The Influences of Wind

Background:

When analyzing SAR images, a number of black streaks or blobs may show on the image, but it does not necessarily mean that it's oil. The influence of wind on ocean currents may cause black streaks on the image. When analyzing images for oil slicks, the differentiation from wind and oil can be a daunting task. Fortunately, data on wind speed and direction can help those analyzing SAR images in determining if the black spot on the image in question is anthropogenic. 

Sources of Wind Data:

One source of wind data is derived from satellites data to determine wave roughness.

In order for NOAA and NESDIS to determine wind speed and direction, an Advanced Scatterometer (ASCAT) on a satellite, and a Geophysical Model Function (GMF) are utilized.1, 3 The results of these processes are reported online, here.

Knowledge about ASCAT and GMFs is important in order to more effectively use this resource. ASCAT is a tool installed onto a satellite; this device sends radio signals towards the surface of the ocean then detects and measures the energy of the reflected signal.2 If the wind is stronger in an area then surface roughness will be higher and, as a result, more signal will return to the satellite. On the other hand, still ocean waves will result in a poor return of radio signal. This detection method produces a Normalized Radar Cross Section (NRCS) used in determining more about the wind direction and speed. The NRCS, is utilized in a GMF, which produces the wind speed and direction, for a single plot. Many of these calculations are congregated at, STAR, Center for Satellite Application and Research, which offers a map of global wind profiles.

Wind speed and direction of Hurricane Katrina in the Gulf of Mexico; Image Courtesy of NASA



Another source for wind data is the National Data Buoy Center(NDBC), which offers oceanic data, such as depth, wind speed, wind direction and current. The NDBC reports data from buoys from around the world, but is especiially concentrated along the coast of the united states and The Gulf of Mexico. Each buoy measures different parameters, wind speeds and directions, ocean currents and directions, and so on. 

How to use this when looking for oil:

ASCAT imagery works similarly to SAR imagery, what we use to detect slicks on the surface of water. Oil and other surfactants, for the same reasons as still water, return very few radio signals. On a SAR image both still water and oil display as black. Sometimes there is so much interference of still water, that it is difficult to identify streaks of oil because identification of oil slicks is very dependent upon contrast. However, utilizing maps of wind speed and direction can help determine if the area being searched for an oil slick was under the influence of still water, storm systems, or even everyday wind fronts. 

The most important detail when preforming these comparisons, is to be sure that the dates are the same. Oceanic wind patterns are relatively constant, however, day to day, in some areas, the wind direction is not.

Some things to keep in mind:

Satellite calculations of wind speed carries some assumptions. Wind speed data recorded at the shoreline may be different than the actual wind speed. Wind speed in open areas of the ocean increase exponentially every ten meters. Interference from land masses causes the wind to increase at a different rate, for this reason, the actual wind speed is inaccurate near the coast.3 The coastal wind speed in this image claims to be higher than open ocean, this is probably because of the wave turbidity near the coast (remember that the satellite detects wave roughness). 

Patrick Barnett

-Skytruth



References:

1. http://manati.orbit.nesdis.noaa.gov/products/ASCAT.php

2. http://visibleearth.nasa.gov/view.php?id=15397

3. http://www.nutech.dtu.dk/sitecore/content/Risoe_dk/Home/business_relations/Products_Services/Software/VEA_windmaps/GMF.aspx?sc_lang=en



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