Simulation and parameterization of large turbulent eddy transport in the hurricane boundary layer

Hurricane boundary layer (HBL) processes, especially, the structure of the coherent large eddy circulations (LECs) and their induced turbulent fluxes, are not well understood. In this study, we have introduced an innovative large eddy simulation (LES) framework in a weather hindcasting mode developed from a multiple scale nested Weather Research & Forecasting (WRF) model. The unique WRF-LES ensures the high resolution data being generated in an appropriate environment as a hurricane evolves. Using the WRF-LES, we investigated the structure of the HBL LECs and the associated turbulent transport during the landfall of Hurricane Ivan. The simulation shows that the HBL LECs exist in a mean stable environment and consist of well defined updraft and downdraft. The analyses indicate that statistically the HBL LECs are only slightly skewed and on average the updrafts and downdrafts of the HBL LECs are relatively evenly distributed. The inversion base basically envelopes the upper boundary of LECs. The trough in between two adjacent LECs is where most entrainment takes place, whereas the crest of the LECs is where boundary layer air detrains out of the HBL. In such a way, LECs directly connect the surface, the HBL, and the main body of a hurricane vortex and enhance the exchange of energy, moisture, and momentum between them. It is found that the current boundary layer schemes significantly under-estimate the resolved HBL turbulent transport apparently due to the fact that the effects of the HBL LECs have not been included in the parameterizations. Based on the structure of the HBL LECs simulated by the WRF-LES, in this paper we proposed a conceptual updraft-downdraft model that can be implemeted in weather forecasting models to parameterize the turbulent fluxes induced by the HBL