Observed Surface Drag Coefficient Under High Wind Speed Conditions and the Relationship With Coherent Structures

Shao, Xin; Zhang, Ning; Peng, Zhen; Zhao, Kun; Luo, Yuan; Song, Xiaoquan

doi:10.1029/2021jd035301

Cited by 4 publications

(20 citation statements)

References 61 publications

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“…In the nontyphoon case, although some large, organized structures occur aloft, they rarely penetrate the near‐surface layer. These findings are consistent with a case study illustrating the effects of vertical coupling of turbulent eddies during typhoon passage (Shao et al., 2022).…”

Section: Resultssupporting

confidence: 92%

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

Lan

Xie

et al. 2023

JGR Atmospheres

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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.

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Section: Resultssupporting

confidence: 92%

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

Lan

Xie

et al. 2023

JGR Atmospheres

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“…This is because the momentum flux is positively related to energy dissipation rate (Katul et al, 2013). Therefore, it can speculate that u * would exhibit near-constant behavior with increasing wind speed phenomenologically, which is in agreement with observations in Fang et al (2020) and Shao et al (2022), as the consequence of the antagonistic relationship between the increasing wind speed and the decreasing efficiency of shear-generated energy transferring to u * . By assuming u * keeps constant when U 10 > 15 m s −1 , Equation 1 yields the relationship between U 10 , z 0 , and modified roughness length 𝐴𝐴 𝐴𝐴 ′ 0 :…”

Section: Methods Simulations and Datasupporting

confidence: 85%

“…Note that 𝐴𝐴 𝐴𝐴 ′ 0 = 𝐴𝐴0 when U 10 < 15 m s −1 . Considering that the recorded maximum wind speeds are around 20 m s −1 in Shao et al (2022) and 30 m s −1 in Fang et al (2020), the key assumption in our method is expected to be valid up to a wind speed of 30 m s −1 . Treating u * as nearly constant over 30 m s −1 can be regarded as an extrapolated approximation to account for the rotation effects on energy cascade over land.…”

Section: Methods Simulations and Datamentioning

confidence: 99%

“…Treating u * as nearly constant over 30 m s −1 can be regarded as an extrapolated approximation to account for the rotation effects on energy cascade over land. The observations in Shao et al (2022), which were collected from a flat area with short longan trees, are utilized to examine the performance of new formula. Observations from the Typhoon Lekima at the Cixi meteorological tower site (30.456°N, 120.964°E), surrounded by a mud flat, are also used in this study.…”

Section: Methods Simulations and Datamentioning

confidence: 99%

“…dropsondes). In a more recent study, Shao et al (2022) identified a decrease of C d at land surface when the wind speed exceeds 15 m s −1 . Fang et al (2020) also documented a leveling off or slight decrease in onshore friction velocity u * (thus a decrease in C d ) with increasing wind speed up to approximately 30 m s −1 in typhoon conditions.…”

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confidence: 97%

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The Impact of Observed Drag Reduction Over Land on Typhoon Forecasting

Shao,

Zhang,

Tang

2023

JGR Atmospheres

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The surface drag coefficient plays a crucial role in determining the momentum exchange between the Earth’s surface and the atmosphere. Previous studies have observed a tendency for the drag coefficient to decrease at high wind speeds over land, primarily due to the inverse energy cascade. This study aims to examine the impacts of the reduced drag coefficient on the typhoon forecasting skills in terms of the track, the central pressure, and the 10‐m maximum wind speed. Drag reduction is approached through the logarithmic wind profile and a new formula that considers the reduction in roughness length. The results show that the track and the 10‐m maximum wind speed forecasts are improved, but these improvements are primarily evident over plain areas, as the typhoon structure is significantly disrupted by the topography over mountainous areas. Additionally, the reduced drag coefficient results in less dissipation of the typhoon’s wind fields over land. As the wind speeds at coastal areas are enhanced compared with those in operational model, the changes in the Coriolis force are responsible for the track prediction improvement. These findings highlight the importance of parameterizing coherent processes in current numerical models.

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

Shao,

Zhang,

Tang

2023

Geophysical Research Letters

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The knowledge of the observed inverse energy cascade in typhoon boundary layers holds significant implications for understanding the formation, enhancement, and deformation of typhoons. This study reveals that the observed inverse energy cascade originates from the rapid rotation of typhoons. The transition from the direct energy cascade regime to the inverse energy cascade regime can be characterized by the Zeman length scale. The turbulence structures behave as two dimensional above the Zeman length scale. Through symmetry analysis, it is discovered that the ratio of the inverse energy cascade flux to the direct energy cascade flux is proportional to the turbulence Rossby number with a power of −2. These findings offer a framework for incorporating the inverse energy cascade into the turbulence parameterizations and improving typhoon modeling.

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Observed Surface Drag Coefficient Under High Wind Speed Conditions and the Relationship With Coherent Structures

Cited by 4 publications

References 61 publications

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

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

The Impact of Observed Drag Reduction Over Land on Typhoon Forecasting

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

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