Improving the horizontal mixing parameterization in TC simulations of HAFS

Tropical cyclone simulations are highly sensitive to the parameterization of horizontal mixing. Wang et al. (2021, Weather and Forecasting, 36(2)) proposed a new length scale, based on vorticity length scale, to account for the effect of rotating flow on the horizontal mixing. In contrast, most TC models use a fixed fraction of horizontal grid spacing as the horizontal mixing length. Their tests in then-operational HWRF demonstrated that the modified length scale can improve the track and intensity simulations of Hurricane Harvey (2017).

In this study, we continue the effort to improve the horizontal mixing parametrization for TC simulations using 30-m high-resolution simulations of an idealized TC. The horizontal mixing length is further analyzed and modified using the explicit simulations of sub-grid fluxes. In addition, it is found that the traditional downgradient eddy diffusivity method cannot represent the horizontal fluxes in many cases. The nonlocal or counter gradient contribution to horizontal fluxes is assessed and a new parameterization is proposed. HAFSv2 is used to test the impact of the proposed modification on HAFS performance.