Estimating downwind turbulence intensity using wind and wave measurements during hurricanes: Fid-Bau Portal

Estimating downwind turbulence intensity using wind and wave measurements during hurricanes

Shen, H. / He, Y. / Hsu, S.A.

Highlights • Under high wind conditions, relationship between overwater downwind turbulence intensity (TI) and wind speed is obtained from measurements. • Based on the proposed TI parameterization, overwater friction velocity ( $U_{*}$ ) can also be obtained. • The formula is verified by state-of-the-art results from the eddy-covariance, the wind gust and the wind-current interaction methods. • In addition, the proposed formula is shown to be valid up to $U_{10}$ = 56 m s⁻¹ as compared to recent analysis on Super Typhoon Megi (2010).

Abstract Analyses of simultaneous measurements of wind speed at 10 m, $U_{10}$ , significant wave height, $H_{s}$ and peak wave period, $T_{p}$ from 3 National Data Buoy Center (NDBC) buoys during the passage of 6 hurricanes over the wind seas when $U_{10}$ ≥ 9 m s ⁻¹ indicate that overwater downwind turbulence intensity (TI) can be formulized as ${T I = - 6.06 \times 10^{- 5} U}_{10}^{2} + 4.93 \times 10^{- 3} U_{10} + 3.95 \times 10^{- 2}$ , here $U_{10}$ is the wind speed at 10 m height over sea surface. It is found that TI = 0.08 when $U_{10}$ = 9 m s⁻¹, and it peaks up to approximately 0.14 when $U_{10}$ ≈ 40 m s ⁻¹. Thereafter, TI saturates as $U_{10}$ continuously increases. From this TI formulation, overwater friction velocity ( $U_{*}$ ) is inferred as $U_{*} = 0.4 {0 U}_{10} (- 6.06 \times 10^{- 5} U_{10}^{2} + 4.93 \times 10^{- 3} U_{10} + 3.95 \times 10^{- 2})$ . This friction velocity parameterization is further verified by 3 independent methods: the direct eddy covariance, the wind gust and the wind-current interaction. It shows that this proposed $U_{*}$ formula is valid up to $U_{10}$ = 56 m s⁻¹ during Super Typhoon Megi (2010) as validated by measurements based on the wind-current interaction method.