An Examination of Residual Wind Fluctuations Observed at 10 m over Flat Terrain

Abstract

This study investigates the behavior of wind fluctuations observed at the 10-m level over a flat terrain site located some 100 km east of the Rocky Mountains. The purposes were to assess residual fluctuations in order to ascertain effects attributable to the nonhomogenous, nonstationary character of turbulence and to evaluate influences of gravity waves. Residual wind fluctuations were defined for purposes of this study as the differences between observed half-hourly average standard deviations of wind fluctuations (σv, σu, σw) and those that are expected to occur in association with simultaneous wind speeds and static stabilities. These latter fluctuations were estimated from equations developed by Leahey, Hansen, and Schroeder (LHS). Results of the analyses showed, as expected, that residual distributions for nonwesterly wind conditions were nearly Gaussian. Standard deviations for residuals of horizontal fluctuations, attributable to the nonhomogenous, nonstationary nature of turbulence, were 0.165 and 0.210 m s?1 for stable and unstable situations, respectively. For residuals associated with vertical fluctuations they were, respectively, 0.065 and 0.075 m s?1. Residuals for horizontal and vertical wind fluctuations observed when winds were from the mountains showed a greater tendency for the positive bias associated with gravity waves. This tendency was most evident under unstable conditions when gravity wave influences on horizontal fluctuations were apparent about 25% of the time. These influences are explained as being associated with mountain lee waves occurring at the planetary boundary layer's capping inversion. They are evidenced at the 10-m level because atmospheric mixing processes occurring in thermally unstable atmospheric situations bring momentum generated from these waves downward to the ground. Nonstationary and nonhomogenous atmospheric turbulence effects result in wind fluctuations whose half-hourly average standard deviations differ from those predicted by the LHS equations. Differences under stable atmospheres and low to moderate wind speeds are typically less than 50% of predicted values. They decrease as a percentage of predicted values with increasing wind speed and decreasing stability.