Understanding Atypical Mid-Level Wind Speed Maxima in Hurricane Eyewalls

Hurricane Patricia (2015) and a few other recent intense tropical cyclones exhibit an atypical eyewall wind profile, with a mid-level (4-5 km) local maximum in wind speed in addition to the characteristic maximum near the top of the boundary layer (0.5-1 km). In ongoing work, we have used idealized axisymmetric simulations with the CM1 model to reproduce this atypical structure and to show that the dual maxima are manifestations of unbalanced tangential wind jets, which are ultimately a consequence of surface friction acting on what would otherwise be a quasi-balanced vortex. The strength of both the boundary layer and mid-level jets are related to the storm intensity and size, as well as the magnitude of the vertical turbulent diffusivity. Calculations using an idealized boundary layer model reproduce this sensitivity, and confirm the hypothesized mechanism by which a vertical oscillation in the radial winds and unbalanced flow results in multiple maxima in tangential winds.

Here, we further extend our axisymmetric results to more realistic three-dimensional simulations, using both idealized configurations with CM1, and real data simulations using COAMPS-TC. We also show that above the lowest few hundred meters in simulated TCs, it is possible to estimate the strength of the eyewall supergradient jets with reasonable accuracy, given only information on the structure of the tangential, radial, and vertical winds. We hypothesize that this could be potentially useful in estimating observed supergradient flow (as well as the gradient wind itself) from Doppler radar retrievals of the wind structure. Finally, we use the existing Doppler wind dataset of Stern et al. (2014) to examine the relationship between the mid-level wind profiles and storm intensity and size.