Why is the tropical cyclone boundary layer not "well-mixed"?

Plausible diagnostics for the top of the tropical cyclone boundary layer include (i) the top of the layer of strong frictional inflow and (ii) the top of the “well-mixed” layer; that is, the layer over which potential temperature θ is approximately constant. Observations show that these two candidate definitions give markedly different results in practice, with the inflow layer being roughly twice the depth of the layer of nearly constant θ. Here, we will present an analysis of the thermodynamics of the tropical cyclone boundary layer derived from an axisymmetric model. We show that the marked dry static stability in the upper part of the inflow layer is due largely to diabatic effects. The radial wind varies strongly with height, and therefore so does radial advection of θ. This process also stabilizes the boundary layer, but to a lesser degree than diabatic effects. We also show that this differential vertical advection contributes to the observed superadiabatic layer adjacent to the ocean surface, where the vertical gradient of the radial wind is reversed, but that the main cause of this unstable layer is heating from turbulent dissipation. The top of the “well-mixed” layer is thus distinct from the top of the boundary layer in tropical cyclones. The top of the inflow layer is a better proxy for the top of the boundary layer, but is not without limitations. These results may have implications for boundary-layer parameterisations that diagnose the boundary layer depth from thermodynamic, or partly thermodynamic, criteria.