Analysis of a simulated tropical cyclone eyewall replacement cycle

Eyewall replacement cycles (ERCs) have a marked effect on the intensity and the overall size of tropical cyclones, and are thereby important to predicting the likely impact of a storm. They are also a fascinating part of the internal dynamics of the storm. While the overall dynamics of ERCs has been understood for several decades, there are still unanswered questions, and much recent research has focussed on the boundary layer. While several groups have succeeded in simulating ERCs with high-resolution NWP, interpreting simulations from such models can be difficult, and is often aided by reference to more idealised models.

Here, we analyse the high-resolution simulation of a tropical cyclone prepared by Nolan et al (2013) as a “nature run” for data assimilation research. That simulation contained a complete eyewall replacement cycle, including the initial formation and contraction of the outer eyewall, and the dissipation of the inner eyewall. The analysis tools include the nonlinear diagnostic boundary-layer model of Kepert and Wang (2001), which is used to diagnose the boundary-layer contribution to the secondary circulation. We show that nonlinear Ekman pumping, as diagnosed by that model, explains much of the distribution and relative strength of the azimuthal-mean updraft out of the boundary layer. The Ekman pumping is determined by the gradient wind structure, so it appears that the ERC is influenced, but not initiated, from within the boundary layer.