Why is the Tropical Cyclone Boundary Layer not Well-Mixed?

Plausible definitions for the tropical cyclone boundary layer include (i) the depth of the inflow layer and (ii) the depth over which potential temperature is approximately constant. Observations show that these two candidate definitions are markedly different in practice, with the inflow layer being roughly twice the depth of the well-mixed layer. This discrepancy is a hindrance to our understanding, and has a potentially large impact on those boundary layer parameterisations that rely on the diagnosis of the boundary layer depth. Here, we will present an analysis of the thermodynamics of the tropical cyclone boundary layer derived from an idealized axisymmetric model, and demonstrate why the two depths are different. We show that the marked dry static stability in the upper part of the inflow layer is largely due to diabatic effects. As the radial wind varies strongly with height, so does the radial advection of the potential temperature, which also contributed to the stabilization of the boundary layer. Additionally, this differential vertical advection of stability contributes to the observed superadiabatic layer adjacent to the ocean surface, where the vertical gradient of the radial wind is reversed. However, this unstable layer is mainly due to heating from turbulent dissipation. The results will be compared with dropsonde observations, and implications for boundary layer parameterisation will be discussed.