A Scale-Aware Horizontal Mixing-Length Scale and Its Impact on Simulations of Harvey (2017) and Lane (2018) in HWRF

The horizontal diffusivity is usually parameterized as a function of horizontal length scale and flow deformation. In current HWRF, the length scale is specified as a fraction of grid size. It, however, may overestimate horizontal diffusion in the case that the grid size is larger than or close to the local flow mixing scale. To address this potential issue, we proposed a more realistic, flow-dependent and scale-aware horizontal length scale. The proposed length scale is tested using triple-nested HWRF to simulate hurricane Harvey (2017) and Lane (2018). Two experiments with different length scales are made. One experiment uses a constant length scale of ~750 m as in the operational HWRF. The other uses the newly proposed length scale. Results suggest that the simulation with the new length scale significantly improves the simulated 5-day track of Harvey (initialized from 2017082012), compared with that using the constant length scale. The intensity errors from both simulations are close. The simulations of Lane (2018) shows intensity errors have been significantly reduced using the new formulation, while the performance of track simulation is not changed. Idealized simulations have been designed to analyze the impact of the new length scale on storm structure.