seviri water turbidity proposal -

SEVIRI Water Turbidity

D1.1 User Requirements Document

Final Deliverable (after discussion at Key Point I teleconference)

Issue 1; Rev. 1 18 May 2015

Prepared by Kevin Ruddick (RBINS) Quinten Vanhellemont (RBINS) Contributions from all project partners

2 SEVIRI Water Turbidity

Table of Content 3.1 OCEAN COLOUR REMOTE SENSING APPLICATIONS AND USERS .............................................................................. 8 3.2 METEOSAT SECOND GENERATION (MSG) SPINNING ENHANCED VISIBLE AND INFRARED IMAGER (SEVIRI) ................. 8 3.3 WATER PARAMETERS THAT CAN BE DERIVED FROM SEVIRI DATA ....................................................................... 11 3.4 ADVANTAGES AND LIMITATIONS OF SEVIRI WATER PRODUCTS .......................................................................... 11 3.5 METHODOLOGY FOR COMPILATION OF USER REQUIREMENTS ............................................................................ 18 4.1 MONITORING OF EUROPEAN COASTAL WATER QUALITY AND THE MSFD ............................................................. 18 4.2 MONITORING OF EUROPEAN LAKES AND THE WATER FRAMEWORK DIRECTIVE ..................................................... 19 4.3 MONITORING OF AFRICAN COASTAL WATER QUALITY ....................................................................................... 21 4.4 WATER QUALITY OF AFRICAN LAKES ............................................................................................................. 22 4.5 SEDIMENT TRANSPORT .............................................................................................................................. 24 4.6 ECOSYSTEM MODELLING (EUTROPHICATION) ................................................................................................. 24 4.7 OFFSHORE DIVING OPERATIONS ................................................................................................................... 26 4.8 CARBON BURIAL BY COCCOLITHOPHORES ....................................................................................................... 27 4.9 SUPPORT FOR VALIDATION OF POLAR-ORBITING OCEAN COLOUR SATELLITE MISSIONS ............................................. 27 4.10 OTHER APPLICATIONS ................................................................................................................................ 27 5.1 SUMMARY OF PARAMETERS TO BE GENERATED FROM SEVIRI DATA .................................................................... 28 5.2 OTHER PARAMETERS NOT CONSIDERED ......................................................................................................... 29 5.3 PRODUCT ACCURACY ................................................................................................................................. 29 5.4 LATENCY OF DATA: HISTORICAL OR NEAR REAL TIME ........................................................................................ 30 6.1 SUMMARY OF APPLICATIONS AND USER REQUIREMENTS .................................................................................. 30 6.2 LIMITATIONS OF THE SEVIRI POTENTIAL FOR OCEAN COLOUR PRODUCTS ............................................................. 31 6.3 IMPLICATIONS OF THE USER REQUIREMENT STUDY FOR THE PROJECT .................................................................. 31

6.3.1 New products .................................................................................................................................. 31 6.3.2 Motivation to improve signal:noise and lower detection limit........................................................ 31 6.3.3 Motivation to improve spatial resolution by use of HRV band ........................................................ 32

6.4 OTHER RECOMMENDATIONS – EVALUATION OF PRODUCTS BY A FEW USERS ......................................................... 32 6.5 MEDIUM AND LONG TERM VISION ............................................................................................................... 32


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6 SEVIRI Scope of this Document

1 Scope of this Document This report is the User Requirements Document (URD) of the EUMETSAT-funded SEVIRI-WT project and corresponds to deliverable D1.1 from Task A of the Statement of Work (EUM/TSS/SOW/14/762098). The SEVIRI-WT project will implement and validate a pre-operational processor to generate historical and near real-time Meteosat Second generation (MSG) Spinning Enhanced Visible and Infrared Imager (SEVIRI) ocean colour products, such as water turbidity and light attenuation every 15 minutes for Europe, Africa and the Atlantic Ocean as well as some of the Middle East and Brazil. This document summarises the User Requirements for this processors and ensure that the most appropriate products will be developed for the benefit of the users. The intended readership of this document is:

• EUMETSAT managers needing to understand the relevance of the SEVIRI-WT development for environment and climate monitoring

• EUMETSAT engineers needing to understand what information the SEVIRI-WT development team may need, e.g. with regards to sensor performance (noise, calibration, etc.) and/or IT processing environment

• The SEVIRI-WT project team needing to design algorithms to satisfy user requirements and to design a processor implementing such algorithms

• Potential users of data from the SEVIRI-WT development, e.g. value-adding satellite remote sensing organisations and projects (including the Copernicus Marine Core Service and the Ocean Colour Carbon Change Initiative) or end-users themselves

2 Change Log

Version Date Short Description Author

1.0 draft 20.4.2015 First draft for discussion at Key Point I Teleconference

Kevin Ruddick Quinten Vanhellemont Seviri-WT Team

1.0 draft rev 1

18.5.2015 Updates following Key Point 1 Teleconference of 27.4.2015 including: • Figures 2 and 3 inserted to show spatial

resolution over full disk. • Figures 5, 6, 7 and 8 inserted (replacing old

Figure 3) showing more clearly the superimposition of the viewing zenith angles on the Rrs climatologies and showing the current detection limit.

• Specific comment on spatial and temporal resolution in each section of Chapter 4.

RBINS, BC

7 SEVIRI Change Log

• Refinement of Table 2 and further comment on spatial resolution “requirement” in section 5.1.

• Mention of NetCDF data format in new section 5.5.

• Strengthened requirement for NRT data for ecosystem model forecast (user feedback received after Key Point 1 Teleconference).

• Other minor updates • Updated section 4.3 with results from GEO

WQ Summit • Updated section 4.4 on floating vegetation • Updated section 4.7 with DAN Divers Alert

Network • Updated section 5.3 with accuracy

assessment scheme

1.0 19.5.2015 Final deliverable for EUMETSAT

8 SEVIRI Introduction

3 Introduction 3.1 Ocean Colour remote sensing applications and users There is an established user need for a range of water quality products from ocean colour sensors, see e.g. MARCOAST and GMES PURE User Requirements [Albert et al, 2014; Brockmann et al, 2008; Ruddick et al 2008a, 2008b], CoBiOS User Requirements [Kaas and Perters, 2012], or Freshmon User Requirements [Stelzer, Koponen, and Heege, 2011]. These user needs, especially those relating to the national obligations of EU Member States to report on water quality under the Water Framework Directive and the Marine Strategy Framework Directive, have guided much of the development of ocean colour products over the last 10 years and have been largely responsible for motivating the funding of the Sentinel-3 satellite by the European Union as part of the Copernicus Service Space Segment. In the context of the SEVIRI-WT project we focus on the subset of the complete ocean colour remote sensing user needs, which is within the capability of the SEVIRI sensor taking account especially of its reduced spectral and spatial resolutions. An idea of these capabilities can be obtained from the various precursor studies to process and exploit SEVIRI data for “ocean colour”1 applications.

3.2 Meteosat Second Generation (MSG) Spinning Enhanced Visible and Infrared Imager (SEVIRI)

Meteosat Second Generation (MSG) is a series of meteorological satellites in geostationary orbit providing data continuously every 15 minutes since 2004 for Europe, Africa and the Atlantic Ocean as well as some of the Middle East and Brazil – see Figure 1. These satellites are designed to be continuously operational and have a backup satellite in space and further units on the ground so that damage to any single satellite does not interrupt the data stream. More information on the MSG series can be found in [Schmetz et al, 2002]. The MSG satellites are platform for the Spinning Enhanced Visible and Infrared Imager (SEVIRI) sensor which has 3 “narrow” solar-reflective spectral bands (1 Visible, 1 Near Infrared, 1 Short Wave Infrared), and one broadband Visible/Near Infrared “High Resolution Visible” (HRV) band given in Table 1, as well as 8 Thermal Infrared bands used for cloud detection, cloud properties and Land and Sea Surface Temperature measurement. More information on SEVIRI can be found in [Aminou, 2002]. Information on the HRV band in particular can be found in [Deneke, 2010].

