore, the remotely sensed derived averaged intensity gives probably no real impression of the local impact of sulphur plumes on marine life. A combination of all available methods like the detection by local observations, in situ measurements and remote sensing approaches is required.

Investigation of hydrogen sulphide eruptions along the Namibian coastline using different remote sensing systems

Hydrogen sulphide eruptions with their typical turquoise discolorations at the water surface are a unique phenomenon along the Namibian coastline. The remote sensing techniques of ocean colour sensors and microwave scatterometers were used for the investigation of such events. The studies with ocean colour sensors showed that the turquoise discolorations near the Namibian coast were neither linked to dust deposition into the water column by desert storms nor to the reflection of bright material in shallow water areas. In addition, other coloured marine events like algae blooms and river outflows were differentiable from the hydrogen sulphide eruptions by their special optical properties. Quasi-true colour images and spectral identification methods were utilised to monitor and investigate the spatial and temporal distribution of sulphide events. In the past years, they were sometimes and locally limited discovered. Newest remote sensing observations including our own investigations have established that the occurrence of sulphide events is more frequent and longer lasting. The north-westerly direction of propagation and their velocity between 12 cm s-1 and 15 cm s1 were derived from an event on 14 April 2004. Lastly, the microwave scatterometer remote sensing was applied to investigate the relation of sulphide events to oceanographic conditions. The events from May 2004 were clearly related to strong coastal upwelling.

Remote sensing of the El Hierro submarine volcanic eruption plume

Submarine volcanic eruptions took place at the island El Hierro (Canary Islands) between October 2011 and March 2012. The event produced plumes of discoloured waters due to the discharge of volcanic matter, magmatic gases, and hydrothermal fluids. The expelled materials, which behaved like oceanic tracers, were detected from the site of the volcano to the open sea by remote-sensing techniques using different level-2 (L2) products of the Moderate Resolution Imaging Spectroradiometer (MODIS) sensor. In order to assess the effect of the anomalous turbidity, three atmospheric correction schemes were evaluated: SeaWiFS Data Analysis System (SeaDAS) standard, near-infrared–shortwave-infrared (NIR-SWIR), and Management Unit of the North Sea Mathematical Models (MUMM). The comparison between them verified that the SeaDAS standard atmospheric correction was the most suitable. The downwelling diffuse attenuation coefficient at 490 nm (Kd(490)) was used as a measure of plume intensity in comparison with other submarine volcanic eruptions. The MODIS-derived Kd(490) values verified that the intensity of the El Hierro plume was moderate. Only in some specific situations did the values barely exceed 0.4 m–1. The remote-sensing reflectance (Rrs) was used for the characterization of the affected waters. The Rrs spectra also allowed a comparison with other volcanic and sulphide events reported in previous studies. Similarities were found, both with submarine volcanic eruptions in the southwestern Pacific and with sulphide events at the Namibian coast, in composition and properties of optically active water constituents. A classification schema based on Kd(490) values and Rrs ratios was developed and used in connection with MODIS red–green–blue (RGB) composites as well as surface current velocities from altimeter missions to investigate the spatio-temporal development of the volcanic plume. The spreading and transport of volcanic material observed at the ocean surface was caused by the predominant surface currents coupled with different mesoscale eddies. Discoloured waters were identified more than 200 km away from the eruption site. Field data from oceanographic surveys verified the high concentration of sulphur compounds in affected waters and confirmed the overestimation by the MODIS default algorithm of chlorophyll-a concentrations in the volcanic plume.

Remote Sensing Applications in Coastal Zone Management

Satellite data of both sea surface temperature and ocean colour provide the opportunity to study dynamic features and the distribution of water constituents synoptically. A number of available and future satellite sensors with different spatial, temporal and spectral resolutions are presented. They allow to monitor both water temperature and quality in special areas influenced by algal blooms (including cyanobacteria) or in river water containing high concentrations of chlorophyll, suspended matter or yellow substances that change the turbidity. The optical properties and water constituents are also related to associated variables such as dissolved and particle-bound nutrients as well as inorganic and organic pollutants, especially in river mouth areas.

