Abstract
This paper reviews the evolution of planetary boundary layer (PBL) parameterization schemes that have been used in the operational version of the Hurricane Weather Research and Forecasting (HWRF) model since 2011. Idealized simulations are then used to evaluate the effects of different PBL schemes on hurricane structure and intensity. The original Global Forecast System (GFS) PBL scheme in the 2011 version of HWRF produces the weakest storm, while a modified GFS scheme using a wind-speed dependent parameterization of vertical eddy diffusivity (Km) produces the strongest storm. The subsequent version of the hybrid eddy diffusivity and mass flux scheme (EDMF) used in HWRF also produces a strong storm, similar to the version using the wind-speed dependent Km. Both the intensity change rate and maximum intensity of the simulated storms vary with different PBL schemes, mainly due to differences in the parameterization of Km. The smaller the Km in the PBL scheme, the faster a storm tends to intensify. Differences in hurricane PBL height, convergence, inflow angle, warm-core structure, distribution of deep convection, and agradient force in these simulations are also examined. Compared to dropsonde and Doppler radar composites, improvements in the kinematic structure are found in simulations using the wind-speed dependent Km and modified EDMF schemes relative to those with earlier versions of the PBL schemes in HWRF. However, the upper boundary layer in all simulations is much cooler and drier than that in dropsonde observations. This model deficiency needs to be considered and corrected in future model physics upgrades.
Highlights
Previous theoretical studies have pointed out the important role of physical processes in the planetary boundary layer (PBL) in governing tropical cyclone (TC) intensification and maximumAtmosphere 2020, 11, 1091; doi:10.3390/atmos11101091 www.mdpi.com/journal/atmosphereAtmosphere 2020, 11, 1091 intensity [1,2,3,4,5,6,7,8,9,10,11]
To study the effects of different versions of the Hurricane Weather Research and Forecasting (HWRF) PBL schemes on hurricane intensity and structure, we ran a series of idealized simulations
This study reviews the PBL schemes that have been used in the operational HWRF model
Summary
Previous theoretical studies have pointed out the important role of physical processes in the planetary boundary layer (PBL) in governing tropical cyclone (TC) intensification and maximumAtmosphere 2020, 11, 1091; doi:10.3390/atmos11101091 www.mdpi.com/journal/atmosphereAtmosphere 2020, 11, 1091 intensity [1,2,3,4,5,6,7,8,9,10,11]. In TC forecast models, small-scale processes such as turbulent diffusion must be parameterized, given that the horizontal grid spacing of current operational models is 2 km or coarser. Research models that are used to simulate TC intensity and structure often have horizontal grid spacing of 0.5–3 km, while scales of turbulent eddies may be as small as 10–100 m. Even with large eddy simulations that can achieve 10–100 m horizontal grid spacing [12,13,14], the lowest boundary condition still requires a parameterization of drag coefficient that is uncertain in hurricane force wind conditions [15,16].
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