Observed Difference in the Axisymmetric Boundary-Layer Structure between Intensifying and Steady-State Hurricanes

In this study, we investigate structural differences in the low-level (<2 km) structure (i.e., the boundary layer) of intensifying (IN) and steady-state (SS) hurricanes by compositing the dropsonde and Doppler radar data collected by NOAA's research aircraft from multiple storms. The IN data sets are from hurricanes with intensity increase of at least 20 kt in 24 h from the initial time, while the SS data sets are from hurricanes with intensity change less than 10 kt in 24h. The analyses show that IN hurricanes have a deeper and stronger inflow layer, moister air in the eyewall above the inflow layer, and more unstable surface layer than SS hurricanes. In addition, inertial stability outside the radius of the maximum wind speed is found to be much smaller in IN hurricanes than that in the SS hurricanes. Reasons for these structural differences in the boundary layer between IN and SS hurricanes are discussed.