An assessment of marine atmospheric boundary layer roll detection using Sentinel-1 SAR data

Abstract

The ability of high-resolution synthetic aperture radar (SAR) to detect marine atmospheric boundary layer (MABL) roll-induced roughness modulation of the sea surface wave field is well known. This study presents SAR measurements of MABL rolls using global coverage data collected by the European Space Agency's C-band Sentinel-1A satellite in 2016–2017. An automated classifier is used to identify likely roll events from more than 1.3 million images that were acquired at two incidence angles of 23° and 36.5° in either VV or HH polarization. Characteristics of the detected rolls are examined for different wind speeds, polarizations, incidence and relative azimuth angles. Roll detection counts are much higher at the higher incidence angle and nearly equivalent for VV and HH polarizations. Detection depends strongly on the relative azimuth with roll detection rates at crosswind being 3–10 times lower than for up- or downwind. All data show a low wind speed threshold near 2 m s−1 and that rolls are most commonly observed at wind speeds near 9 m s−1. For all viewing configurations, we find that rolls induce a wide range of mean surface wind speed modulation with the most frequent value being 8% (±3.5%). Roll detection at crosswind is associated with stronger roll-induced surface wind enhancement. Dependencies of roll detection on the incidence and relative azimuth angles are consistent with rapid short-scale wind-wave adjustments to the roll-induced surface wind gusts. These cm-scale waves are highly directional and provide limited crosswind backscatter at shallower incidence angles. The same roll-induced surface forcing is thus not equally detectable at all viewing geometries or polarizaions. Stronger and possibly longer-duration wind forcing is likely needed to produce detectable roll-induced modulations at crosswind.