Boundary Layer Rolls Derived From SAR Wind Fields in Typhoon Megi

Due to extreme conditions, the boundary layer (BL) structure in tropical cyclones has mostly been analyzed with the help of models (physical parameterization). Nevertheless, a great amount of in situ and remote sensing data is available. ScanSAR modes of RADARSAT and COSMO SkyMed are very useful for tropical cyclone observations not only because of their large swath widths (up to 500 km), but especially due to their high spatial resolution of 100m. Throughout its BL a mature cyclone obtains heat and moisture and in return transfers momentum to the ocean surface. These conditions will lead to a BL turbulence, which forms roll vortices. In tropical cyclones these rolls are spatially periodic, secondary vertical transverse circulations and cover the depth of the BL. It has been shown that patterns of these roll vortices can be found in divergence fields of tropical cyclones derived from SAR imagery mean wind fields. We employed a common parameterization of the vertical eddy viscosity profile using the SAR surface winds as a low boundary condition to estimate vertical profiles of the radial and azimuthal winds in the BL. The azimuthal wind velocity indicates the position, size, and strength of the vortices (Figure 1). In between rolls we find a reduction in the surface wind below updrafts and an enhancement below downdrafts. In Figure 1 we find such a velocity reduction at about 40 and 60 km distance to the storm's eye. In between these two points the velocity is increasing which indicates a downdraft. As a result we suspect two interacting rolls between 40 and 60 km distance to the storm's center. Further examples and analysis of this work would be presented at the conference.