Abstract
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 mproduce larger local changes of AAM and radial momentum, leading to more quickly-evolved vortex flows and secondary circulations over Taiwan’s steep terrain.
Highlights
There have been many observational and modeling studies of tropical cyclones (TCs), our understanding on the evolution of TC’s tangential and radial flows during the landfall process is rather limited
Gradient-wind balance (GWB) assumption for the tangential wind has been widely used in theoretical studies of TCs (Shapiro and Willoughby 1982; Emanuel 1986); whether the GWB model is valid to describe TC’s tangential winds at mid-levels over terrain remains an open question
Both sounding observations from La Seur and Hawkins (1963) and aircraft measurements from Willoughby (1990, 1991) found that the GWB model is a good approximation to the azimuthally-averaged tangential winds in the inner-core region for flows above the marine boundary layer (MBL)
Summary
There have been many observational and modeling studies of tropical cyclones (TCs), our understanding on the evolution of TC’s tangential and radial flows during the landfall process is rather limited. Gradient-wind balance (GWB) assumption for the tangential wind has been widely used in theoretical studies of TCs (Shapiro and Willoughby 1982; Emanuel 1986); whether the GWB model is valid to describe TC’s tangential winds at mid-levels over terrain remains an open question Both sounding observations from La Seur and Hawkins (1963) and aircraft measurements from Willoughby (1990, 1991) found that the GWB model is a good approximation to the azimuthally-averaged tangential winds in the inner-core region for flows above the marine boundary layer (MBL). ZLY indicated that their conclusions are both correct for the levels of observations they analyzed, because of significant differences in the agradient forces and flows between the layers near the top of the MBL and the layers above These studies are only for oceanic TCs, and it remains unknown whether the GWB model is still applicable when a TC makes landfall. While the TC vortex flow over the ocean has been extensively studied in the context of balanced dynamics, it is still uncertain how the unbalanced forces and flows are generated and distributed in the inner-core region as a TC encounters mountainous topography, such as the Central Mountain Range (CMR) in Taiwan
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