Dynamics of Tropical Cyclone Intensification: Deep Convective Cyclonic “Left Movers”

Deep convective processes play an important role in tropical cyclone (TC) formation and intensification. In this study, the authors investigate the interaction between discrete buoyant updrafts and the vertically sheared azimuthal flow of an idealized TC vortex by adapting the updraft–shear dynamical framework to the TC. The authors argue theoretically that deep updrafts initiating near the TC radius of maximum wind (RMW) may propagate with a component left of the mean tangential flow, or radially inward toward the TC center. Results suggest that these unique TC updrafts, or “left movers” with respect to the mean azimuthal flow, may play an active role in TC intensification. The notion that updraft-scale convection may propagate with a component transverse to the mean flow is not at all new. Cyclonic midlatitude supercell thunderstorms often deviate from their mean environmental flow, always to the right of the environmental vertical shear vector. The deviant motion arises owing to nonlinear interactions between the incipient updraft and the environmental vertical shear. Although significant differences exist between the idealized TC considered here and real TCs, observational and high-resolution operational modeling evidence suggests that some intense TC updrafts may propagate with a radially inward and right-of-shear component and exhibit structural characteristics consistent with theory. The authors propose that left movers constitute a unique class of intense TC updrafts that may be favored near the TC RMW where local vertical shear of the TC azimuthal winds may be maximized. To simulate these left movers in a realistic way, mesoscale TC forecasting models must resolve nonlinear interactions between updrafts and vertical shear.