Spectral Band Central Wavelength Wavelength range Spatial Resolution

(nadir)

VIS0.6 635nm 0.56-0.71µm 3km*3km NIR0.8 810nm 0.74 – 0.88µm 3km*3km SWIR1.6 1640nm 1.50 – 1.78µm 3km*3km HRV 750nm 0.37 – 1.25µm 1 km*1 km

Table 1: Solar-reflective Spectral bands of SEVIRI. Data from [Govaerts and Clerici, 2004].

1 The term « ocean colour » is used here very loosely. Normally “ocean colour” would refer to radiometric data with at least five visible and two near infrared bands [IOCCG, 1998] and would imply stringent signal:noise requirements on the sensor. SEVIRI has only one visible band and one near infrared band (in addition to the very wide visible/near infrared “High Resolution Visible” band) and is much noisier than any ocean colour sensor. However, the term is useful here because the SEVIRI-WT project will generate products that are very similar in nature to a subset of products typically retrieved from true ocean colour missions.


The spatial resolution of SEVIRI depends on viewing zenith angle (as with polar-orbiting sensors) and hence on distance from the sub-satellite ground point, thus on latitude and longitude – see Figure 2 and Figure 3. This degradation of spatial resolution is particularly severe for viewing zenith angles exceeding 60°. The difficulties in performing an accurate correction for atmospheric scattering and for reflection/transmission at the air-sea interface also limit the applicability of SEVIRI data for high latitudes [Ruddick et al, 2014], although the degradation of spatial resolution will probably be the most severe limitations for users.

Figure 1: Area visible from SEVIRI at (0°N, 0°E). The SEVIRI-WT processor will provide data up to view zenith angle of ~62° (to be clarified in Task B. Scientific Development).


Figure 2: Spatial resolution of the SEVIRI VIS0.6 band over the full disk for a (0°N, 0°E) orbit, shown as the length of the pixel diagonal, assuming rectangular pixels. Pixels are stretched in the viewing azimuth direction and with stretch amplitude determined by the viewing zenith angle (shown in Figure 1). Thus for the extreme cases pixels along longitude 0° are stretched North-South and pixels along the equator are stretched East-West. Reproduced from [Vanhellemont et al, 2013].


Figure 3: Spatial resolution of the SEVIRI VIS0.6 band shown (top left) West-East pixel length as function of longitude, (top right) South-North pixel length as function of latitude, (bottom) pixel diagonal as function of viewing zenith angle. The colour on these graphs is function of the number of pixels in the full disk. Bottom graph reproduced from [Vanhellemont et al, 2013].

3.3 Water parameters that can be derived from SEVIRI data Although SEVIRI lacks the spectral and radiometric resolution necessary for some ocean colour applications, it has been demonstrated that many useful marine products can be derived from the SEVIRI optical bands, particularly in turbid waters [Neukermans et al, 2009; Neukermans et al, 2012; Vanhellemont et al, 2013]. For example data can be provided for Total Suspended Matter (TSM), Turbidity, euphotic depth, diffuse attenuation of Photosynthetically Available Radiation (KPAR), coccolithophore blooms, etc.

3.4 Advantages and Limitations of SEVIRI water products SEVIRI has the great advantage over conventional polar-orbiting ocean colour sensors (MODIS, VIIRS, MERIS … OLCI) of providing high frequency data, every 15 minutes instead of about once per day. Thus, it becomes possible to both measure fast-varying processes such as tidal variation of suspended sediments (Figure 4) and to acquire data on days with scattered clouds where a single look per day is insufficient. For example, taking account of cloudiness for the Southern North Sea


SEVIRI will provide data about 200 days/year, compared to about 100days/year for MODIS [Ruddick et al, 2014].

Figure 4: Half-hourly subset of SEVIRI images on 2014-09-08 showing tidal variability of turbidity in the Southern North Sea and changes in cloud cover. Processed as [Neukermans et al, 2012; Vanhellemont et al, 2014]. The limitations of the SEVIRI data, compared with other sources of ocean colour data (MODIS, MERIS, VIIRS … OLCI), are the limited spatial resolution, the lack of spectral bands for measurement of chlorophyll a concentration, and the lower sensitivity: Spatial resolution is 3km at nadir (0°N, 0°E) and increases with latitude/longitude from nadir. The SEVIRI-WT project may investigate ways to improve this spatial resolution by a factor 3 (1km at nadir) if there is sufficient demand from this User Survey. In the visible SEVIRI measures only red reflectance and cannot easily distinguish between mineral and algal particles. Thus the SEVIRI-WT project promises to develop only products such as Total Suspended Matter (TSM), Turbidity, coccolithophore blooms [Vanhellemont et al, 2013], and (in turbid waters only) euphotic depth and diffuse attenuation of Photosynthetically Available Radiation (KPAR) [Neukermans et al, 2012]. User Requirements for other products, such as chlorophyll a, are still welcome and may help guide future developments. SEVIRI was designed for meteorological applications such as cloud detection and does not have the sensitivity necessary for the clearest oceanic waters. Due to this low sensitivity there is a detection limit of ~1.0 FNU (Formazine Nephelometric Units) or ~1.0 mg/m3 suspended particulate matter concentration or about 0.0013 for Rrs at .64µm. The SEVIRI-WT project will clarify more precisely this detection limit and, if required by users, will develop methods to push this detection lower for clearer waters. An idea of the regions of turbid water where SEVIRI will be most useful can be achieved from Figure 5. These areas include: the Southern and Central North Sea, Irish Sea, English Channel, French Atlantic Coast, Northern Adriatic, Eastern Tunisian Coast, Nile outflow, Western Black Sea, Mauritania and Sierra Leone/Liberian coasts, Lake Victoria and other large African Rift valley lakes, possibly sections of the Mozambique and North-West Madagascar coasts, possibly the Benguela


upwelling area (Western coast of South Africa and Namibia), and the Amazon outflow area (French Guyana, Surinam) and possibly the coast of Uruguay. The same data is shown in Figure 6 and Figure 8 after masking with a detection limit of Rrs=0.0013. Clearly lowering of this detection limit would greatly enhance the applicability of SEVIRI for aquatic applications and this shall therefore be a priority for further development in this project.


Figure 5: MODIS-AQUA monthly average Rrs 645nm for June 2014. Data downloaded from http:///oceancolor.gsfc.nasa.gov in April 2015. Contours for viewing zenith angles of 60° (solid), 70° and 80° (both dashed) are superimposed.

http://oceancolor.gsfc.nasa.gov


Figure 6: MODIS-AQUA monthly average Rrs 645nm for June 2014 as Figure 5 but with grey masking of areas below detection limit of Rrs645=0.0013. Data downloaded from http:///oceancolor.gsfc.nasa.gov in April 2015. Contours for viewing zenith angles of 60° (solid), 70° and 80° (both dashed) are superimposed.



Figure 7: MODIS-AQUA monthly average Rrs 645nm for December 2014. Data downloaded from http:///oceancolor.gsfc.nasa.gov in April 2015. Contours for viewing zenith angles of 60° (solid), 70° and 80° (both dashed) are superimposed.



