Brief communication: A note on the variance of wind speed and turbulence intensity

This paper addresses the issue that several papers in the peer-reviewed literature on wind energy applications have used an incorrect equation that equals the variance of wind speed (sigma_U^2) to the sum of the variances of the wind components. This incorrect equation is often used to calculate turbulence intensity (TI), which, as a consequence, is often incorrectly estimated too. While exact analytical equations do not exist, here two approximate analytical equations are derived for sigma_U^2 and TI as functions of the variances and means of the wind components. Both formulations are validated with samples from a prior field campaign and perform satisfactorily.