Mapping Surface Oil Extent From the Deepwater Horizon Oil Spill Using ASCAT Backscatter

The 2010 Deepwater Horizon oil spill in the Gulf of Mexico covered a sufficiently large area to be observed by the European Space Agency Advanced Scatterometer (ASCAT) on MetOp-A. In this paper, ASCAT data and numerically computed winds from the European Centre for Medium-Range Weather Forecasts (ECMWF) are used to map the spatial extent of oil on the ocean surface over the duration of the spill event. Surface oil alters the ocean radar scattering properties, resulting in a difference between the measured backscatter and the backscatter that would be measured if oil were not present. Wind scatterometers infer the near-surface wind speed and direction using the wind geophysical model function (GMF) in conjunction with measured radar backscatter. The oil-altered backscatter error propagates through the wind retrieval process to create a difference in ASCAT-inferred winds and actual winds. Numerically computed vector winds from ECMWF are compared against ASCAT-inferred vector winds. The GMF is applied to ECMWF winds to create a predicted backscatter value to compare against ASCAT-measured backscatter. Large differences in wind or backscatter indicate areas of the ocean surface affected by oil. An objective function is developed to choose an appropriate threshold level to flag the oil-contaminated regions. Data from other sensors corroborate the ASCAT oil extent mapping.