NOAA CoastWatch East Coast Node
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MODIS Chlorophyll-a

NOAA CoastWatch produces near real-time ocean color products from the Moderate Resolution Imaging Spectroradiometer (MODIS) on-board the NASA Earth Observing System (EOS) Aqua satellite. The global raw satellite data (Rate Buffered Data or RBD) are obtained from the NASA Goddard Space Flight Center (GSFC). NOAA processes the RBD to Level 0 datasets, then to Level 1A radiances, and then to Level 1B calibrated, geolocated radiances. CoastWatch produces derived geophysical products (Level 2), including chlorophyll, from the calibrated Level 1B radiances. These products are then mapped (Level 3) to the CoastWatch geographic regions.

The MODIS chlorophyll products available from East Coast Node are derived using a 3-band ocean color algorithm (OC3) with the NOAA-MSL12 software. Atmospheric correction is performed using the NOAA NIR-SWIR combined approach: the SWIR atmospheric correction improves chlorophyll values in coastal waters, while the iterative NIR atmospheric correction is used for open ocean waters (Wang and Shi, 2007). For the near real-time constraints of operational data processing, NOAA uses forecast ancillary data (ozone, wind speed, etc.) from the Global Forecast System (GFS) and predicted ephemerides. This NOAA OC3 NIR-SWIR chlorophyll product is available from 28 August 2014 to the present, with usually a one-day lag.

Previously available chlorophyll products used the following algorithms. These products are still generated by CoastWatch but are only available at East Coast Node by request:

    (1) OC3 algorithm from NASA SeaDAS software using the iterative NIR atmospheric correction (NASA OC3 NIR): these products generated by NOAA are of operational quality and differ from the same product generated by NASA. Due to the constraint imposed on near-real time generation, NOAA uses forecast ancillary data from the Global Forecast System (GFS) and predicted ephemerides. These ancillary data are different than those used in production of the NASA science products, but the resulting products show good agreement with respect to patterns and gradients which are important for operational applications. Additionally, although NOAA strives to implement the most recent versions of the MODIS calibration tables and software on its near-real time processing system, it cannot always achieve this objective and may be running a different version than that implemented on NASA's science processing system. NASA's science quality MODIS ocean color products are available from NASA's Ocean Biology Processing Group (OBPG). (NOAA-processed NASA OC3 NIR chlorophyll is available only by request from 16 February 2013 to the present, and for some regions back to 8 January 2010.)

    (2) Regionally tuned OC3 algorithm for Chesapeake Bay: the same OC3 algorithm as in the NASA SeaDAS software (processed by NOAA as described above), but with chlorophyll algorithm coefficients calculated from Chesapeake Bay in-situ chlorophyll measurements (Werdel et al., 2007). This regional tuning reduces the high chlorophyll bias in coastal regions of the NASA SeaDAS OC3 standard algorithm, whose algorithm coefficients were calculated using global in-situ measurements. (NOAA-processed NASA OC3 NIR regionally-tuned chlorophyll available only by request from 30 January 2009 to present.)

    Both of these algorithms have occasional occurrences of unrealistic chlorophyll features in coastal regions (see Figure below for example). NOAA CoastWatch East Coast Node therefore only makes the NOAA OC3 NIR-SWIR product available on a near real-time basis through this web site.

    Figure: MODIS chlorophyll concentration for Gulf of Maine, 24 August 2014, showing an unrealistic chlorophyll front in the NOAA-processed NASA SeaDAS OC3 NIR product (left) and in OBPG's processing of the NASA SeaDAS OC3 NIR product (middle). This unrealistic feature is absent in the NOAA OC3 NIR-SWIR product (right).

References:

Wang, M. and W. Shi, 2007, The NIR-SWIR combined atmospheric correction approach for MODIS ocean color data processing, Optics Express, vol. 15, no. 24, 15722.

Werdell, P.J., et. al., 2007, Approach for the long-term spatial and temporal evaluation of ocean color satellite data products in a coastal environment, Proc. Of SPIE Vol. 6680, 66800G.


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