Observed Atlantic Hurricane Boundary Layer Structural Asymmetries with Respect to Environmental Conditions

Asymmetries in the tropical cyclone boundary layer (TCBL) with respect to environmental vertical wind shear are examined for hurricanes of variable size, motion, strength, and intensity. GPS dropsonde data from reconnaissance aircraft is used in tandem with Doppler radar data to investigate several hurricanes with varying environmental shear and storm characteristics. Radial and shear-relative azimuthal profiles of kinematic and thermodynamic fields will comprise the bulk of the analysis. Analyses are limited to hurricanes, as azimuthal and radial profiles lose applicability in analyses of storms that do not have near-axisymmetric structure, such as less organized tropical cyclones (TCs) of sub-hurricane intensity.

This research expands upon the work of Zhang et al. (2013), which composited dropsondes from 19 hurricanes and found kinematic and thermodynamic asymmetries in the inner core TCBL between mean shear-relative quadrants. Namely, the deepest and strongest mixed- and radial inflow layers were observed in the downshear-right quadrant, with progressively shallower layers counter-clockwise until a minimum in the upshear-right quadrant. Zhang et al. (2013) posited that the asymmetries in the composite data are likely linked to convective asymmetries associated with environmental wind shear. Similarly, Riemer et al. (2010) proposed that vertical shear impacts the TC inflow layer through downdrafts in the convective asymmetry, importing air with low equivalent potential temperature into the boundary layer outside of the eyewall.

However, variation in TCBL asymmetry among different storms is less understood. DeMaria et al. (2005), Tang and Emanuel (2010), and Riemer et al. (2010) have noted the differing responses throughout TCs to the environmental wind shear depending on their strength and intensity. The difference in TC response to wind shear implies a dependency of TCBL asymmetry on TC strength and intensity. The examination of individual hurricanes—each with their own measures of strength and environmental influence—will provide further insight regarding TCBL structure, their response to the outside environment, and their relation to the storm in which it is contained.