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NOAA CoastWatch produces near real-time ocean color products, including
chlorophyll, from the Moderate Resolution Imaging Spectroradiometer (MODIS)
on-board the NASA Earth Observing System (EOS) Aqua satellite.
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 30 October 2022.
Previously available chlorophyll products used the following algorithms.
These products are 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 30 October 2022, 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 is
available only by request from 30 January 2009 to 30 October 2009.
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 through the East Coast Node FTP server.
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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