Diagnosis of Secondary Eyewall Formation Mechanisms in Ensembles of High-Resolution Hurricane Simulations

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 convective 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 are uniformly drier relative to the members that do not, leading to significant surface fluxes that fuel anomalous convection downshear. The convection lofts ice that is advected cyclonically, saturating the upshear side of the storm, and flushes cool, dry air into the low levels right of shear. As low-level dry air rotates cyclonically, we hypothesized that enhanced convergence between the cold pool and low-level inflow initiated inner rainband convection that became the secondary eyewall.

To test our hypothesis, as well as the other SEF hypotheses noted above, we will analyze another SEF case, Hurricane Earl (2010), using the NCAR Advanced Hurricane WRF 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 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.