Time Scales in the Unstable Atmospheric Surface Layer

Abstract

Calculation of eddy covariances in the atmospheric surface layer (ASL) requires separating the instantaneous signal into mean and fluctuating components. Since the ASL is not statistically stationary, an inherent ambiguity exists in defining the mean quantities. The present study compares four methods of calculating physically relevant time scales in the unstable ASL that may be used to remove the unsteady mean components of instantaneous time signals, in order to yield local turbulent fluxes that appear to be statistically stationary. The four mean-removal time scales are: (t c ) based on the location of the maximum in the ogive of the heat flux cospectra, (t M R ) the location of the zero crossing in the multiresolution decomposition of the heat flux, (t * ) the ratio of the mixed-layer depth over the convective velocity, and (t ) the convergence time of the vertical velocity and temperature variances. The four time scales are evaluated using high quality, three-dimensional sonic anemometry data acquired at the Surface Layer Turbulence and Environmental Science Test (SLTEST) facility located on the salt flats of Utah's western desert. Results indicate that t c ≈ t M R and t * ≈t, with t c achieving values about 2-3 times greater than t * . The sensitivity of the eddy covariances to the mean-removal time scale (given a fixed 4-h averaging period during midday) is also demonstrated.

Interactions within the turbulent boundary layer at high Reynolds number

“…As discussed above, the surface roughness is highly dependent on the time of year and playa condition, leading to estimates of aerodynamic roughness length z 0 or equivalent sand roughness k s , which can vary essentially between the smooth and transitionally rough regimes for different sets of experiments (Metzger & Holmes 2008). For instance, near-wall hot-wire measurements by Metzger in 2003(included in Metzger et al 2007) are characterized by k + s as high as 50, while sonic anemometer measurements further from the wall (e.g.…”

A Simple Model for the Afternoon and Early Evening Decay of Convective Turbulence Over Different Land Surfaces

“…The sonic anemometer used in this study was located 2.3 m above the ground. For more details about the SLTEST site, see Metzger and Holmes (2008). At the SLTEST site, 2 days met the selection criteria for our analysis: 11 July and 21 July 2002.…”

Monin-Obukhov Functions for Standard Deviations of Velocity

“…Although the salt flat was ideally uniform for winds from the north (Metzger and Holmes 2008), the tower and transect had been installed near a raised parking area, on which stood several large instrument trailers (see McNaughton et al 2007, for a photograph of the set-up). Comparison of statistics from sonics along the transect (at z = 3 m) during northerly flow revealed an influence of these obstacles that could readily be detected at the 3-m sonic on the tower (viz.…”