Dependence of turbulent and mesoscale velocity variances on scale and stability

The scale dependence of velocity variances is studied using data collected from a grassland site, a heather site, and four forested sites. The depende nce of velocity variances on averaging time, used to define the fluctuation quantities, is modeled. The crosswind velocity variance is emphasized, bec ause it is more difficult to model than the other two components and is cru cial input for applications such as dispersion modeling. The distinction be tween turbulence and mesoscale variances is examined in detail. Because mes oscale and turbulence motions are governed by different physics, meaningful study of the behavior of velocity variances requires adequate separation o f turbulence and mesoscale motions from data. For stable conditions, the ho rizontal velocity variances near the surface exhibit a spectral gap, here c orresponding to a very slow or nonexistent increase of variance with increa sing averaging time. This "gap region,'' when it occurs, allows separation of mesoscale and turbulence motions; however, the averaging times correspon ding to this gap vary substantially with stability. A choice of typical ave raging times for defining turbulent perturbations, such as 5 or 10 min, lea ds to the capture of significant mesoscale motions for very stable conditio ns and contributes to the disagreement with turbulence similarity theory. F or unstable motions, the gap region for the horizontal velocity variances s hrinks or becomes poorly defined, because large convective eddies tend to " fill in'' the gap between turbulence and mesoscale motions. The formulation developed here allows turbulence and mesoscale motions to overlap into the same intermediate timescales. The mesoscale variances are less predictable , because a wide variety of physical processes contribute to mesoscale moti ons. Their magnitude and range of timescales vary substantially among sites . The variation of the behavior of turbulence variances among sites is sign ificant but substantially less than that for the mesoscale motions.