Variations of Wind Fluctuations Observed at 10 m over Flat Terrain under Unstable Atmospheric Conditions

Abstract

Turbulence data collected at the 10-m level during convective conditions at a site located amid flat terrain in Alberta have been analyzed with respect to wind speed U and normalized static stability S?n. These two parameters have been assumed to respectively represent mechanical and thermal forces that engender atmospheric turbulence at ground level. Observations of atmospheric turbulence show wide scatter in the value of the standard deviations of transverse, longitudinal and vertical wind fluctuations (σv, σu, σw) for the same apparent conditions of mechanical and thermal forces (i.e., wind speed and static stability). It has been assumed that the large scatter is attributable to random localized effects such as those caused by the breaking of internal gravity waves. For this reason the present analysis has been restricted to median values of σv, σu, and σw in an effort to discern a pattern of behavior that may be explained in terms of U and S?n. Equations have been empirically developed for median standard deviations of wind fluctuations in terms of wind speed and static stability. One-to-one correlation coefficients between predicted and observed data were typically in excess of 0.90. Results of this study complement findings of a previous study done with the same database except for stable atmospheric situations. Information from the two studies allows for the estimation of parameters (σv/U, σu/U, σw/U) for use in plume dispersion models under a wide range of wind and stability conditions. Estimation procedures depend upon easily measured meteorological variables (U, S?n). Illustrations of the dependencies have been provided.