Observing the effects of roll-vortices on the Tropical Cyclone Boundary Layer (TCBL) using Synthetic Aperture Radars (SAR)

The Tropical Cyclone Boundary Layer (TCBL) is uniquely favorable for the formation of highly asymmetric coherent structures called roll-vortices. They are kilometer-scale eddies that are aligned in the mean tangential wind direction and are generally associated with a combination of shear and convective instability. Roll-vortices can cause periodic enhancements of winds as well as surface fluxes and may assist the intensification process via vertical transport of tangential momentum. Furthermore, the up-gradient transfer of surface heat and momentum fluxes associated with roll-vortices can energize the large-scale motions, possibly resulting in an increased convective activity. It is therefore important to understand and quantify the effects of roll-vortices in setting the overall TCBL structure as well as near-surface winds.

Synthetic Aperture Radars onboard low-earth orbit (LEO) satellites such as Sentinel-1A/B and RadarSat-2 have often been used to identify TCBL roll-vortices. Based on the positive correlation between microwave backscattering and sea surface roughness, sea surface wind speed can be retrieved at high spatial resolution (1–3 km), without any saturation for high wind speed. However, the spatial distribution of roll-vortices in the TCBL and their subsequent impact on the vertical momentum and enthalpy fluxes are not well understood.  In this study, we use standard two-dimensional spectral analysis to detect roll-vortices in SAR-observed winds over multiple tropical cyclones (TCs) and use TC-relative composites to analyze their spatial distributions. We will also study the vertical wind and thermodynamic structures in the vicinity of SAR-observed roll-vortices using collocated aircraft observations whenever available. This may also allow us to extract empirical relationships between the roll-averaged vertical profiles of near-surface fluxes and maximum near-surface winds.