Can dual-frequency altimetry be used to identify surface slicks?

David Woolf, Susanne Ufermann
Southampton Oceanography Centre, Southampton, UK, SO14 3ZH
dkw@soc.soton.ac.uk, ufermann@soton.ac.uk

ABSTRACT

Surface slicks are characterised by exceptionally low decimetre-scale and shorter surface roughness. They are a common feature of the sea surface with many effects on air-sea exchange and can also be indicators of pollution events or natural phytoplankton blooms. They are readily apparent to even a casual observer in situ and also appear in swath radar imagery. Here, we present a preliminary investigation of slick features (i.e. very low steepness of decimetre-scale waves) apparent in dual-frequency radar altimeter data.

Data from satellite altimeters include a measurement of maximum backscatter, «sigma-0, σ_o». Radar backscatter at nadir incidence over natural waters is most commonly related to wind speed, but is affected by a number of factors. In the absence of excessive atmospheric absorption (e.g., heavy rain), the backscatter should be inversely proportional to the mean square slope of surface waves, with an effective «low-pass filter» to waves longer than a critical wavelength that depends on the electromagnetic frequency of the radar. The steepness of waves within a band-limited range can be calculated from the backscatter at two different frequencies.

Data are retrieved from the RADS database (http://www.deos.tudelft.nl/altim/rads/). Only TOPEX data were used in this study though dual-frequency data is now available also from JASON and the RA2 altimeter on Envisat. Data come in the form of one-second averages for each parameter, corresponding to a surface track length of 6km. The effective footprint diameter may be as little as 2km (the theoretical limit for a flat sea) but increases to 5.5km for a 3-metre swell. A few areas and seasons are selected in order to cover cases from very low to very high natural slick occurrence (as expected from a broad link to productivity/chlorophyll). Low surface slope corresponds to high backscatter for nadir incidence, and loci of high backscatter (both at Ku and C band) are apparent in TOPEX altimeter data at all sites. The theoretical low-pass limits of the TOPEX altimeters are surface waves of approximately 6cm and 16cm wavelength for Ku band and C band respectively. By subtracting the two estimates of low-pass-filtered mean square slope, the method is refined to detect occurrence of low amplitude decimetre-scale waves even in the presence of fairly steep longer waves. These are «candidate» slick areas. Any association of high concentrations of these loci to sites with high biological productivity is not immediately obvious. The averaging over 6km of track and the footprint diameter of a few km limit altimeters to the study of relatively large (>10km) slicks compared to the features often observed in SAR imagery. However, there are many instances where a number of consecutive values of inferred decimetre-scale wave steepness are very low, suggesting large slick features. However, as the association of these conditions with contributing environmental factors (e.g., high productivity) is unclear, we suggest that a firm conclusion that altimeters can detect large slicks reliably must await a study that includes coincident in situ observations.

Last Update: 2005-03-16