Understanding the Inland Evolution of Tropical Cyclone Wind Field through Idealized Numerical Modeling Experiments

The post-landfall evolution of the tropical cyclone low-level wind field, including wind field size and structure in addition to maximum wind speed, is critical to the magnitude and extent of inland impacts due to wind and rainfall. However, the physics of this evolution remains poorly understood and a simple physically-based model for it does not currently exist. This work deconstructs the physics of landfall by decomposing the bottom boundary into the simplest form of its two relevant physical components: surface drag and surface heat flux. The effects of these two components on the evolution of low-level wind field are analyzed via a suite of highly-idealized numerical simulation experiments using the Bryan cloud model (CM1) in both axisymmetric and 3D geometry. The experiments are conducted in a simplified state of radiative-convective equilibrium governed by a minimal number of thermodynamic parameters, such that the maximum potential intensity is a simple function of external parameters and our results may be placed directly within this existing theoretical framework.

Experiments are performed testing the effects of varying surface drag coefficient, latent heat fluxes, sensible heat fluxes, and their combinations on the evolution of the complete circulation. Preliminary results indicate that changes in both surface fluxes and surface drag strongly influence the inner-core wind field in the vicinity of the radius of maximum wind, though with distinct structural evolutions. Meanwhile, the outer region of the low-level wind field is strongly sensitive to surface drag yet largely insensitive to surface heat fluxes. The underlying physics that govern these varied responses are examined, including the testing of existing theoretical models for the low-level wind structure typically applied to storms over the ocean. Key experimental results will be compared to observations from historical landfall events. This work lays the foundation for the development of a simple physically-based model for the post-landfall evolution of the wind field for real storms in nature.