Future remote sensors for chlorophyll a. Dimitry Van der Zande, Quinten Vanhellemont & Kevin Ruddick

Transcription

1 Future remote sensors for chlorophyll a Dimitry Van der Zande, Quinten Vanhellemont & Kevin Ruddick Workshop Scheldt Commission: eutrophication 20 th October 2016

2 Ocean colour from space ESA MERIS 7 May 2008 Clear water Algae (CHL) Turbid waters

3 Ocean colour from space Photons passing through the atmosphere are attenuated due to: - Rayleigh scattering (high in blue) - Reflection (and absorption) by aerosols - Absorption by ozone (UV absorption) - Sea surface light interactions Only photons passing through water are useful (1-35%), rest is noise Water is generally darker than the atmosphere

4 Ocean colour sensors Current CHL Monitoring for the BCZ Next generation CHL monitoring for the BCZ Suitable for highly turbid coastal waters Adapted from Blondeau-Patissier et al. (2014) Data gap for BCZ turbid water monitoring for

5 Ocean colour sensors Sensor/Sate llite/type Agency Launch Date Swath (km) Spatial Resolution (m) Bands visible/total Spectral Coverage (nm) MERIS MODIS Aqua ESA (Europe) , ,200 NASA (USA) , ,000 12/ ,050 9/ ,385 VIIRS NASA (USA) , / ,800 OLCI/ Sentinel-3A ESA/EUME TSAT (Europe) , /1, ,020

6 Ocean colour sensors MERIS (ESA) MODIS AQUA (NASA) WQ reporting?

7 The Sentinel missions Family of missions to fulfill the operational needs of the Copernicus programme for land, ocean and atmospheric monitoring Each Sentinel mission is based on a constellation of two satellites to fulfil revisit and coverage requirements. Sentinel-1: polar orbiting, all-weather, day-and-night radar imaging mission Sentinel-2: polar orbiting, multispectral high-resolution imaging mission for land monitoring Sentinel-3: multi-instrument mission for marine monitoring Sentinel-4: payload devoted to atmospheric monitoring (geostationary orbit) Sentinel-5: dedicated to atmospheric monitoring Sentinel-6: radar altimeter to measure global sea-surface height (operational oceanography and climate studies)

8 Sentinel 3 Ocean mission sea-surface topography sea- and land-surface temperature ocean and land colour with high-end accuracy and reliability Sentinel 3a was launched on 16th of February 2016 Sentinel 3b planned to launch in 2017 Sentinel 3c planned to launch before 2020 Daily revisit time, Spatial resolution = 300m Currently in commisioning phase, Level 2 data (incl. CHL) expected at the end of 2016 Not clear if CHL data will be usable for the 2016 growing season RBINS is part of the S3 Cal/Val team

9 Sentinel 2 Land mission Polar-orbiting, multispectral high-resolution imaging mission for land monitoring Imagery of vegetation, soil and water cover, inland waterways sea- and land-surface temperature Spectral quality is sufficient (in terms of S/N) to observe coastal (turbid) waters. Sentinel 2a was launched June 2015 Sentinel 2b planned to launch in day revisit time, Spatial resolution = m RBINS is working on in house atmospheric correction algorithms to enable the use of S2 data

10 medium resolution vs high resolution ESA MERIS CHL product (1km resolution) S2 RGB and SPM product (10m resolution)

11 Sentinel 2 CHL products (Oostende) Spuikom S2 image for 1 st of May 2016 processed with ACOLITE Gons (2005): Red-edge algorithm to derive chlorophyll a concentration from the red band chlorophyll a absorption

12 Sentinel 2 CHL products (Markermeer) S2 image for 25 th of September 2016 processed with ACOLITE

13 Sentinel 2 CHL products (Markermeer)

14 Sentinel 2 CHL products (Markermeer)

15 Validation of new sensors for operational use Algo Image processing Daily water leaving reflectance maps Daily NRT CHL map products from MERIS CHL P (µg/l) CHL mean 2009 (µg/l) Classification Level 1 validation Level 2 validation Reporting Percentage of surface of Belgian part of North Sea (%) Chlorophyll a concentration (P90)-method 0- period period Statistics <6.67 µg/l µg/l µg/l >15 µg/l Chlorophyll a concentration Level 3 validation WQ product

16 Level 1 Validation: AERONET-OC stations 1 Zeebrugge-MOW1 site nearshore turbid water Thornton site offshore clearer water 2

17 Level 1 Validation: AERONET-OC stations

18 Level 2 Validation: match ups between in situ and satellite CHL In situ (seaborne) Chl a observations are collected on Belgica cruises water samples are taken at a depth of 0.5m-1m Chl a determined using HPLC method (ISO certified measurements) Actual pixel is considered Max Time diff = 1h Strict Quality Control

19 Level 2 Validation: match ups between in situ and satellite CHL To ensure an intercomparison between RS data and in situ observations it is necessary to keep collecting in situ samples Currently match-ups are collected within the standard WQ monitoring programme by optimizing sampling timing. Future: dedicated validation cruises for RS algorithm testing by comparing sea born spectral measurements with in situ samples.

20 Level 3 Validation: Multi-temporal products CHL-P90 Indicator of choice for Belgium: Belgian part of the North Sea Parameter: chlorophyll a (90 percentile in µg/l) Period: growing season (March-October) for 6 years Area: Belgian part of the North Sea CHL-90P classes to be used: (µg/l) (µg/l) (µg/l) (µg/l) > (µg/l)

21 MSFD D5 indicator: Eutrophication Multi-temporal CHLa products (e.g. CHL-P90): sampling issues Irregular availability of satellite chlorophyll-a observations (space & time) due to cloudiness, quality flagging, sensor malfunction, etc. This impacts the multi-temporal product accuracy and is dependent on: 1. the availability of observations during the actual algal bloom 2. a proportional distribution of observations in the bloom and nonbloom period Develop optimized method to take the irregular sampling into account when generating CHL-P90 products

22 Conclusions Current eutrophication monitoring based on MERIS data due to optically complex waters present in BE coastal zone. Data available from 2003 to 2011, data gap for period Sentinel-3 direct replacement of MERIS sensor, L2 data will become available end 2016 The Sentinel programme guarantees ocean colour data for the next 20 years by using multiple satellites RBINS active in Sentinel-3 CAL/VAL team and ready to integrate S3 data in current WQ service (HIGHROC project) S2-MSI offers new opportunities with high resolution CHL products. RBINS has worked on atmospheric correction (ACOLITE) to enable use of this land sensor in coastal zones In order to use Remote Sensing data for eutrophication monitoring it is key to validate the processing chain at 3 levels to get an overview of the general accuracy of the final RS product (CHL-P90) in situ data will always be necessary

23 Any questions? Pléiades, 0.5-2M resolution