Mean Wind Profiles in the Boundary Layer of Hurricane Larry (2021) Derived from SAR and Compared to IWRAP Observations

Modern satellite C-band SAR on Sentinel-1 can measure co-pol (VV) and cross-pol (VH) normalized radar backscatter cross section (NRCS) for most of the inner core of tropical cyclones. From these data km-scale surface wind vectors can be retrieved with reasonable accuracy up to ~80 m s^-1 and can directly measure the orientation of the rolls that routinely form in the tropical cyclone boundary layer (TCBL). We use this unique data to infer the mean wind profiles within the TCBL. A NOAA NESDIS P-3 research flight was timed to under-fly a Sentinel-1 overpass of Hurricane Larry on Sep. 7, 2021. In addition to the standard instrumentation such as tail Doppler radar, flight level sensors and SFMR, the P-3 was also equipped with the IWRAP downward-looking conical scanning Ku-band radar. Hence, this flight provided an excellent dataset for advancing SAR research.

We use a simplified TC similarity boundary layer model to estimate a corresponding pressure gradient vector for each surface wind vector and employ ordinary least-squares to find a best fit surface pressure pattern corresponding to the SAR image. The surface pressure pattern is validated by comparing measured and derived pressure differences between all possible pairs of drop sondes. The RMS (~2 hPa) is much lower than typical, although the total drop sonde count was lower than typical. A field of gradient winds can be directly calculated from the pressure surface. Hence, the SAR provides both the upper and lower boundary conditions needed by the full similarity TC boundary layer model.

It is well-established that TCBL rolls are associated with large-scale shear instabilities that reach a quasi-equilibrium state. The basic characteristics of these rolls are fairly sensitive to the shape of the mean wind profiles, especially the cross-wind (largely radial) component. The roll orientation angle provides a sensitive metric that is easy to measure using SAR. For fixed boundary conditions the mean wind profiles are in turn sensitive to the vertical profile of eddy viscosity. Using mesoscale model parameterizations as guide, we survey a reasonable range of eddy viscosity formulations to maximize the agreement between predicted and measured roll orientations. Thes profiles are compared to IWRAP wind profiles. We also present a visualization of the roll perturbation winds extracted from IWRAP.