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Discussion papers
https://doi.org/10.5194/acp-2018-610
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/acp-2018-610
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 06 Sep 2018

Research article | 06 Sep 2018

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This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Chemistry and Physics (ACP).

Role of eyewall and rainband eddy forcing in tropical cyclone intensification

Ping Zhu1, Bryce Tyner1, Jun A. Zhang3,4, Eric Aligo2, Sundararaman Gopalakrishnan3, Frank D. Marks3, Avichal Mehra2, and Vijay Tallapragada2 Ping Zhu et al.
  • 1Department of Earth and Environment, Extreme Event Institute, Florida International University, Miami, FL 33199, US
  • 2Environmental Modeling Center, NCEP, NOAA, College Park, MD 20740, US
  • 3Hurricane Research Division, AOML, NOAA, Miami, FL 33149, US
  • 4Cooperative Institute for Marine and Atmospheric Studies, University of Miami, Miami, FL 33149, US

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.

Ping Zhu et al.
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Short summary
Producing timely and accurate intensity forecasts of tropical cyclones (TCs) continues to be one of the most difficult challenges in numerical weather prediction. The difficulty stems from the fact that TC intensification is not only modulated by environmental conditions but also largely depends on TC internal dynamics. This study shows that the asymmetric eyewall/rainband eddy forcing above the boundary layer plays an important role in spinning up a TC vortex including rapid intensification.
Producing timely and accurate intensity forecasts of tropical cyclones (TCs) continues to be one...
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