The Influence of Boundary Layer Processes and Gestrophic Winds on the Diurnal Variation of the Climatological Near-Surface Winds Distribution over Land

Knowledge of factors influencing near surface wind speed (SWS) probability distribution (PDF) and persistence of wind directions (PWD) are essential for understanding wind resource variations over land. The influences of boundary layer processes and background gestrophic winds on diurnal varying SWS PDF and PWD are investigated combining 10-year wind tower observations and single column climate model (SCM) simulations at Cabouw, Netherlands in the summer season of JJA. The observed diurnally varying vertical structure of the leading three climatological moments of SWS(mean, standard deviation, and skewness), the PWD, and the wind power density can be successfully simulated using the single column version of the Canadian Centre for Climate Modelling and Analysis (CCCma) fourth generation atmospheric general circulation model (CANAM4) with a new semi-empirical diagnostic turbulence kinetic energy (TKE) scheme representing down-gradient turbulence transfer process and a simple stochastic representation of intermittent turbulence at the boundary layer inversion. It is found that the mean and the standard deviation of wind speed are most influenced by large-scale "weather" variability, while the shape of the PDF is influenced by daytime downward turbulent mixing in unstable PBL (leading to a Weilbull-like PDF), and by nighttime intermittent mixing process in stable boundary layer (leading to more skewed PDF). The later warms the land surface and cools the bottom level air temperature at night. The wind direction persistence is found to be most influenced by geostrophic winds variations at the Cabauw site.