A shear reduction mechanism for tropical cyclogenesis

Vertical wind shear is one of the primary inhibitors to tropical cyclogenesis in the presence of a lower-level percursor vortex. Discussions concerning the strength and evolution of the near-storm environmental shear are common in studies investigating both tropical cyclone formation and intensity changes. This work presents a rapid shear reduction mechanism involving anticyclonic wave breaking at low latitudes. An understanding of this mechanism is important because the evolution of the near-storm environmental shear is quick -- occurring on time scales of six to 12 h -- and can therefore lead to rapid changes in cyclone development or intensity.

A conceptual model of the anticyclonic wave breaking mechanism is presented in conjunction with a case study of Hurricane Alex (2004). It is shown that the rapid shear reduction near a low-level precursor combined with broad cyclonic vorticity in the near-storm environment to allow the "surprise" development and intensification of Hurricane Alex near the Eastern Seaboard. Given the storm's impact on the Outer Banks, such rapid evolution is shown to be potentially dangerous when it occurs near the North American continent. A climatology of similar events identifies other systems for which an anticyclonic wave break has led to either the development of intensification of the tropical vortex, demonstrating the general applicability of this rapid shear reduction mechanism.