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The "color" of the ocean is determined by the interactions of incident light with substances or particles present in the water. The most significant constituents are free-floating photosynthetic organisms (phytoplankton) and inorganic particulates. Phytoplankton contain chlorophyll, which absorbs light at blue and red wavelengths and transmits in the green. Particulate matter can reflect and absorb light, which reduces the clarity (light transmission) of the water. Substances dissolved in water can also affect its color.
What is ocean color data?
The phrase "ocean color data" refers to accurate measurements of
light intensity at visible wavelengths. As ocean color data
is related to the presence of the constituents described above, it
may therefore be used to calculate the concentrations of material in
surface ocean waters and the level of biological activity.
Ocean color observations made from Earth orbit allow an oceanographic
viewpoint that is impossible from ship or shore -- a global
picture of biological activity in the world's oceans.
What is SeaWiFS?
The Sea-Viewing Wide Field-of-view Sensor (SeaWiFS) is a
spectroradiometer, which means that it measures radiance
in specific bands of the visible light spectrum. The advantage of observing
the oceans with a space-based spectroradiometer is the global coverage that a
satellite provides. The disadvantage is that interfering optical effects,
primarily light scattering in the atmosphere, must be accounted for to
provide an accurate measurement of the water-leaving radiance.
Uses of ocean color data
Ocean color data has been used for numerous different oceanographic
research activities. The type of research activity generally determines
the type of data that will be most useful to researchers. SeaWiFS ocean
color data is available in several different types, called "levels" after
standard remote-sensing terminology. Below is a brief description of each
ocean color data level, along with appropriate research applications.
LAC and GAC
These two acronyms stand for Local Area Coverage (LAC) and
Global Area Coverage (GAC), and indicate the resolution and acquisition mode
for remote-sensing data. LAC indicates fine resolution data, which in
the case of SeaWiFS is data with 1.1 kilometer resolution. SeaWiFS broadcasts
LAC data to ground stations, and also stores a small amount in onboard
memory for areas of special interest. GAC is coarse resolution data obtained
by subsampling the LAC data every fourth scan line and every fourth picture
element (pixel), which gives a resolution of 4.5 km. SeaWiFS acquires 15
GAC swaths a day, each swath 1500 km wide starting from the Arctic Ocean
and following the orbital ground track to Antarctica. The GAC data is
transmitted to a ground station and then to the SeaWiFS Project once a
day.
Level 1 Data
Level 1 data consist of the radiances (i.e., the light intensity)
measured by the sensor at each of the wavelengths that the sensor detects.
SeaWiFS has eight visible wavelength bands. Instrument and satellite
data, primarily for the purposes of calibration and navigation, are included.
Level 1 data is useful for investigating the optical properties of ocean
waters, and for the development of algorithms that use the radiances to
calculate important oceanographic parameters.
Level 2 Data
Level 2 data consist of geophysical parameters -- values calculated
from the radiances detected by the sensor. Some of the geophysical values
are normalized water-leaving radiances at the various detector wavelengths.
These radiances may then be used to calculate other parameters, such as the
concentration of chlorophyll (which indicates how much phytoplankton is
present) or the diffuse attenuation coefficient (which is a measure of water
clarity). The calculations are based on bio-optical oceanographic research
that takes place before and during the mission. Another set of geophysical
parameters are atmospheric optical values that are used in the atmospheric
correction process.
Level 2 data may be displayed in false color images that highlight the full range of values. Level 2 "browse" images are based on the concentration of chlorophyll, which is of interest to numerous researchers, as it indicates areas of high biological production and the boundaries between low and high productivity waters. This data may also indicate regions where nutrient concentrations are elevated, where current interactions are particularly energetic, or where sediment is being transported.
Other potential applications of Level 2 ocean color data:
Level 3 Data
Level 3 ocean color data is reduced resolution global geophysical
data that has been statistically assimilated into a global grid consisting of
9 x 9 km grid squares, or "bins". The daily global Level 3 product
consists of the reduced resolution data acquired during all of the orbits in
a given day. This data is used to generate a weekly, monthly, and
annual Level 3 product, in which the values of each geophysical parameter
in each bin are averaged over the appropriate time period.
Level 3 data is utilized by researchers interested in global
phenomena and phenomena occurring over large regions, such as El niño
in the Pacific Ocean. Some research groups may use Level 3 data to estimate
global and regional primary productivity. Other applications include
investigations of the cycling of carbon in the oceans and the global
variability of ocean optical properties.