Effects of Wind Direction on Variations in Friction Velocity With Wind Speed Under Conditions of Strong Onshore Wind

Abstract

The effects of wind direction on variations in friction velocity with wind speed are studied under moderate (≥9 m/s) to strong (≥22 m/s) onshore wind conditions using 20‐Hz ultrasonic wind data from a coastal tower at three different heights. The effects of different averaging time intervals of 20, 10, 2, and 1 min on the variations are also investigated. Three typhoons passed by the tower during the 150 hr of observations. Regardless of wind direction, friction velocity increases with increasing wind speed, and linear regression shows that the rate of increase is ~50% less than previously reported. However, a leveling‐off or decrease in friction velocity with increasing wind speed is found under strong wind conditions using a bin‐averaged method. Wind direction affects the variations in friction velocity with wind speed. Friction velocity increases with increasing wind speed at a similar growth rate to that of previously published results, with a leveling‐off or decrease at wind speeds higher than 22 m/s when the wind blows roughly normal to the shoreline. Form drag induced by sea waves with longer wavelengths is suggested to be responsible for the effects of wind direction on the variations in friction velocity with wind speed. An averaging time interval of 1 min yields representative variations in friction velocity with wind speed using the observations described in this study.

Variations in Friction Velocity with Wind Speed and Height for Moderate-to-Strong Onshore Winds Based on Measurements from a Coastal Tower

“…A barometer at 8.5 m above ground level and a thermometer and hygrometer at 10 and 70 m above ground level were also deployed with the output frequency of one data point per minute. More detailed information on the tower and the local topography can be referred to Fang et al (2018).…”

An Evaluation of the Constant Flux Layer in the Atmospheric Flow Above the Wavy Air‐Sea Interface

“…Since its commission (Fisher & Spiess 1963), FLIP has been deployed for several air-sea interaction campaigns where multiple levels of atmospheric variables were measured, such as during SCOPE (Fairall, Bradley, Hare, Grachev, & Edson 2003), the MBL/ARI experiment (Miller, Friehe, Hristov, Edson, & Wetzel 1999), COPE (Grachev, Fairall, Hare, Edson, & Miller 2003), and HiRes (Grare, Lenain, & Melville 2013). At shorelines (recently, Fang et al 2018;Katz & Zhu 2017;Shabani, Nielsen, & Baldock 2014;Zhao et al 2015) or inland waters (Li, Bou-Zeid, Vercauteren, & Parlange 2018), towers have been deployed with turbulence profiles and some assessment of the gradients were conducted. However, these evaluations were limited in scope and tended to assume that the flux variance was randomly distributed, as in a mean ± standard deviation adequately flagged divergent flux gradients.…”

“…(2015); Katz and Zhu (2017); Fang et al. (2018); Li et al. (2018), and due to their proximity to the land‐sea boundary are not representative of the open ocean.…”

“…However, these evaluations were limited in scope and tended to assume that the flux variance was randomly distributed, as in a mean ± standard deviation adequately flagged divergent flux gradients. Furthermore, profiles tended to be limited in their number of observing levels, <4 for Shabani et al (2014); Zhao et al (2015); Katz and Zhu (2017); Fang et al (2018); Li et al (2018), and due to their proximity to the land-sea boundary are not representative of the open ocean.…”

Study of Dynamic Response Characteristics of the Wind Turbine Based on Measured Power Spectrum in the Eyewall Region of Typhoons

“…On the other hand, the wind direction in the eyewall region of the typhoon varies dramatically [26]. Although the wind turbine would be shut down in typhoons, and the yaw system would still be in working condition so as to adjust the position of the rotor to keep it against the incoming flow, there still exists a delay for a certain period of time (typically, 10 min) due to the statistical time consumed for the wind direction changes [27].…”