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

Abstract

The drag coefficient ( 𝐴𝐴 𝐴𝐴𝑑𝑑 ) is an important parameter of the land surface layer that represents the efficiency of momentum exchange between the Earth's surface and is thus important in numerical models for the climate simulation and weather forecasting, such as the prediction of typhoons (Emanuel, 1995) and sandstorms (Okin, 2005). Previous studies documented that 𝐴𝐴 𝐴𝐴𝑑𝑑 levels off over land (Raupach & Thom, 1981;Wieringa, 1993

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

“…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).…”

The Impact of Observed Drag Reduction Over Land on Typhoon Forecasting

“…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 :…”

“…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.…”

“…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.…”

“…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.…”