Ocean turbidity

Ocean turbidity
Visualisation of the Ocean Turbidity of the ocean just before Hurricane Bob (August 14, 1991)

Ocean turbidity is a measure of the amount of cloudiness or haziness in sea water caused by individual particles that are too small to be seen without magnification. Highly turbid ocean waters are those with a large number of scattering particulates in them. In both highly absorbing and highly scattering waters, visibility into the water is reduced. The highly scattering (turbid) water still reflects a lot of light while the highly absorbing water, such as a blackwater river or lake, is very dark. The scattering particles that cause the water to be turbid can be composed of many things, including sediments and phytoplankton.

Contents

Measurement

There are a number of ways to measure ocean turbidity, including autonomous remote vehicles, shipcasts and satellites.

From a satellite, a proxy measurement of the water turbidity can be made by examining the amount of reflectance in the visible region of the electromagnetic spectrum. For the Advanced Very High Resolution Radiometer (AVHRR), the logical choice is band 1, covering wavelengths 580 to 680 nanometers, the orange and red. In order to make derived products that are comparable over time and space, an atmospheric correction is required. To do this, the effects of Rayleigh scattering are calculated based on the satellite viewing angle and the solar zenith angle and then subtracted from the band 1 radiance. For an aerosol correction, band 2 in the near infrared is used. It is first corrected for Rayleigh scattering and then subtracted from the Rayleigh corrected band 1. The Rayleigh corrected band 2 is assumed to be aerosol radiance because no return signal from water in the near infrared is expected since water is highly absorbing at those wavelengths. Because bands 1 and 2 are relatively close on the electromagnetic spectrum, we can reasonably assume their aerosol radiances are the same.

In these images the turbidity is quantified as the percent reflected light emerging from the water column in a range of 0 to 8 percent. The reflectance percentage can be correlated to attenuation, Secchi disk depth or total suspended solids although the exact relationship will vary regionally and depends on the optical properties of the water. For example in Florida Bay, 10% reflectance corresponds to a sediment concentration of 30 milligram/liter and a Secchi depth of 0.5 meter. These relationships are approximately linear so that 5% reflectance would correspond to a sediment concentration of approximately 15 milligram/liter and a Secchi depth of 1 meters. In the Mississippi River plume regions these same reflectance values would represent sediment concentrations that are about ten times or more higher.

Hurricanes

As one would expect, the majority of these images reveal large increases in turbidity in the regions where a hurricane has made landfall. The increases are primarily due to sediments that have been resuspended from the shallow bottom regions. In areas near shore some of the signal may also be due to sediments eroded from beaches as well as from sediment laden river plumes. In some cases a post-hurricane phytoplankton bloom due to increased nutrient availability may perhaps be detectable.

The examination of the turbidity after the passing of a hurricane can have potentially many uses for coastal resource management including:

  • identifying regional "hot spots" where the erosion could be expected to be most severe
  • estimating the total sediment concentration that has been mobilized by the hurricane
  • determining the spatial extent of the sediment mobilization
  • identifying the extent and contribution of river plumes
  • assessing and predicting potential ecosystem impacts

With regard to these uses, determining the regions of high turbidity will allow managers to best decide on response strategies as well as help ensure that post-hurricane resources are most effectively utilized.

Interpreting images

Only a small fraction of the light incident on the ocean will be reflected and received by the satellite. The probability for a photon to reflect and exit the ocean decreases exponentially with length of its path through the water because the ocean is an absorbing media. The more ocean a photon must travel through, the greater its chances of being absorbed by something. After absorption, it will eventually become part of the ocean's heat reservoir. The absorption and scattering characteristics of a water body determine the rate of vertical light attenuation and set a limit to the depths contributing to a satellite signal. A reasonable rule of thumb is that 90 percent of the signal coming from the water that is seen by the satellite is from the first attenuation length. How deep this is depends on the absorption and scattering properties of both the water itself and other constituents in the water. For wavelengths in the near infrared and longer, the penetration depth varies from a meter to a few micrometers. For band 1, the penetration depth will usually be between 1 and 10 meters. If the water has a large turbidity spike below 10 meters, the spike is unlikely to be seen by a satellite.

