Perturbations to the Spatial and Temporal Characteristics of the Diurnally Varying Atmospheric Boundary Layer Due to an Extensive Wind Farm
The effect of extensive terrestrial wind farms on the spatio temporal structure of the diurnally evolving atmospheric boundary layer is explored. High resolution large eddy simulations of a realistic diurnal cycle with an embedded wind farm are performed. Simulations are forced by a constant geostrophic velocity with time varying surface boundary conditions derived from a selected period of the CASES 99 field campaign. Through analysis of the bulk statistics of the flow as a function of height and time, it is shown that extensive wind farms shift the inertial oscillations and the associated nocturnal low level jet vertically upwards by approximately 200 m; cause a three times stronger stratification between the surface and the rotor disk region, and as a consequence, delay the formation and growth of the convective boundary layer (CBL) by approximately 2 h. These perturbations are shown to have a direct impact on the potential power output of an extensive wind farm with the displacement of the low level jet causing lower power output during the night as compared to the day. The low power regime at night is shown to persist for almost 2 h beyond the morning transition due to the reduced growth of the CBL. It is shown that the wind farm induces a deeper entrainment region with greater entrainment fluxes. Finally, it is found that the diurnally averaged effective roughness length for wind farms is much lower than the reference value computed theoretically for neutral conditions.
Boundary Layer Meteorology