The impact of a modified subgrid-scale 3D turbulence closure on tropical cyclone dynamics in a convection-permitting simulation

Two idealized tropical cyclone simulations with the traditional downgradient diffusion closure and the H-gradient closure were analysed. Based on the rotating-convection paradigm, we compared the kinematic and thermodynamic structure differences between these simulations. The results suggest that the simulation with the H-gradient closure has a stronger boundary layer inflow with a shallower depth, and stronger convergence near the center. The cyclone intensity in the H-gradient closure simulation is stronger. Further, the H-gradient closure simulation leads to a more static stable layer of near-surface air. Analyses of the momentum equations reveal that in the H-gradient closure simulation, the horizontal diffusion term leads to a weaker deceleration of the tangential flow near the RMW, and the vertical diffusion term tends to accelerate the inflow more. Another finding is that the stress tensor and scalar sub grid flux in the H-gradient closure are quite different from those in the traditional downgradient diffusion closure. It is significant that the patterns of the stress tensor and scalar sub grid flux in the H-gradient closure simulation are much closer to the LES simulation benchmark than the corresponding patterns for the traditional downgradient diffusion closure. The results highlight the limitation of the traditional downgradient diffusion closure. The kinematic structure in the H-gradient closure simulation looks similar to the low-Km (Km is the vertical eddy diffusivity for momentum) case in previous studies although the thermodynamic structure looks similar to the high-Ks (Ks is the vertical eddy diffusivity for scalar) case. Notably, since the ratio of Km and Ks is set to a constant value in the traditional downgradient diffusion closure we used, the traditional downgradient diffusion closure cannot replicate the H-gradient closure's pattern.