Figure 8: MODIS-AQUA monthly average Rrs 645nm for December 2014 as Figure 7 but with grey masking of areas below detection limit of Rrs645=0.0013. Data downloaded from http:///oceancolor.gsfc.nasa.gov in April 2015. Contours for viewing zenith angles of 60° (solid), 70° and 80° (both dashed) are superimposed.


18 SEVIRI Applications

Merging of high temporal resolution SEVIRI data with the high spatial resolution data of polar-orbiting ocean colour sensors and/or with in situ measurements and sediment transport and/or ecosystem models can combine the benefits of all sources of data. This is outside the scope of the SEVIRI-WT project but could be achieved by suitable post processing of the SEVIRI-WT output.

3.5 Methodology for compilation of User Requirements The following Chapters integrate information from:

• existing sources e.g. those of recent satellite remote sensing projects such as GMES-PURE (including the associated User Requirements Database), GMES-Downstream/MARCOAST, FP7/COBIOS, FP7/HIGHROC

• prior requests to RBINS for SEVIRI products and • responses to a User Questionnaire sent by email to a few potential users.

Information is still pending from some sources (Ocean Colour CCI, the new Copernicus Marine Service, EARSC, EUROGOOS, etc.) and will be added in a subsequent revision of this deliverable.

4 Applications 4.1 Monitoring of European coastal water quality and the MSFD The requirements for monitoring of coastal and marine water quality in Europe are driven by the European Union (EU) Water Framework Directive (WFD) [European Commission, 2000] and Marine Strategy Framework Directive (MSFD) [European Commission, 2008]. This section will concentrate on the MSFD requirements of relevance to the SEVIRI-WT project. The WFD focus on inland waters and waters within 1 nautical mile of the coast will be impossible to satisfy with the coarse SEVIRI spatial resolution except for the largest of inland waters, which are considered in section 4.2. The main goal of the MSFD is to achieve Good Environmental Status of EU marine waters by 2020. The Directive defines Good Environmental Status (GES) as: “The environmental status of marine waters where these provide ecologically diverse and dynamic oceans and seas which are clean, healthy and productive” [European Commission, 2008, Article 3]. The Directive defines GES via 11 qualitative descriptors of which the most relevant to the SEVIRI-WT project are:

• Descriptor 5: Eutrophication: “Human-induced eutrophication is minimised, especially adverse effects thereof, such as losses in biodiversity, ecosystem degradation, harmful algae blooms and oxygen deficiency in bottom waters” Eutrophication is a process driven by the enrichment of water by nutrients, especially compounds of nitrogen and/or phosphorus, leading to: increased growth, primary production and biomass of algae; changes in the balance of organisms; and water quality degradation.

• Descriptor 7: Hydrographical Conditions: “Permanent alteration of hydrographical conditions does not adversely affect marine ecosystems” Hydrographical conditions are characterized by the physical parameters of seawater: temperature, salinity, depth, currents, waves, turbulence, and turbidity (related to the load of suspended particulate matter).

• Descriptor 1: Biodiversity: “The quality and occurrence of habitats and the distribution and abundance of species are in line with prevailing physiographic, geographic and climatic conditions.”


This overarching descriptor is impacted by aspects relating to the other descriptors including 5 and 7.

The EU Member States have defined, for each descriptor, “criteria” and “indicators” to make the descriptors more concrete and quantifiable, along with a coordinated monitoring programme for the ongoing assessment of GES. These indicators cover many biological, chemical and physical parameters of which (only) a few are measurable by optical remote sensing, because of the optical properties of algal and non-algal particles suspended in the water. These optically-active parameters can be measured from satellites with spatial coverage far surpassing what is possible with shipborne or moored instruments. Based on prior indicators from the Oslo and Paris Commission for the Prevention of Marine Pollution (OSPAR) and from EU activities to harmonise indicators for the WFD, Descriptor 5 includes as indicator 5.2.1 the 90 percentile of Chlorophyll a concentration over the algae growing season and, in some regions, as indicator 5.2.2 the water transparency related to increase in suspended algae [Sanden & Håkansson, 1996]. The latter is still often measured via the Secchi depth. These eutrophication indicators are included here for completeness and for future reference in the context of the forthcoming Meteosat Third Generation (MTG) satellite which will embark the Flexible Combined Imager (FCI) having blue (0.44µm) and green (0.51µm) spectral bands and hence enhanced performance compared to SEVIRI. However, it is suspected a priori that SEVIRI data is unlikely to contribute significantly to eutrophication monitoring except perhaps in the most extreme high biomass algal blooms. More relevant to the SEVIRI-WT project is the MSFD descriptor 7 for which turbidity or suspended particulate matter concentration are included in the typical indicators used to determine the spatial extent of areas and habitats affected by permanent alteration of hydrographical conditions. Indeed turbidity is specifically listed in the MSFD Annex III Indicative List of Characteristics for which monitoring is required. For MSFD applications the user is generally a water quality manager representing the national government of an EU Member State and typically editing or contributing to or following procedures defined in documents such as [Belgische Staat, 2012; DEFRA, 2014; BLMP, 2011]. The spatial resolution required for coastal WFD and MSFD applications is as high as possible, although 1km resolution would already be a significant improvement on shipborne monitoring methods. 5km resolution will still allow monitoring of offshore areas, but will be insufficient for coastal waters. The temporal resolution required for coastal WFD and MSFD applications depends on the time scale of the relevant processes. For chlorophyll a, monthly sampling is generally necessary to ensure coverage of seasonal variability. For turbidity and suspended matter tidal variability is important in many areas – monthly sampling will then be highly aliased and daily or even hourly sampling may be necessary. Data is ideally required over many years, or even decades if trends are to be identified. Near real time is not generally required for WFD/MSFD reporting applications.

4.2 Monitoring of European lakes and the Water Framework Directive The EU Water Framework Directive (WFD) sets the requirements for monitoring of inland lakes as well as rivers, transitional waters, nearshore waters and groundwater bodies in Europe. In the SEVIRI-WT project we focus on large inland lakes that fall within the spectral resolution of the sensor. According to the size typology given in Annex II of the WFD, all lakes greater than 0.5km2 in surface area fall under the requirements of the Directive and might need to be included in the water status assessment and monitoring. Of the various elements that should be monitored according to the WFD, those of relevance to the SEVIRI-WT project are:

• Phytoplankton, where for High Environmental Status planktonic blooms should occur at a frequency and intensity consistent with type-specific physico-chemical conditions and


average phytoplankton abundance should be consistent with those conditions and should not alter the type-specific transparency conditions. For Good Environmental Status only slight changes in the composition and abundance of planktonic taxa and only slight increase in the frequency and intensity of type-specific planktonic blooms may occur [WFD, Annex V, 1.2.2]

• General hydromorphological conditions, where for Good Environmental Status transparency (and other parameters not accessible by remote sensing) “must not reach levels outside the range established so as to ensure the functioning of the ecosystem and the achievement of […] the biological quality elements”.

