Boundary layer structure and dynamics beneath a translating tropical cyclone

Recent advances in observational technology have shown that the tropical cyclone boundary layer is considerably more complex than hitherto thought. Examples of interesting phenomena include the now well-known low level jet, and strong asymmetries in the radial and vertical flow components. Moreover, a good understanding of how the boundary layer works is essential to predicting near-surface winds from higher level data, as input to oceanic models and for damage modelling.

Here we present a linear analytic model of the tropical cyclone boundary layer. We show that the boundary layer flow can be understood as the superposition of three components; a symmetric one due to the cyclone itself, and two wave number one components due to the interaction of the cyclone with the underlying surface. The symmetric component has depth of one to two km in the periphery, decreasing towards the core. These two asymmetric components have distinct depth and amplitude scales, with one being shallower and much weaker than the symmetric component, and the other one being several times deeper, and of comparable strength.

The properties of these components will be discussed, particularly in light of what they reveal about the dynamics of the low-level jet, near-surface wind estimation, tropical cyclone asymmetries, and vertical motion forcing. Predictions from the linear model will also be compared to simulations using our previously presented high-resolution numerical model of the tropical cyclone boundary layer.