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

摘要

Hurricane boundary layer (HBL) processes, especially the structure of the coherent large eddy circulations (LECs) and their induced vertical transport, are not well understood. This paper introduces a large eddy simulation (LES) framework in a weather hindcasting mode developed from a multiple scale nested Weather Research and Forecasting (WRF) model. Using the WRF–LES, this study investigated the structure of the HBL LECs and the associated vertical transport during the landfall of Hurricane Ivan (2004). The simulation shows that the HBL LECs exist in a mean stable environment and consist of well-defined updraft and downdraft. Statistically, the HBL LECs are only slightly skewed with the updrafts and downdrafts relatively evenly distributed spatially. 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 underestimate the resolved turbulent fluxes due to the fact that the effects of LECs have not been included in the parameterizations. On the basis of the statistical structure of LECs simulated by the WRF–LES, this paper proposes a conceptual updraft–downdraft model that can potentially be implemented in weather forecasting models to parameterize the fluxes induced by the HBL LEC transport.