The WFD defines concepts of “surveillance monitoring” at least for a year for parameters indicative of all quality elements, “operational monitoring” for those quality elements most sensitive to human pressures and “investigative monitoring” for cases where specific water quality problems are encountered or suspected (exceedance of environmental objectives, accidental pollution, etc.). The frequency of monitoring is left largely to Member States according to the conditions and variability within their own waters, although the general principle is that the frequency of monitoring should allow a reliable assessment of the status of all water bodies. In practice monthly monitoring is typical to ensure reasonable coverage of the seasonal variability of phytoplankton and submerged vegetation. The spatial coverage of monitoring is not specified in the WFD. In general, there is a severe mismatch in the natural spatial variability of ecosystem parameters within a lake and the ability of monitoring organisations to cover (or even understand) this spatial variability using in situ measurement techniques. A typical practical approach is to make in situ measurements at a single location, although it is clear that is many situations this will be inadequate. Remote sensing techniques obviously have a major advantage in their ability to make measurements at many locations simultaneously and hence take account of any spatial variability, at least down to scales that correspond to the spatial resolution of the sensor. In the SEVIRI-WT project the relevant parameters for monitoring of European lakes are turbidity and/or transparency, often measured in situ via the Secchi depth e.g. [Ekholm and MIttika, 2006]. Visibility of the bottom and/or presence of bottom vegetation may also be relevant although such parameters have not yet been considered for SEVIRI. Massive blooms of cyanobacteria are also relevant for WFD monitoring (and recreational use of lakes), but no related parameters have yet been considered for SEVIRI. An obvious limitation of SEVIRI for inland water applications is the limited spatial resolution. Only the very largest European lakes will be measurable – see Figure 9. Even these will be perhaps too small for SEVIRI, especially taking into account the degradation of North-South spatial resolution at this latitude and the problems of mixed pixels and adjacency pixels. A second limitation in the European context is the high viewing zenith angle for higher European latitudes, which would effectively rule out monitoring of lakes in Sweden, Finland and Eastern Europe. The most promising European lake targets for the SEVIRI-WT project are thus Lake Balaton (Max. length 77km, Max. width 14km), Lake Geneva (73km, 14 km) and the Bodensee/Lake Constance (63 km, 14 km) [Wikipedia, consulted April 2015]. All three of these lakes are still quite small compared to the SEVIRI spatial resolution, indicating an enhanced interest for the SEVIRI HRV band. The spatial resolution required for European lake applications is as high as possible, preferably 100m although 1km resolution would already be a useful for some lakes. The temporal resolution required for European lake applications is typically weekly-monthly for WFD type monitoring although higher resolution, effectively (i.e. cloud-free) daily, is needed for harmful algae blooms, particularly cyanobacteria. Data is ideally required over many years, or even decades if trends are to be identified. Near real time is not generally required for lake monitoring applications, except if harmful algae bloom detection is needed.


Figure 9: European lakes. Reproduced from http://www.eea.europa.eu/themes/water/european-waters/lakes/lakes. The main potential targets of interest in this project (considering the viewing zenoth angle constraint) are: Lake Balaton (19), Lac Léman (20) and Bodensee/Lake Constance (21) and the Djerdab Reservoir (Red 19) and the Ijsselmeer (Red 9).

4.3 Monitoring of African coastal water quality Little information has been obtained regarding national monitoring programmes for African coastal water quality. The continent lacks the harmonised legislative framework provided by the WFD and MSFD in Europe. The User Requirements survey provided an expression of some needs, including tracking of coastal harmful algal blooms as well as needs, which will be detailed in later sections, to understand sediment transport (section 4.5), to have information on underwater visibility for maritime operations (section 4.7) and, more generally, better information on high frequency physical/biological interactions to supplement information from polar-orbiting sensors. In April 2015 the GEO Water Quality Summit took place in Geneva2. Participating from African countries was low, only 3 of the 52 participants came from the African continent, representing South Africa, Madagascar and Ghana. On the other hand there were many representatives from developed countries as well as UN organisations (UNEP, UNESCO, WMO, WHO). The summit discussed as one of the key advances that should be achieved for the next summit is to increase the active participation of less developed counties, specifically African countries, in using Earth Observation for monitoring

2 see http://www.geo-water-quality.org/geo-water-quality-summit

http://www.eea.europa.eu/themes/water/european-waters/lakes/lakes

http://www.eea.europa.eu/themes/water/european-waters/lakes/lakes


water quality. The discussion pointed out that, for those countries, availability of drinkable water (i.e. water quantity and quality in the sense of contamination) is the key issue, as opposed to Europe or US, where a good ecological state is the goal. The SEVIRI-WT project could provide support to the South African national policy initiative Operation Phakisa and to a future Regional Implementation Centre for GMES-Africa Marine Services, the latter serving as a relevant distribution mechanism within Africa. Further examples of coastal processes associated with water quality in North Africa include possible anthropogenic modifications of turbidity in the Gulf of Gabes in Tunisia [Katlane et al, 2013] and the outflow of the Nile Delta into the Mediterranean, which is clearly visible in the MODIS Rrs645nm data. The spatial resolution required for African coastal water quality applications is as high as possible, although 1km or even 5km resolution would already be useful in many regions. The temporal resolution required for African coastal water quality applications is typically weekly-monthly although higher resolution, effectively (i.e. cloud-free) daily, is needed for harmful algae blooms. Data is ideally required over many years, or even decades if trends are to be identified. Near real time is not generally required, except for harmful algae bloom detection.

4.4 Water quality of African lakes In the African context, environmental legislation and monitoring is generally less developed and harmonised than in Europe, where the Water Framework Directive has both imposed legal obligations on Member States and has led to some harmonisation of monitoring programmes. It is therefore generally more difficult to identify users and user requirements, although, as a corollary the environmental problems may be more acute, affecting potentially human health and either affecting or affected by food supply (fisheries, aquaculture). International interest in preservation of biodiversity in Africa is also significant since the Rio Summit of 1992 and there may be a complicated range of socio-economic, political and environmental interests to be considered in the management of African lakes. This management may be supported by a better scientific understanding of limnological conditions (habitat) and cause-effect relationships between the physical-chemical environment and the various species. For African inland waters the obvious first targets for the SEVIRI-WT project would be the largest lakes, namely Lake Victoria/Nam Lolwe/Nalubaale (max. length 337km, max. width 250km), Lake Tanganyika (max. length 673 km, max. width 72 km) and Lake Lake Malawi/Nyassa (max. length 560 km, max. width 75 km). All three of these lakes are clearly visible at the SEVIRI spatial resolution, even without use of the HRV band. They also benefit from a favourable viewing zenith angle because they are closer to the satellite sub-point than European lakes. With regards to known environmental problems which would motivate monitoring by SEVIRI, Lake Victoria is known to have undergone major anthropogenic change over the period 1980-2010 [Witte, 2013] including increased predation pressure due to the introduction of the Nile perch, and decreases in water transparency and dissolved oxygen due to eutrophication. These changes have led to a collapse of, e.g. haplochromine cichlids in the Manza Gulf in 1986-1990, with recovery of some species in the 1990s and 2000s. The effects of overfishing have also been debated but are considered [Witte, 2007] to have had potentially a negative impact on haplochromine cichlids locally but not to be a major driver for lake-wide loss of cichlid diversity. The bays of Lake Victoria have experienced a steady eutrophication throughout the past decades. One of the most obvious consequences of this eutrophication is the intensive proliferation of water hyacinths (Eichhorna crassipes) along their shores (see Figure 10), and in particular in the Winam Gulf (or Nyanza Gulf). Water hyacinths were first spotted in Lake Kyoga in May 1988, and arrived in the Winam Gulf by 1990 from the Kagera River [Albright et al., 2004; Gordon et al., 2009]. Due to the abundance of space, nutrients and illumination, their high reproduction rate and the lack of natural enemies, a massive accumulation in many beaches and bays resulted, most notably in the Winam


Gulf. The weed carpets have several negative socio-economic and environmental effects. They disrupt fishing activities, transport, irrigation and water purification, provide a breeding ground for carriers of human diseases, and affect biodiversity. Detection of floating vegetation, such as water hyacinths, from space is rather easy due its “land like” optical signal.

