Constructing a climatology of Marine Atmospheric Boundary Layer Roll Vortices Using Sentinel-1 SAR Vingettes

Roll vortices that are approximately aligned with the mean wind are a
common feature of the turbulent atmospheric boundary layer (ABL) in
near-neutral to moderately unstable stratification. These conditions
are frequently found in the marine ABL (MABL), which is the focus of
this study. The organized secondary circulation induced by the roll
instabilities usually spans the depth of the MABL and contributes a
non-local contribution to the MABL fluxes of momentum, heat and water
vapor. Parameterizing the contribution of rolls to the MABL fluxes
requires numerous observations in order to characterize the structure
of these rolls and both the MABL conditions under which they form and
for which they do not form. Although the physics of rolls are
becoming better understood through observations, numerical and
theoretical modeling, at present there is no global roll
climatology. This is due to a lack of global all-weather,
all-condition sensors of sufficiently high resolution.

In this study we use very high resolution synthetic aperture radar
(SAR) imagery of the sea-surface to detect the impact of rolls on the
surface cm-scale wavefield. While MABL roll detection in SAR has a
history dating back to Seasat, coverage and image quality have proven
to be serious obstacles. When present, rolls usually induce a
quasi-periodic pattern in the sea surface roughness that can be
detected by SAR. The Sentinel-1 constellation (S-1A, S-1B) launched by
European Space Agency (ESA) in April of 2014 and 2016 routinely
collects ∼120,000 (20 x 20 km, 5 m pixel) SAR wave mode vignettes (WV)
in each month globally over the ocean. These high quality SAR images
provide routine and detailed sea surface imprints of many geophysical
phenomena in nearly all weather conditions at two incidence angles at
sampling intervals of 200 km. The broad coverage of the S1 WV data
provides a new and unique opportunity to characterize MABL rolls at
global scale. We will present the development of our automated
geophysical classification model and a first look at the frequency of
roll occurance and the concomitant MABL conditions for the year 2016.