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Abstract. The fundamental mechanism underlying tropical cyclone (TC) intensification may be understood from the conservation of absolute angular momentum, where the primary circulation of a TC is driven by the torque acting on air parcels resulting from asymmetric eddy processes, including turbulence. While turbulence is commonly regarded as a flow feature pertaining to the planetary boundary layer (PBL), intense turbulent mixing generated by cloud processes also exists above the PBL in the eyewall and rainbands. Unlike the eddy forcing within the PBL that is negative definite, the sign of eyewall/rainband eddy forcing above the PBL is indefinite and thus provides a possible mechanism to spin up a TC vortex. In this study, we show that the Hurricane Weather Research & forecasting (HWRF) model, one of the operational models used for TC prediction, is unable to generate appropriate sub-grid-scale (SGS) eddy forcing above the PBL due to lack of consideration of intense turbulent mixing generated by the eyewall and rainband clouds. Incorporating an in-cloud turbulent mixing parameterization in the PBL scheme notably improves HWRF's skills on predicting rapid changes in intensity for several past major hurricanes. While the analyses show that the SGS eddy forcing above the PBL is only about one-fifth of the model-resolved eddy forcing, the simulated TC vortex inner-core structure and the associated model-resolved eddy forcing exhibit a substantial dependence on the parameterized SGS eddy processes. The results highlight the importance of eyewall/rainband SGS eddy forcing to numerical prediction of TC intensification, including rapid intensification at the current resolution of operational models.
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Evaluating the impact of parameterized turbulent mixing and boundary layer structure on hurricane intensification
This talk presents results from the authors’ recent work on evaluating the role of turbulence and boundary-layer parameterizations on hurricane intensification. We show that observation-based modification of these physical parameterizations significantly improved the HWRF intensity forecast. Turbulent mixing in both the vertical and horizontal directions are found to be crucial for hurricane spin-up dynamics in 3D numerical simulations and HWRF forecasts. Vertical turbulent mixing regulates the inflow strength and the location of boundary-layer convergence that in turns regulates the distribution of deep convection and the intensification of the whole hurricane vortex. Convergence of angular momentum in the boundary layer that is a key component of the hurricane spin-up theory is also found to be regulated by vertical turbulent mixing in connection to the boundary layer inflow. Horizontal turbulent mixing, on the other hand, mainly influences the eddy momentum flux inside the radius of the maximum wind speed in the angular momentum budget. The effect of horizontal turbulent mixing on the convergence of angular momentum is on smoothing the radial gradient of the angular momentum when the horizontal mixing length is large. In a sheared storm, both the vertical and horizontal turbulent mixing affect vortex and shear interaction in terms of the evolution of vortex tilt and boundary-layer recovery processes.
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- Award ID(s):
- 1822128
- PAR ID:
- 10089104
- Date Published:
- Journal Name:
- 23 Symposium on Boundary Layers and Turbulence
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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