How Does Horizontal Diffusion Influence the Intensification and Maximum Intensity of Numerically Simulated Tropical Cyclones?

Recent studies have demonstrated the sensitivity of simulated tropical cyclone (TC) intensity to horizontal diffusion in numerical models. It is unclear whether such sensitivity comes from the horizontal diffusion in or above the boundary layer. To address this issue, both an Ooyama-type model and a full-physics model are used to conduct sensitivity experiments with reduced or enlarged horizontal mixing length (lh) in the boundary layer and/or in the free atmosphere. Results from both models show that enlarging (reducing)lh throughout the model domain considerably reduces (increases) the TC intensification rate and quasi-steady intensity. A new finding is that changing lh above the boundary layer imposes a much greater influence than that in the boundary layer. Large lh above the boundary layer is found to effectively reduce the radial gradient of tangential wind inside the radius of maximum tangential wind and thus the inward flux of absolute vorticity, reducing the positive tangential wind tendency and the TC intensification rate and the steady-state intensity. In contrast, although larger lh in the boundary layer reduces the boundary layer tangential wind tendency, it also leads to the more inward-penetrated inflow and thus enhances the inward flux of absolute vorticity, which offsets part of the direct neg-ative contribution by horizontal diffusion, making the net change in tangential wind tendency not obvious. Results from three-dimensional simulations also show that the resolved eddies contribute negatively to TC spinup when lh is small, while its effect weakens when lh is enhanced either in or above the boundary layer.