MyOcean Remote Sensing Reflectances

Summary

Provider: MyOcean (CNR)

Contact person: ???

Contact for production: MyOcean Service Desk Send mail

Algorithm development:

Water-leaving radiance (Lw) is the primary retrieved level-2 quantity after atmospheric correction of level-1 data. The data are mainly converted into normalised water-leaving radiance (nLw) or remote sensing reflectance (Rrs) and used as inputs to algorithms to compute Chl-a and other in-water parameters (e.g. Kd490) but may be used individually or as a three-band false-colour composite to show area of enhanced brightness, for example, due to coccolithophore blooms. The MyOcean products are RRs i.e. ratio of upwelling radiance emerging from seawater to downwelling radiative flux in air, at various wavelengths depending on the sensor. These are produced for the Mediterranean by the Group for Satellite Oceanography (GOS-ISAC) of the Italian National Research Council (CNR), in Rome, and globally by ACRI.

Units are sr-1.

Data Portal: MyOcean 2

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Platform & processing

Mediterranean

The Group for Satellite Oceanography (GOS-ISAC) collects Level-0 raw data from ESA and/or NASA as soon as they are available an classed as Near Real Time (NRT). Delayed Time (DT) processing mode is performed four days after satellite overpasses as soon as ancillary data are made available for downloading from NASA. Standard masking criteria for detecting clouds or other contamination factors are routinely applied, i.e., land, cloud, sun glint, atmospheric correction failure, high total radiance, large solar zenith angle (70deg.), large spacecraft zenith angle (56deg.), coccolithophores, negative water leaving radiance, and normalized water leaving radiance at 555 nm.[McClain et al., 1995].

Global Ocean, European Seas / North Africa

The ESA Envisat MERIS radiance products provided by ACRI are the outputs of the case 1 / open ocean or case 2 / coastal water algorithms at 1.2 km resolution from 2002 to date. The NASA radiances are case 1: Aqua MODIS at ~1.1 km resolution and SeaWiFS at 2km resolution. ESA data are processed by ESA to level 2 using specific software (see http://www.odesa-info.eu/distrib/); NASA data are processed to level 2 using NASA specific software. The user version of the processing software is called SEADAS and is available at http://oceancolor.gsfc.nasa.gov/seadas. SeaDAS can also process MERIS data.

Algorithm

Mediterranean

Level 0 data are processed by CNR to level 3 using specific software; NASA data are processed to level 3 using NASA specific software. The user version of the processing software is called SeaDAS v6.1 and is available at http://oceancolor.gsfc.nasa.gov/seadas/. MERIS Level 2 data are processed to level 3 using SeaDAS.

Global Ocean, European Seas / North Africa

Input is the level 2 MODIS, and SeaWiFS and MERIS data (where available). MODIS and SeaWiFS have the atmospheric correction procedure of Gordon and Wang 1994 whilst MERIS uses that of Antoine and Morel (2005), but in bright water uses that of Aiken and Moore (2000).

All operational SeaWiFS and MODIS/Aqua processing includes an out­-of-­band correction to normalize differences in their spectral response functions. The 412 and 443 nm bands have differences on the order of 1 to 3% in Case 1 water. The 555/551 nm bands may have differences of up to 11%. This normalization is applied to remove spectral band­pass effects in Lw λ). Essentially, it adjusts for out­-of-­band contribution in the derived radiances, leaving nominal nLw (λ) values. The normalization, however, assumes Case 1 spectral dependence, so waveband differences may increase in turbid conditions. The Remote Sensing Reflectance for each wavelength (nnn) is derived from the normalised water leaving radiance (nLw) Rrsnnn = nLwnnn/F0nnn where F0 is the mean solar irradiance. All operational SeaWiFS and MODIS ­Aqua processing includes an out-­of-­band correction. After normalization, the 412 and 443 bands are nearly identical in Case 1 water. The 555/551 nm differences are reduced to around 8%

The global merged fully normalised water leaving radiances (at 412, 443, 490, 510, 531, 550-565, 620, 665-670, 681 and 709 nm.) produced for MyOcean are obtained by the merging of MERIS, MODIS and SeaWiFS data using an error-weighted averaging. MERIS reflectances read from the L2 products are initially converted into fully normalised water leaving radiances (the normalisation is not applied to the MERIS 681 fluorescence band and to the 709 nm band). The MERIS 551 and MODIS 560 bands are spectrally adjusted to 555 nm, using an interspectral conversion LUT which is a function of the CHL1 concentration. The main data input are post atmospheric correction level 2 MERIS, MODIS SeaWiFS binned data. The algorithm changes depending on the processing version - please see latest information from NASA (e.g. http://oceancolor.gsfc.nasa.gov/REPROCESSING/R2009/) and ESA.

Product uncertainties

Case 1 retrievals will have an error in case 2 waters depending on the nature of the water. Temporally the water may change through phytoplankton growth/decay, changes in nutrient inputs etc. When the MODIS or MERIS processing changes the archive is also usually reprocessed so archives should be consistent with current NRT data.

