Time and Space Scales in the Tropical Cyclone Boundary Layer

It is often convenient to assume that the boundary layer is in equilibrium with the forcing conditions, which typically consist of the characteristics of the underlying surface and the flow immediately outside of the boundary layer. Assuming that the boundary layer is “slaved” to the adjacent free flow and the surface in this manner enables useful simplifications for developing theory and understanding. Dimensional arguments suggest that the tropical cyclone boundary layer adjusts to changes in its forcing faster than the cyclone evolves, supporting such an assumption. Those arguments can be extended to show that the boundary layer only responds to features in the forcing larger than a certain scale; that is, that the boundary layer dynamics contain an implicit low-pass filter.

This talk will begin by using a time-dependent boundary-layer model to confirm the dimensional arguments, and to more thoroughly characterise the spatial filtering. The low-pass filtering property is shown to affect the radius at which the eyewall updraft forms, with implications for tropical cyclone intensification. It also affects the efficiency of a recently proposed positive feedback mechanism for secondary eyewall formation (SEF) in tropical cyclones, and thereby enables a prediction of which region of the storm is most favourable for SEF.