MyOcean Secchi Disk Depth Estimate

Summary

Provider: MyOcean (ACRI)

Contact person: ???

Contact for production: MyOcean Service Desk Send mail

Algorithm development:

The Secchi depth is the depth at which the pattern on a disk (typically black and white quadrants) is no longer visible i.e. the Secchi depth is reached when the reflectance equals the intensity of light backscattered from the water. This depth is taken as a measure of the transparency of the water and is related to water turbidity. The Secchi disk method is a common method for determining turbidity in-situ.

Units are in metres (m)

Data Portal: MyOcean 2

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Platform & processing

Processed by ACRI in association with MyOcean. (In theory, software is available)

Algorithm

The Secchi disk depth is derived from the merged, GSM, chlorophyll-a estimates.

The merged Secchi disk depth is computed directly from the merged chlorophyll-a, from the GSM model, using the following equation:

Zsd = 8.50 − 12.6 X + 7.36 X2 − 1.43 × X3

with X = log10 (Chlsurf )

The merged coefficient is generated from the 1/24° (4km); GSM chlorophyll products.

See Doron et al. 2006, Morel et al. 2007.

Product uncertainties

Cloud cover limits coverage of the sea-surface. The error of the merged product was not assessed.

Case 1 products may not work 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.

Product dissemination

These products are not distributed via GEONETCast. The data available through MyOcean is distributed via the internet

Typical delay of near-real-time product is a minimum of 24hrs from acquisition of data by ACRI, and the delayed time product is 31 days.

Product validation

The GSM algorithm has been peer reviewed. The Secchi disk algorithm has been peer reviewed(see references). GSM chlorophyll was tested, but Secchi disk depth was not part of the GlobColour validation plan. Similarly GSM chlorophyll was checked with other models, the Secchi disk depth was not.

Data characteristics

Global Ocean

Resolution: 4km, 25km and 100km.

Map projection: All resolutions now use regular equirectangular.

European Seas / North Africa

Coverage: 20°N to 85°N ; 45°W to 68° E. Can also be subsetted to the Mediterranean area (30°N to 46°N ; 6°W to 36.5° E) for download

Resolution: 2km

Map projection: regular equirectangular

Availability

Depends on processing type:

near-real-time: daily product, 31 day archive;

delayed time: daily product, 31 day archive;

reanalysis: daily, 8day, monthly product,

Flags for land/cloud, flags for bad radiometric quality of spectral bands (i.e. saturated pixels, sun glint etc.) have already been applied to input data. A separate flags product is available for download and documented here

Notes

Information about processing version changes are not widely disseminated. If processing changes the archive should usually be reprocessed for consistency with current NRT data although whether this mechanism is in place is not known.

Useful references:

Maritorena, S., Siegel, D. A. (2005). Consistent merging of satellite ocean color data sets using a bio-optical model. Remote Sensing of Environment 94:429-440. [pdf]

Morel, A., Huot, Y., Gentili, B., Werdell, P.J., Hooker, S.B. and B.A. Franz (2007). Examining the consistency of products derived from various ocean color sensors in open ocean (Case 1) waters in the perspective of a multi-sensor approach. Remote Sensing of Environment, 111, 69-88.

Doron, M., Babin, M., Mangin, A. and O. Fanton d'Andon (2006). Estimation of light penetration, and horizontal and vertical visibility in oceanic and coastal waters from surface reflectance. Journal of Geophysical Research, volume 112, C06003, doi: 10.1029/2006JC004007.