Dynamics of Eyewall Contraction: Deep Convective "Left-Movers"

Eyewall contraction is a well-known phenomenon that typically occurs in concert with tropical cyclone (TC) intensification. However, little is known about the dynamic mechanisms that facilitate the reduction in the radius of maximum winds (RMW). In this study, we argue theoretically that individual convective elements initiating near the eyewall should propagate left of the mean tangential flow, radially inward toward the center of the TC, even with no consideration of the secondary TC circulation. In support of this argument, we provide observational and cloud-resolving modeling evidence of intense updrafts that propagate with a radially-inward component. Results suggest these convective elements, or “left-movers” with respect to the mean tangential flow, could play an active role in eyewall and RMW contraction.

The notion that updraft-scale convection may propagate with a component transverse to the mean flow is not new. Evidence strongly suggests that cyclonic supercell thunderstorms deviate from their mean environmental flow, always to the right of the environmental vertical shear vector. The deviant motion stems from the tilting of horizontal vorticity associated with the environmental vertical shear. This tilting favors further updraft growth on the flanks transverse to the shear vector. Here we adapt the basic theory of shear-updraft interaction to the TC, where the vertical shear of interest is that of the mean tangential vortex (we disregard environmental shear). Results suggest that convective elements initiating near the eyewall will deviate to the right of the vertical shear vector, i.e. toward the TC center, via analogous dynamic processes to those occurring in midlatitude supercells that deviate from their mean flow.

Forecasting implications of these results are nontrivial. To simulate these left movers in a realistic way, mesoscale models used to forecast TCs must resolve nonlinear interactions between updrafts and vertical shear. At present, the operational TC models parameterize convection and utilize horizontal (and vertical) resolution that is too coarse to capture updraft-scale processes realistically. Until these constraints are lifted, it is possible that potentially important eyewall dynamics may be poorly represented.