Diurnal variations of land surface wind speed probability distributions under clear-sky and low-cloud conditions

Abstract

Long-term 10 min wind tower data and ceilometer backscatter data at Cabauw in the Netherlands provide quantitative information on the influence of low clouds on the diurnal evolution of the near-surface wind speed (NSWS) probability distribution (probability density function), the wind power density (WPD), and their vertical structure in the bottom 200 m of the atmosphere. Under clear-sky conditions, pronounced diurnal cycles are identified in the leading three moments of NSWS as well as WPD and the boundary layer thermal structure in all seasons. When low clouds are present, weaker diurnal cycles with a different vertical structure are observed. Under clear skies, skewness at night is positive within the stable air near the surface but negative above 100 m. In the presence of low clouds, wind speeds are positively skewed and the probability of strong winds is higher associated with a larger geostrophic wind speed. Key Points Diurnal variations of near surface wind speed PDF are influenced by low clouds Thermal structure and geostrophic winds affect wind speed PDF shape variations Pronounced diurnal cycle of mean wind power density is suppressed under clouds ©2013. American Geophysical Union. All Rights Reserved.