Diagnosis of Secondary Eyewall Formation Mechanisms in Hurricane Igor (2010)

Despite being a common feature of major hurricanes and exhibiting a strong relationship with storm intensity change, there exists no unified theory explaining secondary eyewall formation (SEF) in tropical cyclones (TCs). In recent years, a number of hypotheses have been proposed for SEF with the most substantive difference among them being the relative roles of internal dynamics and external, environmental forcing. Examples of the former involve vortex Rossby wave-mean flow interaction, anisotropic upscale energy cascade, and unbalanced boundary layer (BL) spinup, while examples of the later include wind-induced surface heat exchange being triggered by external forcing and environmental humidity controlling storm size and rainband structure. This last factor, the moisture distribution and rainband activity outside the TC core, appeared to be important in our previous research examining ensemble forecasts of Hurricane Igor (2010): ensemble members that undergo SEF exhibit a more uniform moisture distribution relative to the members that do not, with the moisture originating in active outer rainband convection upshear of the center. The rainbands flood the boundary layer with low moist static energy air, which propagates cyclonically around the center as a cold pool. We hypothesized that enhanced convergence between the cold pool and low-level inflow reinvigorated the rainband convection that became the secondary eyewall.

To test our rainband reinvigoration hypothesis, as well as the other SEF hypotheses noted above, we will analyze the 16 (out of 96) ensemble members of the NCAR Advanced Hurricane WRF that exhibited SEF in Hurricane Igor (2010) at high spatial (1.33 km) and temporal (10 min) resolution. Specifically, we will: 1) diagnose the role of vertical wind shear in determining the distribution and evolution of moisture and outer rainbands, 2) calculate moist static energy and angular momentum budgets, 3) investigate the existence of vortex Rossby waves and whether they interact with the mean vortex at their stagnation radius, and 4) look for the hallmarks of the unbalanced BL spinup paradigm, e.g., the generation of supergradient winds in the BL, strengthening of the BL inflow, and an eruption of air from the BL to support convection.