Diurnal and semidiurnal variability of coastal wind over Taiwanese waters

The Taiwan Strait is rich in offshore wind energy. Developing offshore wind farms is vital for renewable energy development. Rapid variations in the speed of offshore wind affect the capacity and operations of wind farms. On the basis of the demand for developing offshore wind farms in Taiwanese waters, this study examined in situ coastal wind data and documented the characteristics of the diurnal and semidiurnal oscillations in surface wind as well as the seasonal and spatial characteristics of these oscillations. Wavelet-based rotary spectral analysis and significance level theory were adopted to quantify the occurrence probability and intensity of diurnal and semidiurnal wind fields. The occurrence probability of semidiurnal variations was approximately 50% greater than that of diurnal variations across Taiwan, with persistent occurrence of such variations near the southwest coast. The intensity of the diurnal phenomena was twice that of the semidiurnal phenomena, reaching an average of 27% of the wind energy variation at the potential sites located in Western Taiwan. Diurnal wind velocity oscillations were most intense during the fall but declined as the latitude moved from north to south. The instantaneous rotary spectrum was used to estimate the ellipticity of the wind vector. The analysis indicated that diurnal wind was dominated by land-sea breezes. Moreover, the semidiurnal wind was less influenced by surface thermal effects than by diurnal phenomena. The effects of these phenomena on offshore wind energy conversion are considerable and should not be ignored.