A Framework for Simulating Hurricane Boundary Layers using Large-Eddy Simulation and Its Use in Developing PBL parameterizations for NOAA’s Hurricane Analysis and Forecast System

Accurately representing planetary boundary layer (PBL) turbulent processes in numerical models is critical for improving hurricane forecasts. However, existing PBL parameterization schemes are mostly designed for low-wind conditions, and assessing their uncertainties in hurricane conditions remains challenging, mostly due to very scarce in-situ turbulence measurements. To fill in the gap, this study develops a modeling framework based on a small- domain large-eddy simulation (LES) to better understand the uncertainty of PBL schemes in hurricane conditions. The novelty of this framework includes the usage of a few input parameters to represent the TC vortex and the addition of a simple nudging term for temperature and moisture to account for the complex thermodynamic processes in TCs. This special model setup allows for a fair comparison of PBL schemes under the same controlled thermodynamic conditions against LES. Using this framework, we improved the high-order PBL scheme used in NOAA’s next- generation hurricane forecast model, Hurricane Analysis and Forecast System (HAFS). HAFS retrospective runs during the 2021 Atlantic hurricane season demonstrate that the improved PBL scheme leads to better structure and intensity forecasts than the original PBL scheme. Importantly, the improved PBL scheme shows promise to improve the forecast skill of rapid intensification events, which are notoriously challenging to predict. Avenues for future development of PBL parameterizations will be discussed.