Difficulties in using power laws for wind energy assessment

A 1/7 power law is often used to estimate atmospheric wind profiles. The use of this relationship, while perhaps appropriate in a climatological sense for daytime wind profiles in the lower atmosphere, frequently results in serious underestimates of wind speeds aloft at night. Several years of wind speed data from a 45 m tower and from use of a Doppler acoustic sounder (sodar) indicate that low-level wind speed maxima seem to form on 50 per cent of summer nights and 15 per cent of winter nights in northeastern Illinois. Even with 24 hr averages, the extrapolation of 6 m wind speeds to those at 45 m calculated with the 1/7 power law expression are found to be approximately 15 per cent too small, corresponding to a 40 per cent underestimate of wind power potential.