Cloud cover limits coverage of the sea-surface; similarly sun-glint or high levels of aerosol mask the surface signal. The limitation is the accuracy of the atmospheric correction procedure together with the calibration and signal to noise of the instrument. NASA undertake various calibration and comparison of RRs with in situ data (see http://oceancolor.gsfc.nasa.gov/REPROCESSING/R2009/validation/ for MODIS and SeaWiFS 2009 reprocessing).

Product dissemination

These products are not distributed via GEONETCast currently, but the basic data are distributed over the internet by ESA and NASA respectively. The DevCoCast produced data are also distributed via the internet.

Typical delay of product is a minimum of 6 hrs from acquisition of data by NASA or ESA. Products from are delivered on a best effort basis only.

Product validation

MyOcean state that this product is meant for use for educational purposes and for the managing of the marine safety, marine resources, marine and coastal environment and for climate and seasonal studies.

MyOcean provides validation documents for both CNR and ACRI products. MODIS processing chains are validated by comparison with in situ data with strict time and space matchup criteria (see http://oceancolor.gsfc.nasa.gov/REPROCESSING/R2009/). Operational ESA products are not currently validated against field measurements though comparisons are made (e.g. http://www.obs-vlfr.fr/Boussole/html/calibration/matchup_r.php#meris).

SeaWiFS and MODIS data use a common processing system and NASA undertake intercomparisons.

Data characteristics

Mediterranean

Wavelengths: MERIS 412, 443, 490, 510, 560, 665 and 865nm, MODIS 412, 443, 488, 531, 547, 667 and 869nm

Resolution: 1km

Map projection: cylindrical equirectangular

European Seas / North Africa

Wavelengths: MODIS, MERIS and SeaWiFS merged at 412, 443, 490, 555 and 670 nm

Resolution: 2km

Map projection: cylindrical equirectangular

Global Ocean

Wavelengths:MODIS, MERIS and SeaWiFS merged at 412, 443, 490, 555 and 670 nm

Resolution: 4km, 25km or 100km resolution.

Map projection: All resolutions now use cylindrical equirectangular.

Availability

The typical delay of product is less than 24 hours for Near Real Time (NRT) products. Delayed Time (DT) products are made available to the user with 5 days of acquisition. DT data are processed using precision orbital data and improved meteorological fields used for atmospheric correction when compared to NRT data. Reanalysis (RAN) products for the Mediterranean are the output of period but infrequent (after some years) reprocessings using the same configuration throughout. Global RAN products are available from 30 days after acquisition as daily, 8-day and monthly composites

Notes

Acquisition of SeaWiFS data stopped on 11 December 2010. Since 12 December 2010 this product has been produced using only MODIS and MERIS data: the results is a possible degradation of the product spatial coverage.

Useful references:

Aiken J. and G. Moore (2000) Case 2 (S) Bright Pixel Atmospheric Correction, MERIS ATBD 2.6 (http://envisat.esa.int/instruments/meris/pdf/atbd_2_06.pdf)

Antoine D. and A. Morel (2005) Atmospheric Correction of the MERIS observations Over Ocean Case 1 waters, MERIS ATBD 2.7, http://envisat.esa.int/instruments/meris/pdf/atbd_2_07.pdf

Doerffer, R., and H. Schiller (2007), The MERIS case 2 water algorithm, International Journal of Remote Sensing, 28, 517-535.

Gordon, H. R., and Wang, M. (1994), Retrieval of water-leaving radiance and aerosol optical thickness over the oceans with SeaWiFS: a preliminary algorithm. Appl. Opt. 33:443-452.

McClain, C. R., Arrigo, K. R., Esaias, W., Darzi, M., Patt, F. S., Evans, R. H., et al.(1995). SeaWiFS Algorithms, Part 1. NASA Tech. Memo. 104566, Vol. 28 Greenbelt, Maryland: NASA Goddard Space Flight Center.

Santoleri R., G. Volpe, S. Marullo, B. Buongiorno Nardelli, 2008: Observing the Mediterranean Sea from space: ocean colour algorithms and chlorophyll variability, in Remote Sensing of the European Seas, edited by Vittorio Barale and Martin Gade, ISBN: 978-1-4020-6771-6, doi:10.1007/978-1-4020-6772-3_8, 103-116.

Schiller, H., and R. Doerffer (2005), Improved determination of coastal water constituent concentrations from MERIS data, IEEE Transactions on Geoscience and Remote Sensing, 43, 1585-1591.

G. Volpe, R. Santoleri, V. Vellucci, M. Ribera d'Alcala, S. Marullo, F. D'Ortenzio, The colour of the Mediterranean Sea: Global versus regional bio-optical algorithms evaluation and implication for satellite chlorophyll estimates, Remote Sensing of Environment, Volume 107, 4,625-638, 2007, doi:10.1016/j.rse.2006.10.017.