constraints are required to obtain a satellite-in situ ‘‘matching’’ pair. For example,
a common practice is to restrict the time difference to ±3 h and the 3 9 3 pixel
coefficient of variation \0.15 (Bailey et al. 2000). The latter constraint is to ensure
that the ocean is relatively homogeneous so a satellite pixel (usually *1 km
2 ) can
be represented by a point measurement in the field. Further, to avoid cloud-edge
pixels and other such problems, additional constraints such as wind speed, solar
zenith angle, and satellite viewing angle are also used to find the matching pairs.
Lastly, because R rs (k) is a weighted measure of OSCs within the upper optical
depth with the weighting factor decreasing exponentially with increasing depth
(Zaneveld 1982; Gordon 1992), Chl collected at multiple depths, whenever
available, should be used to calculate a depth-weighted Chl in order to validate the
satellite observations (e.g., Cannizzaro et al. 2013).
Global validation efforts of the SeaWiFS Chl data product showed that for most
open ocean waters, the algorithm performed well, with RMS differences between
SeaWiFS and in situ Chl (after logarithmic transformation) of 0.2–0.3 without significant bias (Gregg and Casey 2004; McClain et al. 2004; Bailey and Werdel 2006).
This translates to about 50–60 % RMS relative difference. Global validation of the
semi-analytical Chl (Maritorena et al. 2002, 2010) using merged data from SeaWiFS, MODISA, and MERIS showed slightly worse performance for Chl between
0.02 and 10.0 mg
-3 , possibly due to residual inconsistencies between sensors.
Online validation tools have been provided by NASA through the SeaBASS
online query (http://seabass.gsfc.nasa.gov/seabasscgi/validation_search.cgi) and
by ESA through the MERis Matchup In-situ Database (MERMAID, http://
hermes.acri.fr/mermaid/matchup/matchup.php), where the area of interest, temporal window, and validation criteria can all be specified. Figure 7.6 and Table 7.2
0.01
0.1
1.0
10.0
100.0
Measured Chl (mg m -3 )
0.01
0.1
1.0
10.0
100.0
Measured Chl (mg m
-3 )
100.0
10.0
1.0
0.1
0.01
Satellite derived Chl (mg m
-3
)
(a)
(b)
100.0
10.0
1.0
0.1
0.01
Fig. 7.6 Global Chl validation results using data from the SeaBASS data archive and online
query (http://seabass.gsfc.nasa.gov/seabasscgi/validation_search.cgi). a SeaWiFS (1997–2010);
b MODIS/Aqua (MODISA, 2002–2011). The default validation criteria have been applied
(Werdell et al. 2005). Validation statistics are listed in Table 7.2
7 Oceanic Chlorophyll-a Content
185
a common practice is to restrict the time difference to ±3 h and the 3 9 3 pixel
coefficient of variation \0.15 (Bailey et al. 2000). The latter constraint is to ensure
that the ocean is relatively homogeneous so a satellite pixel (usually *1 km
2 ) can
be represented by a point measurement in the field. Further, to avoid cloud-edge
pixels and other such problems, additional constraints such as wind speed, solar
zenith angle, and satellite viewing angle are also used to find the matching pairs.
Lastly, because R rs (k) is a weighted measure of OSCs within the upper optical
depth with the weighting factor decreasing exponentially with increasing depth
(Zaneveld 1982; Gordon 1992), Chl collected at multiple depths, whenever
available, should be used to calculate a depth-weighted Chl in order to validate the
satellite observations (e.g., Cannizzaro et al. 2013).
Global validation efforts of the SeaWiFS Chl data product showed that for most
open ocean waters, the algorithm performed well, with RMS differences between
SeaWiFS and in situ Chl (after logarithmic transformation) of 0.2–0.3 without significant bias (Gregg and Casey 2004; McClain et al. 2004; Bailey and Werdel 2006).
This translates to about 50–60 % RMS relative difference. Global validation of the
semi-analytical Chl (Maritorena et al. 2002, 2010) using merged data from SeaWiFS, MODISA, and MERIS showed slightly worse performance for Chl between
0.02 and 10.0 mg
-3 , possibly due to residual inconsistencies between sensors.
Online validation tools have been provided by NASA through the SeaBASS
online query (http://seabass.gsfc.nasa.gov/seabasscgi/validation_search.cgi) and
by ESA through the MERis Matchup In-situ Database (MERMAID, http://
hermes.acri.fr/mermaid/matchup/matchup.php), where the area of interest, temporal window, and validation criteria can all be specified. Figure 7.6 and Table 7.2
0.01
0.1
1.0
10.0
100.0
Measured Chl (mg m -3 )
0.01
0.1
1.0
10.0
100.0
Measured Chl (mg m
-3 )
100.0
10.0
1.0
0.1
0.01
Satellite derived Chl (mg m
-3
)
(a)
(b)
100.0
10.0
1.0
0.1
0.01
Fig. 7.6 Global Chl validation results using data from the SeaBASS data archive and online
query (http://seabass.gsfc.nasa.gov/seabasscgi/validation_search.cgi). a SeaWiFS (1997–2010);
b MODIS/Aqua (MODISA, 2002–2011). The default validation criteria have been applied
(Werdell et al. 2005). Validation statistics are listed in Table 7.2
7 Oceanic Chlorophyll-a Content
185