A Simple Model of the Tropical Cyclone Boundary Layer at Landfall

As a tropical cyclone approaches landfall, it progressively experiences asymmetric friction that directly modifies the boundary layer flow, since the land surface is rougher than the sea. An idealized model of the boundary layer flow within a stationary cyclone, located exactly on the coast, is presented. This model is linearized and utilizes simple representations of the turbulent fluxes. These simplifications enable analytical solutions, which represent the flow as the sum of three components: a symmetric component and two asymmetric components of azimuthal wavenumber 1, which rotate with height. The stronger asymmetric component rotates anticyclonically with height and the weaker asymmetric component rotates cyclonically. The solution is thereby similar to that for a moving cyclone over sea, except that (i) the symmetric component has larger amplitude because the azimuthal-mean surface roughness is greater and (ii) the amplitudes and phases of the asymmetric components are different because of the different surface boundary condition. The linear solutions are compared to simulations using a nonlinear model with more sophisticated representations of the turbulent fluxes, and it is shown how to extract the two asymmetric components from the flow in this latter model. This study complements the analysis of the observed wind asymmetry in Tropical Cyclone Veronica in a companion paper. Significance Statement Landfall is the time at which tropical cyclones generally are most dangerous, so understanding the processes that cause structure change during landfall helps to mitigate impact. We derive a simple model of the near-surface flow in a tropical cyclone over a coastline and compare the results from this model to those from a more complete model, leading to an improved physical understanding of the effects of asymmetric friction on a tropical cyclone.