Momentum Budget Evolution of Typhoon Nari (2001) During the Landfall Process

  • Author(s): Ming-Jen Yang, Tai-Chi Chen Wang, Yi Zhang, and Ching-Yi Weng
  • DOI: 10.3319/TAO.2011.05.31.01(TM)
  • Keywords: Landfall typhoon, Momentum budget, Supergradient wind
  • Citation: Yang, M. J., T. C. Chen Wang, Y. Zhang, and C. Y. Weng, 2011: Momentum budget evolution of Typhoon Nari (2001) during the landfall process. Terr. Atmos. Ocean. Sci., 22, 595-612, doi: 10.3319/TAO.2011.05.31.01(TM)

In order to understand the evolution of tangential and radial flows of Typhoon Nari (2001) during its landfall in Taiwan, absolute angular momentum (AAM) and radial wind budgets of Nari are conducted by analyzing the MM5 simulation results with high spatial and temporal resolutions (2-km horizontal grid spacing and 2-min output interval). The AAM is nearly conserved outside the eyewall and above the boundary layer while Nari is over the ocean; after landfall, the enhanced surface friction and turbulent mixing produces mostly negative local tendencies of AAM above terrain. For the landfall storm, both the radar observation and model simulation indicate that the radial inflows at lower levels become thicker and stronger over land, and the sloping radial outflow jet is maximized at the midlevel above rugged topography. The midlevel radial outflows result from supergradient accelerations of 20 - 35 m s-1 h-1 and supergradient winds of 5 - 9 m s-1. The enhanced imbalance accelerations imply that the gradient wind balance is no longer appropriate to describe tangential winds over terrain. Near the eyewall, the supergradient winds can be as strong as 9 m s-1 above terrain and subgradient winds up to -21 m s-1 are found at the surface on the lee side. The stronger force imbalances of the landfall Nari produce larger local changes of AAM and radial momentum, leading to more quickly-evolved vortex flows and secondary circulations over Taiwan's steep terrain.

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