Understanding Atypical Mid-Level Wind Speed Maxima in Hurricane Eyewalls

In tropical cyclones (TCs), the peak azimuthal-mean wind speeds are generally found within the boundary layer (z=0.5-1 km), and decrease monotonically with increasing height.  Recently, Stern et al. (2014) showed that in a few observed TCs, the azimuthal-mean wind speed in the eyewall actually increases with height over several kilometers, resulting in a secondary local maximum in the mid-troposphere (4-5 km).  Based on a correspondence with similar structures in idealized WRF simulations, it was hypothesized that such atypical wind structures are the consequence of systematic and quasi-steady unbalanced flow.

Here, we present additional evidence of this type of atypical structure, using high-density dropsonde observations from Hurricane Patricia (2015) gathered during the TCI field experiment, along with Doppler wind analyses from NOAA/HRD.  Near peak intensity, Patricia exhibited an absolute wind maximum at 5-6 km height, along with a weaker boundary layer local maximum.

Although Patricia does not seem to be unique, these atypical wind profiles are apparently only found in hurricanes that are very intense and/or very small.  To investigate the dynamics responsible for these profiles, and to understand why they are uncommon, we conduct idealized simulations using the CM1 model, and systematically examine the magnitude and distribution of unbalanced flow and its relationship to storm intensity and size.  We then use an idealized boundary layer model, to further quantify the modulation of frictionally-induced unbalanced jets by the imposed gradient wind profiles.  Finally, we use observational analyses of the gradient winds from Patricia as forcing for the boundary layer model, to determine if this simple model can qualitatively reproduce the atypical mid-level maximum seen in Patricia.