Air-Sea Interactions and Boundary Layer Processes in the Hurricane Inner-Core during Intensity Change

Atlantic basin GPS dropwindsonde data from aircraft reconnaissance missions (Hock and Franklin 1999) between 1998 and 2015 are gathered to construct inner-core boundary layer representations of hurricanes undergoing intensity change. Sounding information within the lowest 2.5 km of the atmosphere are gridded to an axisymmetric radius-height space, which is normalized using radii of maximum winds (RMW) detected with Stepped Frequency Microwave Radiometry data (SFMR; Klotz and Uhlhorn 2014). Dropsonde data are separated into three groups based on intensity change: intensifying, weakening, and steady-state. 12,045 dropsondes from 50 Atlantic hurricanes were collected from the Hurricane Research Division’s storm pages archives, quality controlled in NCAR’s atmospheric sounding processing environment software (ASPEN), and filtered against our composite requirements to yield our dataset containing 3,270 dropsondes.

Analyses of storm-relative winds reveal familiar features, including near-logarithmic wind speed profiles below low-level jets (Franklin et al. 2003; Powell et al. 2003); strong, frictionally-driven near-surface inflow layers that penetrate the eyewall region (Montgomery et al. 2006; Zhang et al. 2011, 2013); and outward sloping of the eyewall with height (Stern and Nolan 2009; Hazelton et al. 2013; Stern et al. 2014). These kinematic analyses also depict differences between composite groups, seen most notably in the horizontal and vertical extent of the low-level jet, near-surface inflow maxima, and the radial profile of tangential wind. Composited thermodynamic fields depict the warm-core nature of hurricanes, nearly moist adiabatic conditions outside of the eye and above the frictional inflow layer, and the reservoir of moist static energy in the low-level hurricane eye.

We discuss the role of surface turbulent fluxes in establishing apparent profiles of entropy (dry and moist) and momentum in the near-surface inflow, and the implications of these processes in hurricane intensity change. We supplement our observational findings using preliminary results from a 7-day simulation of Hurricane Irma using the Advanced Research Weather Research and Forecasting model (WRF-ARW; Skamarock et al. 2008), which we use to examine the air-sea processes occurring in the simulated hurricane boundary layer.