Latitudinal Dependence of Marine Boundary Layer Roll Orientation Detected by Synthetic Aperture Radar
Abstract
Satellite synthetic aperture radar (SAR) images of the sea surface detect variability in the cm-scale ocean surface wave field at meters-scale resolution. Surface waves at these wavelengths are quite sensitive to the local surface wind vector field and the surface imprint of the lower marine boundary layer (MBL) medium and larger scale turbulent eddies can often be detected in SAR images. In near-neutral stratification the MBL turbulence tends to spontaneously generate quasi-equilibrium organized large eddies in the form of periodic roll vortices. The basic characteristics of these rolls are that they align at small angles to the surface wind direction and have cross-wind wavelengths of 2 to 6 times the MBL depth. The rolls generate an overturning circulation in the cross-roll plane and induce a strong modulation of the low-level wind along the direction of the roll axes. This approximately along-wind modulation of the surface wind and its modulation of the cm-scale surface wave field is the signal that is detected by SAR. We examine the characteristics of detected rolls extracted from a vast and nearly global dataset of Sentinel-1A,B Wave Mode imagery. One surprising result is that the roll orientation relative to the mean surface wind direction changes sign at approximately 30 deg. latitude in either hemisphere. Possible physical mechanisms for this effect will be presented.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.A31D..07F
- Keywords:
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- 3307 Boundary layer processes;
- ATMOSPHERIC PROCESSES;
- 3322 Land/atmosphere interactions;
- ATMOSPHERIC PROCESSES;
- 3379 Turbulence;
- ATMOSPHERIC PROCESSES;
- 0426 Biosphere/atmosphere interactions;
- BIOGEOSCIENCES