Observed Features of the Hurricane Inner-core Boundary Layer 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 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. To the best of our knowledge, these differences found in the hurricane boundary layer are new in observational literature. Questions and potential ramifications of these findings---for example, on radial convergence, convection, and possibly intensification---are discussed.

Composited thermodynamic fields illustrate plainly 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. Static stability is variable across composite groups, especially near the top of the frictional inflow layer, in the eyewall region, and in the low-level eye. Equivalent potential temperatures in the hurricane eye and in the lowest 100 m are markedly different between groups. In the context of intensity change, apparent kinematic and thermodynamic signals are broken down to explain physically the differences between composite groups. Hopefully, these explanations will provide guidance and insight for future endeavors to model or otherwise understand the intertwined behavior between a hurricane and its boundary layer.