Large-eddy simulation of the hurricane boundary layer: Evaluation of the planetary boundary-layer parametrizations

The organized roll vortices are the integral part of the hurricane boundary layer (HBL) where they have an important contribution in the vertical transport of momentum, heat and moisture. Large-eddy simulations (LESs) were conducted to explicitly resolve the organized roll vortices in the HBL over the ocean using the Weather Research and Forecasting (WRF) model. The LESs were nested within the WRF mesoscale model to provide two-way up-scale and down-scale exchange at the nest interfaces, and explicitly resolve large eddies. Downdrafts in the organized roll vortices correspond to transport of high-momentum air down to the surface, while updrafts tend to transport low-momentum air upwards. The downdrafts and updrafts therefore lead to a well-organized band-like structure of alternating strong and relatively weak near-surface winds, respectively. The upward legs of the roll vortices are also associated with transport of moist air to the upper levels, while drier air is brought down to the surface in the downward legs. Organized roll vortices contribute a significant portion to the vertical transport of heat and moisture. Three planetary boundary-layer (PBL) parametrizations were examined against the LES. Results indicate that PBL parametrizations cannot capture the band-like structure of alternating strong and relatively weak near-surface winds as they are subgrid scale features, and are unable to adequately represent the surface heat fluxes and wind profiles in the hurricane conditions. PBL parametrizations show overall less variability in the extremes of the wind field.