Implementing a New Mixing-Length Constraint in Operational HAFS v2.2 Upgrade: Impacts on Tropical Cyclone Intensity Forecast

This study introduces a physics-based dynamic constraint for the asymptotic allowable mixing length, implemented in the latest upgrade of NOAA’s operational Hurricane Analysis and Forecast System (HAFS) v2.2. Replacing conventional fixed-value caps, the new constraint varies as a function of local vertical wind shear and planetary boundary layer depth. Compared to fixed-value constraints, the new formulation yields mixing-length distributions in HAFS that align more closely with large-eddy simulations. Furthermore, the resulting eddy-diffusivity profiles show closer agreement with aircraft observations than those obtained using fixed-value constraints. Retrospective forecast evaluations indicate that the variable constraint reduces tropical cyclone intensity errors by 2–8% across most forecast lead times relative to the operational baseline and shows improved rapid intensification prediction performance, without degrading track performance. Experiments with a range of fixed-value constraints further suggest that the variable constraint provides benefits that are not consistently achieved by varying a fixed constraint. While the new constraint improves HAFS intensity forecasts, future development should focus on refining mixing-length models to improve the underlying physics and potentially reduce reliance on an imposed constraint.