Simulated Azimuthal Structure of the Hurricane Boundary Layer in Hurricanes Irma (2017) and Earl (2010) during Intensity Change

Observational studies have shown that the hurricane boundary layer (BL) can possess distinct kinematic and thermodynamic structures specific to shear-relative position (e.g., Zhang et al. 2013) and intensity change (e.g., Ahern et al. 2019). These preceding studies investigated azimuthal structure and structure relative to intensity change seperately (i.e., the latter study examined azimuthal-mean BL structure). We complement these prior works with high-resolution, full-physics numerical case studies of Hurricanes Irma (2017) and Earl (2010), which are used to analyze the complete 3-D structure of the hurricane BL during intensity change. The Weather Research and Forecasting Model for Advanced Research (WRF-ARW) is employed for these simulations.

In our simulation of Hurricane Irma, strong tangential winds were relatively confined to the RMW, associated with weaker inertial stability outside the eyewall. Aside from land interactions, Irma tended to steadily intensify, with an inflow maximum at the radius of maximum winds (RMW) and BL ascent isolated inward of the RMW. A brief (approximately 6-hour) weakening period was associated with shear- and motion-induced asymmetry, whereby drier air was able to descend into the BL inflow near the RMW.

Hurricane Earl’s simulation conveyed a broader tangential wind field, with moderately high inertial stability well outside the RMW. Earl's strong BL inflow spread over a large radial range, which was associated with widespread BL convergence and shallow ascent outside the RMW. During a prolonged decay in Earl's intensity, two regions of BL convergence became apparent: one inward of the RMW, and the other well outside the RMW. Descent of low-enthalpy air into the BL near the RMW occurred during Earl's weakening phases. Despite shear and storm motion of comparable magnitudes to Irma, asymmetries were far more pronounced in Earl's BL. Earl's decline in intensity was also associated with strong low-level outflow in the upshear-right quadrant, which may have led to kinematic and thermodynamic evolution that promoted an outer region of BL convergence, as well as an inner-eyewall collapse and coincident secondary eyewall formation.

The results generally corroborate prior observational studies (e.g., stronger BL inflow downshear and downstream of storm motion, low-entropy BL air immediately outside the RMW during non-intensification), although some exceptions do occur (e.g., strongest inflow is not always in the downshear-right quadrant, storm weakening is not always associated with strong BL convergence outside the RMW). New results show how 3-D hurricane BL mechanisms can affect internal dynamics and thermodynamics, vortex-scale structure beyond the BL itself, and storm intensity.