For very shallow clear water there is a good chance the bottom may be seen. For example, in the Bahamas, the water is quite clear and only a few meters deep, resulting in an apparent high turbidity because the bottom reflects a lot of the band 1 light. For areas with consistently high turbidity signals, particularly areas with relatively clear water, part of the signal may be due to bottom reflection. Normally this will not be a problem with a post-hurricane turbidity image since the storm easily resuspends enough sediment such that bottom reflection is negligible.

Clouds are also problematic for the interpretation of satellite derived turbidity. Cloud removal algorithms perform a satisfactory job for pixels that are fully cloudy. Partially cloudy pixels are much harder to identify and typically result in false high turbidity estimates. High turbidity values near clouds are suspect.

Note: The information in this page has been incorporated from NOAA, allowable under United States fair use laws. Original source of the information is at http://www.csc.noaa.gov/crs/cohab/hurricane/turbid.htm

See also


Wikimedia Foundation. 2010.

Игры ⚽ Поможем написать реферат

Look at other dictionaries:

  • Ocean nourishment — See also: Iron fertilization Ocean Nourishment is a type of geoengineering based on the purposeful introduction of nutrients to the upper ocean [1] to increase marine food production [2] and to sequester carbon dioxide from the atmosphere.… …   Wikipedia

  • Turbidity — standards of 5, 50, and 500 NTU Turbidity is the cloudiness or haziness of a fluid caused by individual particles (suspended solids) that are generally invisible to the naked eye, similar to smoke in air. The measurement of turbidity is a key… …   Wikipedia

  • Ocean chemistry — A diagram showing ocean chemistry around deep sea vents Ocean chemistry, also known as marine chemistry, is influenced by turbidity currents, sediments, pH levels, atmoshperic constituents, metamorphic activity, and ecology. The field of chemical …   Wikipedia

  • turbidity current — Geol. a turbid, dense current of sediments in suspension moving along the slope and bottom of a lake or ocean. Also called density current. * * * Underwater current of abrasive sediments. Such currents appear to be relatively short lived,… …   Universalium

  • turbidity current — turbid′ity cur rent n. gel a turbid, dense current of sediments in suspension moving along the slope and bottom of a lake or ocean …   From formal English to slang

  • turbidity current — Geol. a turbid, dense current of sediments in suspension moving along the slope and bottom of a lake or ocean. Also called density current …   Useful english dictionary

  • Atlantic Ocean — an ocean bounded by North America and South America in the Western Hemisphere and by Europe and Africa in the Eastern Hemisphere. ab. 31,530,000 sq. mi. (81,663,000 sq. km); with connecting seas ab. 41,000,000 sq. mi. (106,100,000 sq. km);… …   Universalium

  • Northwest Atlantic Mid-Ocean Channel — A map of the NAMOC with its major tributary IMOC.[1] The Northwest Atlantic Mid Ocean Channel (NAMOC) is the main body of a turbidity current system of channels and canyons running on the sea bottom from the Hudson Strait, through the Labrador… …   Wikipedia

  • Freshwater environmental quality parameters — are the natural and man made chemical, biological and microbiological characteristics of rivers, lakes and ground waters, the ways they are measured and the ways that they change. The values or concentrations attributed to such parameters can be… …   Wikipedia

  • Eutrophication — The eutrophication of the Potomac River is evident from its bright green water, caused by a dense bloom of cyanobacteria. Eutrophication (Greek: eutrophia healthy, adequate nutrition, development; German: Eutrophie) or more precisely… …   Wikipedia

Share the article and excerpts

Direct link
Do a right-click on the link above
and select “Copy Link”