Figure 10: Mechanical removal of invasive water hyacinths on the Ugandan shore of Lake Victoria, 1998 (image courtesy of Conver BV, The Netherlands) [Witte, 2013] specifically uses Secchi depth measurements as a basis for understanding the impact of changes in water transparency on various species and concluded for haplochromine cichlids that “clearer water seems to support differentiation in feeding techniques as well as year-round spawning, and both may facilitate species coexistence”. In this context, water transparency affects the encounter rate for visual predators and hence feeding efficiency. For example, oral mollusc-shelling species such as Platytaeniodes degeni attack their prey by grasping the foot of the snail. This can only be achieved when light levels are high enough [Seehausen, 2003]. In addition to the eutrophication/transparency debate, floating vegetation (water hyacinths) cover some parts of Lake Victoria and can be seen from space. In the SEVIRI-WT project the relevant parameters for monitoring of African lakes are turbidity, and Secchi depth. There may also be an interest for detecting extreme high biomass or cyanobacteria blooms and/or scums and floating vegetation. As for European lakes, the spatial resolution of SEVIRI will limit the possible applications, although there are many larger lakes in Africa, especially considering the favourable viewing zenith angle, and, for example, features in Lake Victoria are clearly visible in SEVIRI imagery. The spatial resolution required for African lake applications is as high as possible, preferably 100m although 1km resolution would already be a useful for some lakes and even 5km resolution would give useful information for the largest lakes. The temporal resolution required for African lake applications is typically weekly-monthly although higher resolution, effectively (i.e. cloud-free) daily, is needed for harmful algae blooms, particularly cyanobacteria. Data is ideally required over many years, or even decades if trends are to be identified. Near real time is not generally required for lake monitoring applications, except if harmful algae bloom detection is needed.


4.5 Sediment Transport The transport of sediments in coastal waters is of major interest to coastal zone managers because of the importance of environmental and socio-economic impacts. Changes in bathymetry are critical for navigation in shallow waters and in many coastal zones it is necessary to perform continuous maintenance dredging at high cost to ensure sufficient draught for maritime traffic. Changes in coastlines and beaches arising from coastal erosion or sedimentation affect various human activities including tourism and recreation and aquaculture. The type and stability of bottom sediments is an important factor in the habitat of benthic organisms. Many marine pollutants are transported via suspended sediments. An understanding of sediment transport is needed to ensure that coastal zone management decisions, for example relating to dredging/dumping operations, beach nourishment or offshore construction, can be made optimally. Moreover, offshore constructions, e.g. of platforms, wind farms or even artificial islands, are generally associated with a mandatory Environmental Impact Assessment (EIA) which would typically include factors such as possible modification of bathymetry, habitat for marine organisms and turbidity (affecting primary production and underwater visibility for visual predators). The approach to sediment transport problems generally involves coupled hydrodynamical-sediment transport models, often associated with in situ measurements or, more recently, satellite data. [Fettwies et al, 2011, 2012], each method carrying its strengths and weaknesses. Sediment transport models have the advantage of excellent 4D spatio-temporal coverage and the ability to consider prospective scenarios, but the disadvantage of sensitivity to unknown parameters such as settling velocity of particles. In situ measurements have excellent temporal coverage and reliability (assuming sufficient maintenance) and the ability to measure many parameters, including particle size distribution and composition, but lack spatial coverage. Remote sensing measurements have excellent 2D spatial coverage and reasonable temporal coverage but lack information on vertical structure and, in particular, the near-bottom processes that are important in many sediment transport problems. Remote sensing data may typically be used for initialisation and/or validation of sediment transport models. In the SEVIRI-WT project the relevant parameters for supporting sediment transport applications are turbidity and/or suspended particulate matter concentration. SEVIRI products will typically be considered in conjunction with SPM data from polar-orbiting missions, providing the temporal resolution that is lacking in the latter, but at lower spatial resolution. In general the detection limit problem for SEVIRI-derived SPM products will be less critical for many sediment transport applications since it is in turbid waters that these applications are most important. The spatial resolution required for sediment transport applications is highly dependent on the specific application and can be very high. For example, sediment transport in the vicinity of offshore structures or ports may involve processes at scales of metres or tens of metres [Vanhellemont and Ruddick, 2014]. Sediment transport models have typical resolutions of 100m-10km with the coarser resolutions used for large scale transport, e.g. at the scale of the Southern North Sea. The temporal resolution required for sediment transport applications is typically hourly in regions of tidal variation and once-daily data from polar-orbiting sensors may be severely under-resolved. Data is ideally required over many years, or even decades if trends are to be identified or if used in support of long-term sediment transport model simulations.

4.6 Ecosystem Modelling (eutrophication) In coastal waters, eutrophication management has driven the use of ecosystem models simulating the response of phytoplankton communities to increased nutrient loads from rivers [Lenhart et al,


2010]. Starting in the 1980s in the framework of the OSPAR Eutrophication Strategy, more recently superseded by the EU WFD and MSFD, it was decided to identify and monitor Eutrophication Problem Areas and impose reductions on anthropogenic nutrient input to the sea in order to reduce or solve such problems and achieve “Good Environmental Status”. Ecosystem models have been developed to assist in this decision-making by forecasting the probable impact of different nutrient reduction scenarios, e.g. reduction of Nitrogen and/or Phosphorus by X% either individually or together. These models are driven by the light climate, typically represented by scalar quantum Photosynthetically Available Radiation (PAR: 400-700nm), which controls photosynthesis. This light climate depends on both above water downwelling light and on the vertical attenuation of light in the water column, which is typically represented in these models by the vertical attenuation coefficient of PAR (KdPAR). In deep oceanic waters, KdPAR is generally determined by phytoplankton alone and can be modelled as function of chlorophyll a concentration, which is available from the ecosystem model itself. In turbid waters or waters with significant river input and hence Coloured Dissolved Organic Matter (CDOM), KdPAR is a more complicated function of algal particles, non-algal particles (NAP) and CDOM. In turbid waters the non-algal particles are typically the dominant factor determining algal bloom timing and duration, and information on their concentration (or optical effect) is required at the spatial resolution of the ecosystem itself, typically between 1 and 10 km, and at a temporal resolution which ideally takes account of tidal variability. In general, this is not achieved by most ecosystem models, leaving unknown uncertainties. However, there has been a clear trend to improve progressively the representation of the effect non-algal particles in ecosystem models by the use of satellite data. For example, the Belgian eutrophication modelling work started with a box model of phytoplankton processes [Lancelot et al, 2005] with a single value for NAP for the whole region. Next a satellite-derived climatology for NAP was used with a value for each model cell and four values over the year to give a very crude seasonal cycle [Lacroix et al, 2007]. This was further improved by use of a satellite climatology using EOF-analysed satellite data to give input for NAP every day over a test period of 4 years [Sirjacobs et al, 2011]. The next step should be to add higher frequency variability of NAP to this model to better represent the significant tidal variations of NAP during the photoperiod. It has already been demonstrated by [Desmit et al, 2005] that the net effect of light on photosynthesis in an ecosystem model can be quite different if the high frequency tidal and diurnal processes are resolved rather than simply represented by daily-averaged values for PAR and KdPAR. In particular the timing of maximum light penetration (minimum KdPAR) associated with tidal resuspension/advection of SPM with respect to the timing of maximum above water light can be a critical factor not represented by models which do not adequately resolve both processes in time. It is an open question in the ecosystem modelling how to deal with the three separate components affecting KdPAR. In the model of [Lacroix et al, 2007] KdPAR is a function of a) NAP, obtained from satellite data input, b) CDOM, obtained via a local empirical relationship as a function of modelled salinity, and c) CHL, obtained from the ecosystem model variables themselves. This enables prospective scenarios to be carried out which integrate the feedback effect of different modelled CHL on the light climate. Other models may prefer to use simply satellite-derived KdPAR input which integrates the effect of all three components. In the SEVIRI-WT project the relevant parameters for supporting ecosystem (eutrophication) modelling applications are KdPAR or an equivalent parameter such as euphotic depth, and/or Non-algal particle (NAP concentration). For the support of eutrophication-oriented ecosystem models the spectral, radiometric and spatial limitations of SEVIRI are not so critical. Since these models generally simulate CHL themselves it is only terrigenic CDOM that may be missing from the model inputs, although in many cases CDOM can be adequately modelled in other ways, e.g. via a correlation with salinity. As regards the detection limit for SPM, it is in the most turbid waters where SEVIRI products will be best that the SPM input is


