It is well known that the intensity and structure of a tropical cyclone is modulated by the environment in which it is embedded. Large-scale factors that include high mid-level moisture and weak vertical wind shear are well-known, necessary conditions for a tropical cyclone to form (Gray 1975). Whereas high vertical wind shear and intrusion of dry air into an incipient tropical cyclone is likely to inhibit further development, the effects of adverse environments on mature storms is less obvious although it is generally presumed that they would be detrimental to maintenance of intensity.

Numerical studies have clearly documented a relationship between simple structured, environmental vertical wind shear and subsequent simulated tropical cyclone intensity change (Frank and Ritchie 1999; 2001). These studies show that when unidirectional shear is applied to a simulated mature storm, weakening occurs (measured by a rise in the central pressure). Furthermore, the amount of and time lag until weakening occurs is directly related to the strength of the shear.

Preliminary calculations indicate that the environmental shear is rarely unidirectional, nor is it uniformly varying through the troposphere. More complicated vertical shear structure may have a more radical effect on the development of convection in the eyewall that appears to be so important for maintaining the tropical cyclone intensity. This presentation uses the PSU/NCAR mesoscale model (MM5) to explore how complex vertical shear affects tropical cyclone structure and intensity. Here, the idealized simulations of Frank and Ritchie (2001) are extended to more realistically represent the structure of vertical wind shear that is routinely observed in the tropical atmosphere. Two sets of simulations will be presented. The first set investigates the effect of applying an invariant, but realistically structured vertical shear to a tropical cyclone. The second set will examine the effect of varying the vertical shear with time: i.e., the effects of onset and release from vertical shear will be examined. Particular emphasis will be placed on model diagnostics that examine the physical changes in the core of the tropical cyclone.