The Upwelling Area Off Namibia, the Northern Part of the Benguela Current System

In the area off Namibia satellite remote sensing data of the visible and infrared spectral range were used to investigate the upwelling processes and the biological response. Satellite derived Sea Surface Temperature was applied to study upwelling processes in relation to the driving trade winds. The investigations were focussed on the intensity and horizontal extent, the temporal and spatial variability including inter-annual and climate scales. Ocean colour satellite data allow investigation of the response of surface water to the nutrient input into the euphotic zone by upwelling processes. Observations in the area revealed, that the phytoplankton development starts with species absorbing light in the visible spectral range (diatoms and dinoflagellates) and may end in persistent shallow surface filaments with light scattering algae blooms. These blooms of coccolithophores identified by different in situ methods changed the water colour by strong particle scattering to milky turquoise discolorations. Further milky turquoise discolorations were identified as sulphur plumes. Low wind periods support the degradation of organic matter and the development of hydrogen sulphide in the bottom layer. After the onset of the trade winds and the offshore transport of surface water hydrogen sulphide enriched waters reaches with the near-bottom counter current the coast. Upwelling transports the water to the surface where the hydrogen sulphide will be oxidised to elemental sulphur. An algorithm was developed to identify and separate the sulphur plumes from algae blooms and other features on the basis of highly spectrally resolved satellite data from the MERIS sensor. The algorithm identified only coastal plumes as sulphur and that offshore plumes are formed by coccolithophores. The sulphur season is the boreal spring starting in February and reaching the maximum in April.

Spectral effects of atmospheric dust and clouds on estimation of chlorophyll-a concentration from radiation measurements

Atmospheric dust and clouds modify the spectral distribution of the incident solar radiation. The influence of these spectral effects on the determination of chlorophyll-a concentration from the sea surface and remotely sensed radiation measurements made from ships, aircrafts or satellites was studied in the region off Northwest Africa in the Atlantic Ocean. The chlorophyll-a algorithms that are typically used in ocean colour community are based on ratios of reflectance values of different wavelengths. This fact, together with the wavelength dependent effect of atmospheric dust and clouds, causes uncertainties in the estimation of chlorophyll-a concentration. The most frequently used Morel, OC4v4 and OC3M chlorophyll-a algorithms were included in the investigation. The highest and the smallest uncertainties were found for the Morel and the OC3M algorithms, respectively. For the first time, sky conditions with dust and clouds together in the atmosphere were studied. Overestimations of the chlorophyll-a concentration of up to 8.8% were observed in the case of dusty skies without clouds. The concentration was also overestimated between 7.6% and 14.3% for skies with mixtures of dust and clouds increasing the solar irradiance. Underestimations of up to 24.1% and overestimations of up to 12.2% were found for skies with mixtures of dust and clouds decreasing the solar irradiance. Compensations of the influences of spectral effects were observed at special ratios of clouds to dust. Earlier results in relation to the impact of clouds on the estimation of chlorophyll-a concentration were verified. The spectral effects of cloudy skies cannot be neglected because it may result in an error of up to 40% of the estimated value.

Impacts of Saharan Dust on the Marine Environment in the Area off Northwest Africa

The main sources of nutrient supply in the area off Northwest Africa are upwelling and Saharan dust deposition. Their influence on phytoplankton development was evaluated by different methods. Statistical analyses showed that alongshore wind stress and induced upwelling were most significantly responsible for surface Chl-a variability mainly in winter and spring with delay of up to 16 days. Only 5 % of the Chl-a variability was related to the dust input. All strong Saharan dust storms between 2000 and 2008 were identified by remote sensed dust aerosol optical depth. Only some of the events caused a biological response expressed by an increase of surface chlorophyll-a. Time lags between dust deposition and enhancement of chlorophyll-a between 8 and 16 days were determined. The chlorophyll-a concentration was increased up to 2.4 mg m−3. Atmospheric dust modifies the amount and spectral distribution of the water light field. In the applied optical model the dust effect on incident solar radiation was parameterized by radiation measurements. The photosynthetically active radiation was reduced up to 19.3 % in the upper water column. The impacts of spectral effects on photosynthetically active radiation were different in oceanic and coastal regions. A compensation of the spectral effects at water depths of about 7 m was only observed in ocean regions because of different light attenuation in coastal areas. Saharan dust also impacts satellite derived ocean surface wind speeds. The influence depended mainly on the strength of dust storms, the microwave frequency and the wind speed. The impact was higher for stronger dust storms, higher frequencies and lower wind speeds.