Horizontal Subgrid-Scale Diffusion Parameterizations for Tropical Cyclones

Horizontal diffusion in the numerical simulation of Tropical Cyclones (TCs) is the most controlling factor for simulating maximum TC intensity (Bryan & Rotunno, 2009). It also significantly influences the simulated hurricane intensity (Rotunno & Bryan, 2012), maximum wind speed, TC vortex size, inflow layer depth (Zhang & Marks, 2015), and storm structure (Zhu et al., 2014). Appropriate parameterization of horizontal diffusion in the numerical models is of great importance towards better prediction of TCs. However, both researches and parameterization schemes on horizontal diffusion are very limited compared to its vertical counterpart. This study first reviews the existing horizontal diffusion schemes, and argues their intrinsic viabilities for TC simulation. Then, a high-resolution large-eddy simulation dataset is adopted to analyze the subgrid-scale (SGS) horizontal diffusion at different resolutions from hectometer to kilometer scale. The analysis reveals that the established schemes which base on the assumption of down-gradient eddy diffusivity model fail to represent the spatial distribution of SGS horizontal fluxes. On the contrary, scale-similarity models show good agreement with the benchmark LES, especially for the resolved flux components induced by TC circulation. It is suggested that a hybrid model that includes scale-similarity component will be a better solution for the simulation of TC at mesoscale resolution.