Downward Momentum Flux: An Important Mechanism of Typhoon Maintaining Ground Destructive Force

Abstract

Under the background of global climate change, typhoons have been attracting increasing attention due to their extraordinary destructive potential and great impact on the coastal areas of the South China Sea. Although the risk of strong winds related to typhoons has long been of interest, less is known about the underlying mechanism responsible for the severe near‐surface winds. Using eddy covariance data collected at four different heights on a 365‐m meteorological tower located in a coastal region, the characteristics of the convective typhoon boundary layer and the associated turbulence structures are compared with their counterparts in the “textbook” dynamically unstable boundary layer. In the convective typhoon boundary layer, bulk wind shear predominates the generation of mechanical turbulence, enhancing the vertical correlation between vertical layers. The spectral analysis highlights the salient features of turbulent structures under the convective typhoon boundary layer, confirming that the gust disturbance with the nondimensional frequency ranging from 0.003 to 0.3 modulates not only turbulent transports but also the horizontal flow. Such gusts with reduced phase difference enhance the downward momentum transport, mainly responsible for the maintenance of the strong near‐surface winds during typhoon landfalls.

A Physical Model for the Observed Inverse Energy Cascade in Typhoon Boundary Layers

“…The deformation of vertical kinetic energy above the Zeman length scale prompts us to revisit the typical 3D structures in boundary layers, such as rolls, which are prevalent in typhoon boundary layers (Lan et al., 2023; Lorsolo et al., 2008; Morrison et al., 2005), and attached eddies, which are characteristic coherent structures in boundary layer flows (Ghannam et al., 2018; Hutchins et al., 2012; Kline et al., 1967; Li et al., 2021, 2023; Marusic & Monty, 2019). The size of rolls in typhoon boundary layers is approximately 1 ∼ 2 km based on the analysis of Doppler velocity data (Morrison et al., 2005).…”

Evaluation and Projection of Gale Events in North China

“…For daily variation (Figure 2b), the gale events show a single peak with a maximum from 10:00 to 16:00 local time in North China. The largest frequency of gale events reaches > 12% in 13:00-14:00 local time in North China, which is related to the down sent momentum [42,43]. Generally, after the ground is heated in the afternoon, the enhanced vertical exchange of air results in gales being transmitted from high levels to the ground.…”