Observations of Supergradient Winds in the Tropical Cyclone Boundary Layer

Recent studies have highlighted the importance of boundary layer dynamics in tropical cyclone (TC) intensification and structure change. Numerical models suggest that a supergradient jet at the top of the boundary layer (Kepert and Wang 2001) associated with boundary layer convergence and forcing of deep convection may play a critical role in the formation of secondary eyewalls (Huang et al. 2012). Additional TC boundary layer (TCBL) phenomena such as turbulent kinetic energy, horizontal rolls, and momentum fluxes are also important for TC evolution. However, kinematic fields with adequate spatial resolution to resolve TCBL features are difficult to obtain due to limitations in our observation capabilities.

Airborne Doppler radar, dropsondes, flight level in situ sensors, and Stepped Frequency Microwave Radiometers (SFMR) can make measurements in the TCBL, although each of these has its own limitations. A new spline-based, 3-D variational analysis technique called SAMURAI is used to retrieve high-resolution low-level TC winds and pressure gradients from these data as close to the surface as possible. An error analysis and calculated supergradient winds will also be presented using analytic data and some preliminary results from the RAINEX field campaign. The implications for TC intensification will be discussed.