Evidence for and a Theoretical Model of 10-km Wavelength Tropical Cyclone Boundary Layer Roll Vortices

As part of the ONR-sponsored Impact of Typhoons on the Pacific Ocean 2010 field experiment, new methods for retrieving km-scale surface wind vector fields from satellite synthetic aperture radar (SAR) imagery of the sea surface under tropical cyclones were developed and validated. The quality of the SAR wind vector fields is sufficiently good that meaningful derivative fields can be calculated. Throughout all of the SAR images that were processed, we found a clear signature of alternating bands of surface wind convergence that are roughly aligned with the mean flow. The largest features are believed to be associated with the primary rain bands, however an obvious 8 to 10-km wavelength signal was nearly ubiquitous. Although reminiscent of well-documented and theoretically explained hurricane boundary layer roll vortices that have wavelengths around 1.5 km, the wavelength (and hence aspect ratio) of these features is much larger. In this paper we present a non-linear wave-wave interaction model that may explain how the smaller-scale roll vortices can transfer energy into the larger aspect ratio roll vortices. Implications for PBL parameterization and air-sea interaction will be discussed.