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You are here: GES DISC Home Education and Outreach Additional Features Science Focus Ocean Color SCIENCE FOCUS: MORE THAN MEETS THE EYE, PART 4


Step IV:Masks and Flags

Now that the chlorophyll concentrations have been visually presented, it's time to take a look at some of the various conditions that are present in the data.A "condition" only means that a particular pixel in the data file has certain characteristics (it's not a disease). A simple case is where the pixel is located on land.In that case, the land "mask" is applied.When Level 1A data is processed to Level 2, any pixels covered by the land mask are not processed, which saves time.

Other masked conditions are the presence of clouds (which are detected by a high radiance value) and "radiance above the knee".The latter refers to the way that SeaWiFS is capable of imaging both land and water. Water is much, much darker than land, optically speaking, so SeaWiFS has two gain settings that allow it to discriminate varying radiance levels over water and land.The gain setting for water allows the instrument electronics to process light levels linearly up to a certain threshold (the "knee"), at which point a higher linear gain is used.This feature prevents saturation of the sensor over particularly bright areas, and is referred to as a bilinear gain. When an oceanic area is so bright that it exceeds the radiance threshold for the lower gain setting, the "radiance above the knee" mask appears.

SeaWiFS data processing also analyzes the data for several conditions that can make data analysis uncertain. For any pixel with one or more of these conditions, a "flag" specific to that condition is given to that pixel.These conditions include a large angle between the pixel and the satellite (which occurs at the edges of the scanning swath), different types of atmospheric aerosols, failure of the analytical algorithms (i.e., the calculation doesn't return a meaningful value), or the presence of turbid or sediment-laden water. Two types of phytoplankton are also flagged:coccolithophore blooms and Trichodesmium, a species that can fix nitrogen from the atmosphere.Any pixel can have several different flags.

The coccolithophore bloom algorithm was created based on both surface and satellite observations of coccolithophore blooms.It uses nLw values to determine if the characteristic optical signature of a coccolithophore bloom is present.It isn't perfect: areas that are not coccolithophore blooms can be flagged, and areas that very likely are coccolithophore blooms might be missed. Even though that must be kept in mind, the algorithm works quite well.The current coccolithophore bloom algorithm is described in Volume 9 of the SeaWiFS Postlaunch Technical Memorandum Series.

The image displayed below shows the nLw(490) data and features three masks (brown for land, white for clouds, and green for radiance above the knee) and three flags (blue for coccolithophore blooms, yellow-orange for low nLw(555), and red for absorbing aerosols). Look for each of these colors:

SeaWiFS normalized water leaving radiance at 490 nanometers with flags and masks

Most of the masks are found where they probably should be assigned (which is desirable).The only place that the absorbing aerosol flag appears is attached to the tail of the diamond-shaped cloudy area below the Rio de la Plata estuary.In the true-color image, this area appeared as hazy cloud, and not as bright as other clouds nearby.So it is possible that this is a smoke aerosol. The low nLw(555) flag only appears on the edges of clouds.This condition is likely due to the fact that it takes the sensor and electronics a few milliseconds to adjust when the sensor's scan makes a transition at the edge of a bright cloudto dark water.

Now for a closeup view of the area around the Falkland Islands:

Unflagged 490 nanometer
closeup of bloom region Flagged 490 nanometer
closeup of bloom region

The left image is the 490 nm radiance data without flags, and the right image is the 490 nm data with the flags added.In the left image, the black patches interior to the bright areas were suspected to be the "radiance above the knee" mask;they are green in the flagged image, so this is correct.The blue coccolithophore flag has been assigned to a significant area of the bloom.This flag data helps to verify the suspicion that the bright blue-green waters seen in the true-color image are in fact due to a coccolithophore bloom.

However, also note that in the full-size image, the turbid waters of Bahia Blanca have also been flagged as a coccolithophore bloom, and the "radiance above the knee" mask appears inland of the coccolithophore flag.In this case it helps to view the true-color image and see that this is an area of turbid water, which indicates that this is less likely to be a coccolithophore bloom.

A few observations and comments, along with other relevant links, will be found on the final page.

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Last updated: Apr 06, 2016 10:25 AM ET