Ocean Turbidity

What is Turbidity?

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 black water lake, is very dark. The scattering particles that cause the water to be turbid can be composed of many things, including phytoplankton, sediments, and organic by-products.

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 nm, 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 surbtracted 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 infra-red 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. If necessary, a color could be assigned to the aerosols to increase the aerosol signal in band 1.

Why is Turbidity Important?

River Plumes

Many rivers pick up sediments as they travel to the ocean. Because sediments are essentially scattering particles, the river plume will be apparent in the turbidity image. This can be useful for determining where the river water is going, information particularly important in the event of a toxic spill upriver. Because the sediments eventually settle out, turbidity data can also be used to estimate the location and amount of sediment deposition expected in channels. This information could be of interest during high water periods after major storm systems pass.

Sediment Resuspension

In some shallow areas, such as Florida Bay, high winds regularly cause enough vertical mixing to resuspend the bottom sediments. Such events are clearly visible on the turbidity image and could be important in assessing the potential impact of toxics adhering to the bottom sediments.

Sea Grass Beds

In general, sea grass beds decrease the turbidity signal because they absorb light that could have reflected from the bright bottom and into the satellite's view. Using a time series of turbidity images, it should be possible to track changes in the sea grass coverage. Areas that have lost significant sea grass coverage will show increased turbidity over time.

Cautions

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 eventaully 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.

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 turbidty estimates in normally low turbidity areas and near clouds are suspect.