Observational study of power-law approximation of wind profiles within an urban boundary layer for various wind conditions

This paper investigates the accuracy and limitations of wind profile modeling using the power-law (PL), especially for low speed conditions in which air and thermal pollution can prevail. A Doppler lidar system and ultrasonic anemometer were installed to measure wind profiles and turbulence statistics in the urban boundary layer of Tokyo, Japan over seven months. The wind speeds at a height of 67.5 m (u(b)) at average intervals of 10 min were < 6 m/s for 80% of the observation period. For low wind speeds, the difference in wind direction with height is significant, making it difficult to determine the prevailing wind direction. The PL could be used to model the wind profiles for high wind speeds (u(b) > 6 m/s), whereby the power-law index (PLI) converges to 0.25. Although the PL model can be used for an ensemble-averaged profile composed of all profiles from the observed period, the accuracy of the PL decreases for profiles with low speeds and short average time intervals. The PLI on average decreases to similar to 0.21 for low speeds and shows diurnal changes with small PLIs during the daytime. This research quantitatively discusses the application limits of the PL for windprofiles under low speed conditions.