PROJECT TITLE:
Optical and Ancillary Measurements at High Latitudes in Support of the
NASA's EOS Validation Program - NASA Grants NAG5-6466 and NAG5-6512 (period of performance 10/01/1997-09/30/2002)
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PRINCIPAL INVESTIGATORS:
Dariusz Stramski
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1. Project summary
As
part of NASA's "Satellite Remote Sensing Measurement Accuracy,
Variability, and Validation Studies" (NRA-97-MTPE-03) we made optical and
ancillary measurements in the north polar waters of the Atlantic in support of
the MODIS ocean
color validation program. The major goal of our project was to characterize
errors for the ocean color data products and develop understanding of these
errors in order to improve performance of bio-optical algorithms at high
latitudes of the north Atlantic. Our specific objectives were:
(1)
To conduct a comprehensive suite of in situ optical and ancillary measurements
in the north polar waters of the Atlantic.
(2)
To identify errors in SeaWiFS- and MODIS-derived ocean color data products in
the investigated polar waters, especially normalized water-leaving radiances
and pigment concentrations.
(3)
To develop understanding of the errors in chlorophyll concentration derived from
standard SeaWiFS and MODIS chlorophyll algorithms.
(4)
To improve the bio-optical ocean color algorithms for the investigated polar
region.
(5)
To examine effects of bubbles entrained by breaking wind waves on remote
sensing reflectance and to examine the surface whitecap coverage in the
investigated polar waters.
There
were two major reasons for undertaking this research. First, the bio-optical
algorithms for retrieving phytoplankton pigment concentration from
water-leaving radiances in polar regions may differ significantly from those
derived at temperate waters. These differences may be attributable to the
regional variability in the chlorophyll-specific absorption coefficient of
phytoplankton and relative contributions of phytoplankton, detritus, and
soluble materials to the total absorption, as well as regional variations in
the backscattering properties of seawater. Further research in polar regions is
needed to improve our understanding of why and how the high-latitude algorithms
differ from those developed at lower latitudes, and to evaluate whether there
exist differences within the polar regions. Second, the optical effects of air
bubbles in the near surface layers of the ocean have been ignored in the
satellite ocean color algorithms. Preliminary estimates indicate that even
under calm sea conditions (no injection of bubbles by breaking wind waves), the
persistent bubble populations can make sizable contribution to light scattering
by seawater (Stramski, 1994, Ocean Optics XII, SPIE Proc. Vol. 2258).
Therefore, bubbles may influence the remote sensing reflectance and accuracy of
data products derived from ocean color algorithms, especially under strong
winds.
Our
approach involved an integrated program of field measurements, analysis of in
situ data including comparison with SeaWiFS-derived data products, modeling of
radiative transfer, and development of bio-optical algorithms based on our in
situ data. Our field program was based on collaboration with the Institute
of Oceanology, Polish Academy of Sciences. During the cruises in the summer of 1998, 1999, and 2000, we carried
out a diverse suite of measurements of underwater radiometric quantities,
inherent optical properties, and other variables including the chlorophyll-a
concentration. MODIS data are not available for the time period of our cruises,
so a comparison of our in situ data with satellite-derived data was possible
only for SeaWiFS. Most of our results of direct relevance to the ocean color
validation program are described in a paper entitled "Bio-optical
relationships and ocean color algorithms for the north polar region of the