Understanding the Structure and Role of the Turbulent Boundary Layer in Hurricane Intensity Change

The hurricane boundary layer or HBL (traditionally, surface to ~ 1 km height) is of prime importance for vulnerable coastal communities, making accurate forecasts of storm intensity and understanding the basic dynamics of the system. Measurements of this intense and dangerous layer are difficult to make and have relied heavily on instruments dropped from aircraft (“dropsondes”). While valuable sources of wind information, the Lagrangian nature of the dropsonde (not a vertical profile) and coarse horizontal sampling significantly hinders the characterization of winds in hurricanes and limits our understanding of the storm physics. Conventional airborne radars, such as the tail Doppler radar, are very valuable, but they cannot measure the turbulent scales of motion that are essential to the HBL.

New algorithms and analysis of data from the Imaging Wind and Rain Airborne Profiler (IWRAP) can provide the full structure of the HBL at turbulence-resolving scales, filling a crucial measurement gap. In this presentation, the three-dimensional structure of the HBL from IWRAP data in several intense storms from recent hurricane seasons will be presented with a grid spacing of 125 m/30 m in the horizontal/vertical dimensions.

The focus of the analysis will be on characterizing the kinematics of coherent turbulent structures (CTSs) in the HBL across a range of storm intensities and intensity trends. These structures are believed to play an important role in the nonlinear dynamics of the HBL and the larger scale vortex through the cycling of enthalpy, momentum and kinetic energy as described in recent papers from the Geophysical Fluid Dynamics Group (GFDG) at Hampton University. Full TKE budgets using the IWRAP measurements also show that typical assumptions from boundary layer theory (steady-state, horizontal homogeneity and purely dissipative) do not apply to the HBL.