The Dynamics of Tropical Cyclones in Vertical Shear: Insight from Theory and Idealised Models.
The detrimental effect of vertical shear on tropical-cyclone intensity is well known. The detailed mechanisms which bring about the intensity change are perhaps less well understood. Theoretical studies and idealised numerical modelling can be used to improve our understanding of these mechanisms. Many recent studies have based their interpretation on potential-vorticity thinking. In this framework the interaction between a tropical cyclone and environmental vertical shear can be viewed in terms of the interaction of different potential-vorticity anomalies. The structure of a tropical cyclone is characterised by a strong deep cyclonic potential-vorticity anomaly with a broad but shallow anticyclonic anomaly at upper levels. The environmental shear may be characterised by uniform potential vorticity if the shear is constant with height or may contain horizontal potential-vorticity gradients if the shear varies with height, for example in association with an upper-level jet. In either case the tropical-cyclone might be expected to respond to the vertical shear by developing a vertical tilt. In the presence of vertical tilts the vertical interaction of the potential-vorticity anomalies becomes important. The extent to which potential-vorticity anomalies interact in the vertical depends on the Rossby penetration depth. This increases with latitude, horizontal scale and intensity of the cyclone and decreases with environmental static stability.
When a tropical-cyclone enters an environment containing vertical shear both the motion and the structure of the tropical cyclone are affected. Idealised studies have investigated the role of the cyclonic core of the tropical-cyclone vortex, of the upper-level anticyclone and of horizontal potential-vorticity gradients in the environment. The structural changes which occur include vertical tilt of the cyclone, horizontal displacement of the upper-level anticyclone relative to the surface centre, distortion of the potential vorticity of the cyclone producing significant asymmetries and the development of asymmetric patterns of vertical motion and static stability. As discussed by DeMaria, these changes can influence the inner-core convection and so influence the intensity of the tropical cyclone.
Recent theoretical and idealised modelling studies have improved our understanding of both the motion and structural changes of tropical-cyclone-like vortices in vertical shear. However, there are many questions still unanswered which must be addressed in future studies. In particular, the implications of the structural changes discussed above for the inner-core convection and the influence of more complicated environmental shear profiles, e.g. where the shear vector rotates with height, deserve further study. The coordination of further idealised studies with observational work (see Velden) should improve our understanding of the role of vertical shear in tropical-cyclone intensity change.