Posters

Abstract

The atmospheric stratification, caused by the daily cycle of the surface heating and cooling, impacts a lot on the wind profile extrapolation to the hub heights in wind resource assessments. The atmospheric stratification could be an unstable stratification when the surface is heated during the day, or a stable stratification when the surface is cooled at night. The atmospheric stratification is estimated with the Monin-Obukhov length or Richardson numbers etc. which depend on the heat flux. When the heat flux isn't available, it could be obtained by the temperature profile based on Monin-Obukhov similarity theory (MOST). Using the temperature profile and the wind speed, Meteodyn WT calculates the Monin-Obukhov length iteratively from a guessed Monin-Obukhov length. The algorithm searches iteratively the better approximation of the Monin-Obukhov length. Svenningsen [1] proposes a novel method to quantify atmospheric stability in the absent of temperature measurement. Based on the standard wind measurements, the high wind speeds turbulence and wind shear are selected to estimate the neutral stratification in given direction. Then the wind with low turbulence and higher wind shear than the neutral wind is considered as the stable stratification. And the wind with high turbulence and lower wind shear than the neutral wind is considered as the unstable stratification. The method of Meteodyn WT and the Svenningsen's method are compared using the Cabauw site data. At the Cabauw site, the air temperature and the wind speed are measured at heights between 10m and 200m for four years. The monthly and daily wind data analysis shows that the two methods can distinguish the seasonal and daily variation of the thermal stability. The method of Meteodyn WT demonstrates more stable stratification at night regarding the daily variation. And it presents more evident thermal stability in winter regarding seasonal variation. [1] L.Svenningsen, René M.M.Slot and Morten L. Thogersen, A novel method to quantify atmospheric stability, Journal of Physics: Conf.Series (2018)