A Flow-Dependent Horizontal Mixing Length Scale and Its Impact on Track Simulations of Harvey (2017) in HWRF

The horizontal diffusivity is usually parameterized as a function of horizontal length scale and flow deformation. In 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 horizontal length scale. The proposed length scale is tested using HWRF with 18-6-2km resolutions to simulate hurricane Harvey (2017). Three experiments with different length scales are made. Two experiments use constant length scales of 750m and 1500m, respectively. The third one uses the newly proposed length scale. All simulations are initialized from 2017082012. Results suggest that the simulation with the new length scale significantly improves the simulated 5-day track, compared with those using the constant length scales. The simulated tracks are close between the two simulations using the constant length scales. The intensity from the simulation using the new length scale is close to that using the 750-m length scale. The simulation using the 1500m length scale yields a weaker storm as expected. The large-scale flow and storm structure from the simulations will be analyzed to highlight the impact of the new length scale.