important. As regards spatial resolution, most ecosystem models have similar spatial resolution to the SEVIRI resolution. The main missing information for this application is during cloud-free periods, where the data from polar-orbiting satellites is even more critically affected. A good approach to dealing with the data gaps arising from clouds is to use intelligent interpolation methods, such as the EOF-based “DINEOF” method, applied previously to MODIS data to provide input data for ecosystem models [Sirjacobs et al, 2012] and recently tested for the geostationary SEVIRI data [Alvera-Azcárate et al, 2015]. The use of SEVIRI data for deep ocean ecosystem models, such as those dealing with the carbon budgets of the world’s major oceans, is not considered here. In such waters SEVIRI the spectral and radiometric limitations of SEVIRI are expected to be too severe. However, future satellite missions with improved capabilities could render this application feasible for geostationary sensors. The spatial resolution required for ecosystem model applications is usually determined by the model grid, typically 1-10km. The temporal resolution required for ecosystem model applications is typically hourly in regions of tidal variation and once-daily data from polar-orbiting sensors may be severely under-resolved. Data is ideally required over many years, or even decades for long-term ecosystem model simulations or studies of interannual variability. Most such simulations are made in hindcast mode, although there is a growing interest in near real time data for assimilation into short range forecast models.

4.7 Offshore diving operations In coastal waters with offshore constructions and/or associated Environmental Impact monitoring programmes, human divers are typically used either to assist in construction/maintenance or to monitor environmental impacts, e.g. as in the Belgian wind farm monitoring programme [Degraer et al, 2013]. These operations often involve day trips by boat with a go/no-go decision based on expected conditions of sea state, current and underwater visibility. The latter is generally based on the personal experience of divers, but could be improved with suitable model- and/or satellite-based information. The high frequency data from SEVIRI is particularly well-suited to this application because of the need to define optimal time windows for diving, typically lasting 1-3 hours because of tidal variability. This information could be of interest for DAN, the Divers Alert Network (http://www.diversalertnetwork.org). DAN helps divers in need of medical emergency assistance and promotes dive safety through research, education, products and services. In the SEVIRI-WT project the relevant parameter for supporting diving applications is horizontal visibility or a suitable proxy such as turbidity. The main limitation of SEVIRI for supporting diving applications is the lack of data during cloudy periods, although it is noted that SEVIRI here has the significant advantage over polar-orbiting missions in this respect. A promising way forward to deal with cloud-free periods is to combine SEVIRI data with in situ data at a few locations, e.g. in the Southern North Sea the CEFAS Smartbuoys [Capuzzo et al, 2013], and/or with hydrodynamic models containing information on currents and bottom and wind stress. To deal with small scale variations of horizontal visibility it would be necessary to combine the temporal information from SEVIRI with spatial information at a higher resolution, e.g. from polar-orbiting satellites. The spatial resolution required for to support diving operations is as high as possible, down to 1m scale if possible, although it is likely that spatial variability may be identified by other means (experience, data from high resolution polar-orbiting sensors) and the main interest is in temporal variability. The temporal resolution required to support diving operations applications is typically hourly. Data is required in near real time for the last few hours/days and would preferably be combined with a short range (1-2 day) forecast. Historical data may be useful for building up a forecast capability.


4.8 Carbon burial by coccolithophores Coccolithophores are a class of calcifying phytoplankton distinguish by a covering of calcium carbon plates or scales termed coccoliths. They are of particular interest to global climate change, both past [De Varagas et al, 2007] and future, because of their role in oceanic inorganic carbon chemistry, their sensitivity to oceanic acidity [Smith et al, 2012] and of their importance in the long-term sink of carbon via sedimentation and burial [Milliman, 1993]. Interestingly for the SEVIRI-WT project, the coccoliths are highly reflective making this species easily visible from space during blooms [Groom & Holligan, 1987]. In fact, for the deep oceans coccolithophore blooms are probably the only biological process visible with the limited spectral resolution of SEVIRI. In the SEVIRI-WT project the relevant parameter for supporting coccolithophore study is an indicator of coccolith concentration, probably reduced to a simplified yes/no coccolithophore bloom flag as supplement to the red marine reflectance. SEVIRI is ideally suited for detecting the strongest coccolithophore blooms occurring in the worlds’ oceans and has a clear advantage over polar-orbiting satellites of the high frequency of acquisitions allowing study of potential diurnal variability as well as improving data availability for regions/periods of scattered clouds. While dedicated ocean colour missions have the advantage of better spectral resolution, facilitating the identification of these species and avoiding confusion with non-algal particles, suitable algorithm development, possibly using time series and/or multi-sensor information should enhance the SEVIRI capability. The spatial resolution required to support coccolithophore applications is of the order 10km since these blooms are generally large scale. The temporal resolution required to support coccolithophore applications is typically daily-weekly for effective cloud-fee data although hourly acquisitions will help improve detection in periods of scattered clouds. Data is required over many years, preferably decades, but is not required in near real-time.

4.9 Support for validation of polar-orbiting ocean colour satellite missions In addition to the direct marine applications of SEVIRI listed above, there is an interest to exploit high frequency data from SEVIRI to link the polar-orbiting ocean colour missions, setting their data in a temporal perspective. For example, when performing match-up validation of polar orbiting missions it is necessary to define a time window for acceptable synchronicity of a match-up, typically between 1 to 6 hours, [Doerffer, 2012] or to quantify the uncertainty arising from asynchronicity of an in situ measurement with a satellite measurement. This can be achieved by following the temporal variability of SEVIRI data, either for individual match-up pairs or more generally by identifying waters with high temporal variability necessitating close time-matching of in situ with satellite measurements. In the SEVIRI-WT project the relevant parameter for supporting the validation of polar-orbiting ocean colour missions is the red marine reflectance. The spatial resolution required to support ocean colour validation applications is of the order 1-10km. The relevance of SEVIRI is the improved temporal resolution. The temporal resolution required to support ocean colour validation applications is hourly or preferably even less, e.g. 15-minutes. Data is not required in near real-time.

4.10 Other applications The dredging sector was identified as an important application for the products of the FP7/HIGHROC project. Although the spectral resolution of SEVIRI is adequate for such SPM-type products, the spatial resolution of SEVIRI is insufficient for the monitoring of dredging operations. SEVIRI will be useful only for the larger scale problems of optimising locations for dredging/dumping (section 4.5) and for monitoring of very large constructions such as artificial islands and new ports (section 4.1).

