Accurate prediction of the secondary moment of three-dimensional turbulent wind field is important for structure dynamic analysis, while literatures on turbulence scales for offshore wind field are limited. In this study, an offshore wind field measurement campaign is carried out by an offshore meteorological mast using ultrasonic anemometers at 40, 60 and 80m heights, from which the turbulence integral length scales used in von Karman model are evaluated. It is found that turbulence structure at these heights for offshore wind field can be expressed well by von Karman model. The length scales are evaluated by model fitting for auto-spectrum and spatial coherence. The scales for auto-spectrum showed similar values with the IEC design code for longitudinal, lateral and vertical component. Spatial length scales in vertical direction show similar value with the model provided in ESDU considering surface roughness, but were larger compared to previous measurements in suburban area for longitudinal and lateral components. Spatial length scale in vertical direction for vertical component shows similar value with the previous measurement in lateral direction. Finally, a model for the turbulence length scales of three velocity components in von Karman model is proposed for offshore wind field, based on the measured results and previous studies.

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In this study, the accuracy of the WRF (the Weather Research and Forecasting model)-simulated horizontal gradient of offshore wind speed near a coastline is evaluated using in-situ measurements and Envisat/ASAR (Advanced Synthetic Aperture Rader) images. The WRF simulation is conducted for the coastal waters off Shirahama in Wakayama Prefecture and the accuracy is examined in comparison with in-situ measurements from the offshore platform of Shirahama Oceanographic Observatory (referred to as Shirahama) and the Wakayama Southwest buoy (SW-buoy). It is found that WRF overestimates wind speed at Shirahama through the year, while it underestimates wind speed at SW-buoy only for northwesterly winds, which are predominant through the year. For these reasons, WRF tends to have a weaker horizontal gradient of wind speed between these two sites. Compared with wind speed fields obtained from ASAR, it is found that the underestimation at SW-buoy is partly due to the offshore shift of a strong wind zone, formed by the speed-up effect from two upstream strains, in the WRF simulation.

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This study proposed a correction procedure for field observation data obtained by a three-dimensional ultrasonic anemometer. First, wind tunnel experiments were carried out in order to evaluate directional characteristics of the anemometer. Then, by using the experimental results, a correction table was developed which is used for correcting raw observation data. The effectiveness of the correction procedure was verified by applying it to the past field observation data obtained close to a building. From the comparison between raw and corrected data, it was found that the proposed procedure is effective for the mean vertical wind direction of larger than 30 degrees or smaller than -30 degrees.

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