Ocean colour data

Figure 1: SeaWiFS images left: false colour 22 Sept 1997 (from Bowers and Binding, 2006); right: 665nm radiance (from Bowers et al. 2005).

Ocean colour data provide information on near sea surface phytoplankton chlorophyll-a concentration (chl-a) in case 1 and case 2 waters, water-leaving radiances (Fig. 1), turbidity (Kd490) and visibility. Ocean colour data are the most popular “level 2” products provided by NEODAAS. The imagery also shows physical phenomena such as eddies, fronts and current boundaries, and are particularly valuable where SST variability is difficult to observe. Data are produced within 2 hours from Aqua-MODIS received at Dundee. MODIS resolution is approximately 1 km at nadir and three daytime Aqua passes are received providing one or two views of a given location each day. These continue the time series of colour data started in 1997 with SeaWiFS. Processing involves calibration, atmospheric correction (using UK-specific approaches where necessary), computation of biophysical quantities using published or de facto standard algorithms, geo-location, and re-mapping within standard user-specified regions. Global coverage is provided from MERIS and MODIS (at 1 km resolution) and SeaWiFS (at 4 km resolution by subscription) with typically a 6-7 hour delay.

Processing of MODIS, MERIS and SeaWiFS is important since cloud cover, sunglint and orbit/sensor swath characteristics all limit observation of the sea surface on a given day: the three sensors in combination give more opportunities to retrieve a valid pixel (see example in Fig. 2). Archived MODIS, MERIS and SeaWiFS data are processed on customer request and the entire archive is re-processed following implementation of improved algorithms and new releases of processing software (such as SeaDAS) in the latter case to ensure consistency with NASA.

Figure 2: Left to right: MERIS, MODIS and SeaWiFS scenes for southwest UK on 5 June 2007 showing differing coverages.

NEODAAS Plymouth has developed a number of novel ocean colour algorithms including higher resolution (500 m) chl-a estimation from MODIS (Shutler et al., 2007) and inherent optical properties (IOP: the fundamental optical properties of the water) using the CASIX IOP model (Smyth et al., 2006). Total backscatter (bb) and absorption (a) are produced with the latter decomposed into phytoplankton (aΦ) and detrital (ady) components. The model has been applied to MODIS, SeaWiFS and MERIS.

NEODAAS maintains an up-to-date copy of the NASA archive of MODIS and SeaWiFS composite data comprising 4 km and 9 km equal-angle projection, daily, 8-day, monthly and seasonal arrays of variables such as chl-a and water leaving radiance at 555 nm. NEODAAS will also supply ESA GlobColour composites where these are available. To complement these data NEODAAS produces higher resolution (1-km) rolling composites (e.g. last 7 days) for user areas. Composite data are used where customers require time series of data at medium resolution or over synoptic scales and to improve coverage for pre-operational monitoring where cloud-gaps are problematic.

In the future ocean colour data will be obtained from instruments such as VIIRS on follow-on satellites such as NPP and NPOESS that will replace MODIS. NEODAAS will also investigate ocean colour from Indian and Chinese instruments that would “fill the gap” if SeaWiFS (>10 years old) or MERIS and Aqua-MODIS (both 5 years old) should fail. Additional novel and UK-specific products will be implemented as developed through NCEO/CASIX and other projects.

Smyth, T.J., Moore, G.F., Hirata, T. & Aiken, J. (2006) Semianalytical model for the derivation of ocean color inherent optical properties: description, implementation, and performance assessment. Applied Optics, 45(31), 8116-8131.

Shutler, J. D., P. E. Land, T. J. Smyth and S. B. Groom (2007) Extending the MODIS 1 km ocean colour atmospheric correction to the MODIS 500m bands and 500m chlorophyll-a estimation towards coastal and estuarine monitoring, Remote Sensing of Environment, 107, 521-532, doi:10.1016/j.rse.2006.10.004.

Bowers, D.G. & Binding, C.E. (2006) The optical properties of mineral suspended particles: A review and synthesis. Estuarine Coastal and Shelf Science, 67(1-2), 219-230.

Bowers, D.G., K.M. Ellis, and S.E. Jones, Isolated turbidity maxima in shelf seas, Continental Shelf Research, 25 (9), 1071-1080, 2005.