28 SEVIRI Specific Requirements for the SEVIRI-WT project

5 Specific Requirements for the SEVIRI-WT project

5.1 Summary of parameters to be generated from SEVIRI data Based on the summary of applications described in Chapter 4 and on specific user requirements detailed in response to the SEVIRI-WT questionnaire, given by the users of the FP7/HIGHROC project or noted in prior feedback on the publications of the RBINS team, a set of parameters to be generated by the SEVIRI-WT project is given in Table 2 and Table 3. For most applications there is no specific lower limit to the spatial resolution and finer resolution data will be used, if available. E.g. Most MSFD/WFD applications treated by the MARCOAST project were originally based on MERIS 1.2km data, but were updated to 300m as the latter become more easily available. In Table 2 the typical spatial resolution is therefore given as the resolution of (polar-orbiting) ocean colour data currently used for the existing applications, considered to be a more “reasonable requirement” than “as fine as possible”, which could mean ~1m if that were possible.

Application Parameter Spatial Resolution Temporal Accuracy

requirement

Coastal Water Quality (MSFD) TUR, SPM, SD 300m-1km 1h – 10y+ threshold

Water quality of European lakes (WFD)

TUR, SD, XCYA 300m-1km 1h – 10y+ threshold

Coastal Water Quality - Africa TUR, SD, XHAB ~1km 1h – 10y+, NRT scientifically sound

Water quality of African lakes TUR, SD, XCYA 300m-1km 1h – 10y+ scientifically sound

Sediment Transport TUR, SPM 10m-1km 1h – 10y+ absolute Ecosystem modelling (eutrophication)

KdPAR/ZE, SPM 1-10km 1h – 10y+, some NRT uncertainty per

pixel

Offshore diving operations TUR (… HVIS) 1-100m? 10m – 6h, NRT scientifically sound

Carbon burial by coccolithophores

COCCO ~10km? 1h – 10y+ unknown

Support for ocean colour validation

Rrs 300m-1km 5m – 10y+ absolute

Table 2: Summary of SEVIRI parameters required for application described in Section 4. Definitions of the acronyms used for each parameter are given in Table 3. Spatial resolution is given as that typical of existing (polar-orbiting) ocean colour data used for the application. Temporal characteristics are given both as (lower figure) preferred temporal resolution and (higher figure) the duration of a typical temporal analysis. NRT = Near Real Time. Explanation of accuracy see section 5.3.

29 SEVIRI Specific Requirements for the SEVIRI-WT project

Table 3: Summary of parameters required by users. Flag “units” are defined as logical values (TRUE/FALSE or YES/NO). Feasibility column indicates whether the parameter is certain to be possible (“OK”), is probable but pending success of Task B (“TBD” = To Be Developed), is of unknown feasibility pending Task B (“?”), is feasible but with possible severe restrictions, e.g. limited to turbid waters (“OK*”) or is infeasible (“NO”).

5.2 Other parameters not considered One User Response mentioned the need for above-water PAR data for running ecosystem models and this need has been expressed previously by other contacts. Above-water PAR data can indeed be derived better from SEVIRI than from polar-orbiting sensors, because SEVIRI measures the high frequency variability of clouds whereas once-per-day measurements from polar-orbiters require assumptions of constant cloud cover during the day. However, there are other existing initiatives to derive above-water PAR data from SEVIRI (and GOES data) and since this activity would be very different from the ocean colour activities targeted within the SEVIRI-WT project, it is considered out of scope. The complementarity between SEVIRI-WT KdPAR data and the above-water PAR data is however noted because both are used together in ecosystem models.

5.3 Product accuracy In general it has not been possible to define specific requirements for product accuracy, although validation and product confidence is recognised by most users as of high importance. This difficulty to define user requirements for product accuracy is found also in the database of the GMES-PURE User Requirements Survey [Albert et al, 2014], where only two users reported accuracy requirements for SPM (10-25% and <30% respectively) and another two users, possibly the same, reported an accuracy requirement for euphotic depth (20%). It is recommended to provide some kind of uncertainty/confidence product along with data.

The accuracy requirement is actually strongly depending on the user as well as on the application. An example of the user dependency is the case of Africa, where for many waters there are no measurements of water quality at all, and any serious information is better than none. An example for the application requirement is the European Water Framework Directive. It classifies a water body according to 5 classes, and it is very important to get the assessment very precisely when the value on which the assessment if based (e.g. the chl-a 90% percentile) is close the border between 2 classes. However, within a class the accuracy of the value is unimportant as long as the uncertainty does not extend outside the class limits.

Parameter (units) Units Symbol Feasibility

Remote sensing reflectance 640nm at water level sr-1 Rrs OK Suspended Particulate Matter g m-3 SPM OK Turbidity FNU TUR OK Secchi Depth m SD TBD Diffuse attenuation coefficient of PAR m-1 KdPAR OK* Euphotic depth m Ze TBD Coccolithophore bloom Flag? COCCO TBD Extreme High Biomass algal bloom Flag? XHAB ? Extreme cyanobacteria bloom/surface scum/vegetation Flag? XCYA ? Chlorophyll a mg m-3 CHL NO Algal pigment absorption coefficient at 443nm m-1 apig443 NO CDOM absorption coefficient at 443nm m-1 aCDOM443 NO Diffuse attenuation coefficient spectrum m-1 Kd NO Phytoplankton functional types ? PFT NO

30 SEVIRI Conclusions and Implications for the SEVIRI-WT project

Based on these thoughts we added a qualitative accuracy requirement indicator to Table 2, which specifies the level of accuracy requirement for a give parameter. The meanings are:

• threshold: the detection of passing certain thresholds is important; it is not necessary to achieve a constant accuracy over the full range of values

• absolute: an quantitative accuracy measure should be provided which allows to associate an uncertainty with the parameter, i.e. a per pixel uncertainty in the same units as the parameter, or a global quantification such as “30%”.

• scientifically sound: basically there are no accuracy requirements, probably because the current situation that no measurements are available at all. However, the algorithm should be validated and there should be a case by case verification that the applied algorithm is applicable (for example: detection limits should be considered)

• unknown: user requirements on accuracy are not known

5.4 Latency of data: historical or Near Real Time A need for Near Real Time (NRT) data was not identified except for a few applications: extreme high biomass HAB for an African user (section 4.3), planning of offshore diving activities (section 4.7) and certain short range ecosystem model forecasts (section 4.6). In the first two cases NRT means within 1 hour (less severe for the ecosystem model forecast, stated as 03:00 UTC the day after acquisition) and in the case of offshore diving is preferably accompanied with a prediction for the coming 12 hours. More important for most applications is the use of multi-year historical data. For example, for most water quality reporting tasks (WFD, MSFD) and for sediment transport and ecosystem modelling support NRT data is not required. Harmful Algal Bloom (HAB) applications may require NRT data for rapid response, although most HAB applications are outside the scope of the SEVIRI-WT project, with the possible exception of the extreme high biomass HABs and extreme cyanobacteria blooms with surface scums – see the possible XHAB and XCYA parameters in Table 3.

5.5 Data Format Most users did not specify requirements for the format of data to be supplied. One user did specify a requirement for NetCDF format, which is thought to be the most appropriate format for all SEVIRI-WT output data.

