Improved calculation of offshore meteorological parameters for applications in wind energy

Work presented in this thesis demonstrates an improved calculation of offshore meteorological parameters, in particular the wind speed and turbulence intensity, relevant for wind energy applications. An alternative offshore drag law is proposed that anticipates drag coefficients at intermediate wind speeds expected in applications for wind energy, but is also consistent with the functional behaviour of drag coefficients as a function of wind speed expected at tropical cyclone wind speeds. The law is compared with measurements recorded at the FINO1 platform in the North Sea and with those reported in the literature, where differences are attributed to the nature of the water wave field. A correlation equation connecting the air side drag coefficients and water side wave steepnesses is then proposed which can also anticipate drag coefficients expected at tropical cyclone wind speeds using measurements from a buoy recorded during hurricane Rita (2005). The correlation equation interpolates between two hypothesised asymptotic regimes: One whereby drag coefficients scale with the squared wave steepness and the other whereby drag coefficients are constant. At wind speeds relevant for wind energy purposes, two distinctive wave steepness scaling regimes were detected in measurements recorded at FINO1, which are also evident in results reported in the literature. A higher order correlation attempted here, but left open for further development in the future, finds that the unsteady orientation of the wind with the wave direction is likely a further important parameter. This is found by analysing a stable internal boundary layer detected at FINO1 for a period of about a week, which resulted in a diurnal cycle of offshore meteorological parameters including the wind direction. An oscillation was correspondingly found in buoy measurements of wave steepnesses during this period, and hence in drag coefficients. It is demonstrated if higher order wave field effects can be correctly accounted for, an enhanced calculation in the wind speed will result. The nature of the stable internal boundary layer then facilitated investigation throughout a large portion of the boundary layer by the 100 m high FINO1 tower, due to the relatively shallow (O(100) m) height of the internal layer. Comparison with numerical simulations using the Mellor-Yamada-Janji\'{c} planetary boundary layer parametrization within the Weather Research and Forecasting model showed contrasting vertical profiles of turbulent kinetic energy compared with the measurements. This consequently resulted in an overprediction of turbulence intensities as calculated by the model compared with FINO1 measurements at 80 m above the sea surface, and an underprediction of turbulence closer to the surface as inferred from previously reported work in the literature. It is shown here that an improved calculation of turbulence is possible by making changes to the selection of the closure constants and the surface length scale. Comparison with more conventional, less stably stratified periods at FINO1 showed the current Mellor-Yamada-Janji\'{c} parametrization underpredicted the 80 m high turbulence intensity during these periods, whereas the changes proposed here showed an improved calculation of hub height turbulence intensity