6 Conclusions and Implications for the SEVIRI-WT project

6.1 Summary of applications and User Requirements This document has summarised the User Requirements for SEVIRI ocean colour products and will guide subsequent project developments, notably the Product Definition (Task 1, deliverable D1.2), the Scientific Developments (Task B) and the Algorithm Theoretical Basis Document (Task C), which in turn lead to the Product Validation (Task D) and Processor Implementation (Task E).

The applications described here generally correspond to activities where there is already usage of products from dedicated polar-orbiting ocean colour missions. In general, SEVIRI products will complement the latter by adding new information, especially tidal or diurnal variability which may be


severely aliased from polar-orbiting missions, or by providing information on partially cloudy days when such information is lacking from polar-orbiting missions.

6.2 Limitations of the SEVIRI potential for ocean colour products The limitations of SEVIRI are clearly recognised here and have been explained to the potential users contacted specifically for this URD. This sensor was designed for meteorological applications and lacks the spectral resolution for certain ocean colour products (e.g. chlorophyll a concentration), lacks the signal:noise characteristics of dedicated ocean colour missions and lacks the spatial resolution of polar-orbiting missions.

As regards spectral resolution, the SEVIRI-WT project will not attempt to meet any user requirements for chlorophyll-related products but will focus primarily on the subset of products related to suspended particulate matter (SPM concentration, turbidity, PAR attenuation and euphotic depth in turbid waters) as well as the bright waters occurring during coccolithophore blooms.

6.3 Implications of the User Requirement study for the project While there was a reasonable understanding of User Requirements from many previous User Requirements studies for ocean colour products and from the specific experience of RBINS with SEVIRI products, this User Requirements study has refined some elements and may consequently affect the subsequent project activities, including Scientific Development of algorithms (Task B), the Algorithm Theoretical Basis Document (Task C), Product Validation (Task D) and Processor Implementation (Task E). These refinements are noted here.

6.3.1 New products The current user requirements survey has highlighted the interest in certain products that were not foreseen in the SOW of in the proposal, e.g.

• Remote sensing reflectance, which is obviously the fundamental radiometric parameter underlying all user parameters, but which is identified here as of interest in itself to support the validation of polar-orbiting sensors (Section 4.9).

• Secchi depth, which is still used in many inland water monitoring programmes and in African water bodies,

• Horizontal visibility for underwater diving operations, • flagging of extreme high biomass algal blooms, such as the Benguela upwelling (if detectable

by SEVIRI), and/or • flagging of extreme cyanobacteria blooms and/or surface scums/vegetation.

The feasibility of the latter two still needs to be explored and the most appropriate algorithm for the Secchi depth and clarification of it restrictions to turbid water situations will be further explored in the Scientific Development (Task B).

Horizontal visibility was also noted as a user requirement for diving operations, but it is thought that turbidity will be a suitable proxy.

6.3.2 Motivation to improve signal:noise and lower detection limit As regards the poor signal:noise of SEVIRI, this limitation was clearly identified in prior research papers, [Neukermans et al 2009, 2012], and was recognised at the SOW/proposal stage. There will be a corresponding detection limit for the SPM-related products. This is currently thought to be around 1 gm-3 SPM concentration or about 1 FNU turbidity, when using a 5-image moving average temporal


filtering as in [Vanhellemont et al, 2014]. While this detection limit is not a problem for moderately turbid waters like the Southern North Sea, it is clearly desirable to lower the detection limit wherever possible to catch more users in clearer waters. It will be a challenge for the Scientific Development Task B to clarify and quantify this detection limit and push it lower wherever possible.

6.3.3 Motivation to improve spatial resolution by use of HRV band As regards the poor spatial resolution of SEVIRI, again this limitation was clearly identified in prior research papers, e.g. [Neukermans et al, 2009; Vanhellemont et al, 2014], and was recognised at the SOW/proposal stage. The general advantage of a geostationary mission is clearly the temporal resolution, not achievable by polar-orbiting sensors, and engineering trade-offs, associated with the higher orbit generally lead to a coarser spatial resolution, as for SEVIRI. The spatial resolution of a geostationary sensor also varies with viewing zenith angle (VZA) and becomes degraded for high VZA, e.g. greater than 60°, and seriously degraded or even unusable at very high VZA, e.g. greater than 70°. Interestingly, the spatial resolution for Africa is quite favourable because of the lower VZA compared to Europe. There are two promising approaches to improve the spatial resolution of the SEVIRI products beyond the basic resolution of the 0.6µm. Firstly, high temporal resolution SEVIRI data can be combined with high spatial resolution data from polar-orbiters, such as MODIS, VIIRS and OLCI, via suitably-designed “synergy” products. A precursor of such synergy products has been suggested by [Vanhellemont et al, 2014] who combined 15 minute SEVIRI data with 1km MODIS data to get a synergy product with better performance, when compared to in situ data, than the respective input products. Secondly, the High Resolution Visible (HRV) band of SEVIRI has a spatial resolution three times finer than the 0.6µm band and could potentially yield ocean colour products with improved spatial resolution. The design of synergy products is beyond the scope of the SEVIRI-WT project, although it is noted that this will be investigated in the FP7/HIGHROC project. Potential usage of the HRV band will be considered in the SEVIRI-WT project Scientific Development Task B.

6.4 Other recommendations – evaluation of products by a few users Although not required in the SOW and not promised in the project proposal, it is recommended here to involve a very limited number of users in the evaluation of products from the pre-operational SEVIRI-WT processor. Preferably these users would be chosen to cover different applications areas, if possible. Some highly motivated users already exist based on prior contacts with RBINS and based on key users already targeted by the FP7/HIGHROC project demonstration activity to run Oct 2016-Oct 2017.

New contacts are being sought in particular to cover areas not previously considered such as Lake Victoria in Africa, where there is a lively debate regarding the possible impact of eutrophication and water turbidity on biodiversity and fisheries. Owing to the exceptional size of this lake the spatial resolution of SEVIRI should be sufficient to yield useful information and the temporal resolution of SEVIRI will be a significant advantage compared to existing data from polar-orbiting missions. This will be a useful precursor to possible follow-up activities on smaller lakes.

6.5 Medium and long term vision Despite the limitations of the SEVIRI mission, which was not at all designed for ocean colour applications, we believe there is a clear utility for the products that will be generated in this project, for a variety of applications. However, we see this development also as a precursor for follow-on activities with significant improvements in the products.


In the medium term the developments carried out within the SEVIRI-WT will be easily transportable to improved products that can be generated for the Flexible Combined Imager (FCI) on Meteosat Third Generation. The improved spatial resolution of FCI - 1km (and 500m for VIS0.6) – will greatly enhance the utility of data for certain applications. The improved temporal resolution of FCI – 10 minutes full disk and 2.5 minutes Rapid Scan Service over Europe – could also be exploited to bring down the signal:noise level of measurements and hence push down the detection limits expected for the SEVIRI products. The improved spectral resolution of FCI, which adds blue (0.44µm) and green (0.51µm) bands with respect to SEVIRI, may even make it possible to generate chlorophyll-related products.

In the long term it is clear from the successes of the Korean GOCI mission that a dedicated geostationary ocean colour mission over Europe (and Africa) would have significant advantages and complementarity with respect to polar-orbiting missions. In this longer term context SEVIRI would be superseded by a moderate spatial resolution (~250m), high temporal resolution (~1 hour) geostationary mission with low signal:noise level and a spectral band more suitable for ocean colour. The CNES OCAPI mission, currently in phase A, may fulfil such a role, perhaps in the context of Copernicus Next Generation satellites.