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Publication# PIV Measurements in the Atmospheric Boundary Layer within and above a Mature Corn Canopy. Part I: Statistics and Energy Flux

Résumé

Particle image velocimetry (PIV) measurements just within and above a mature corn canopy have been performed to clarify the small-scale spatial structure of the turbulence. The smallest resolved scales are about 15 times the Kolmogorov length scale ($\nu \approx$ 0.4 mm), the Taylor microscales are about $100\nu$, and the Taylor scale Reynolds numbers range between $R_{\lambda} =2000$ and 3000. The vertical profiles of mean flow and turbulence parameters match those found in previous studies. Frequency spectra, obtained using the data as time series, are combined with instantaneous spatial spectra to resolve more than five orders of magnitude of length scales. They display an inertial range spanning three decades. However, the small-scale turbulence in the dissipation range exhibits anisotropy at all measurement heights, in spite of apparent agreement with conditions for reaching local isotropy, following a high-Reynolds-number wind tunnel study. Directly calculated subgrid-scale (SGS) energy flux, determined by spatially filtering the PIV data, increases significantly with decreasing filter size, providing support for the existence of a spectral shortcut that bypasses the cascading process and injects energy directly into small scales. The highest measured SGS flux is about 40% of the estimated energy cascading rate as determined from a -5/3 fit to the spectra. Terms appearing in the turbulent kinetic energy budget that can be calculated from the PIV data are in agreement with previous results. Evidence of a very strong correlation between dissipation rate and out-of-plane component of the vorticity is demonstrated by a striking similarity between their time series. 1. Introduction

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Recent advances in boundary-layer meteorology are beginning to allow the study of atmospheric flow phenomena that have previously been poorly understood. In this dissertation, we study the effects of complex terrain and unsteady regimes on the atmospheric boundary layer (ABL) dynamics, with the help of field measurements and theoretical analyses. We consider transitions generated by sharp surface discontinuities and by the daily cycle of solar heating. When air flows over an inhomogeneous landscape, it encounters a series of parcels (vegetated areas, mountains, cities, etc.) with different mechanical and thermal properties. Each parcel's boundary triggers a transition layer in the atmosphere. Substantial changes in momentum and heat exchanges are found when air flows over an urban area, due to the complex arrangement of buildings and the various thermal properties of the surface materials. To study the daytime heat exchanges between the built environment and the atmosphere, we conducted a field campaign over the EPFL campus in 2006-07 with a wireless network of 92 weather stations and an atmospheric profiler. In this analysis, the heat exchanges are successfully estimated with Monin-Obukhov similarity and the thermal roughness length method. We also illustrate how one carefully-selected station inside the urban canopy can provide a satisfying estimate of the sensible heat flux over the campus. The diurnal cycle of solar heating also induces transitions in the ABL. During the day, the ABL is usually characterized by an unstable stratification and large turbulent exchanges of momentum, heat and moisture. At night, the ABL is stably stratified and weak turbulent exchanges with the surface are typically observed. This dissertation presents a simple model to track the decay of atmospheric turbulence during the evening transition period, when the ABL shifts from its daytime to its nighttime regime. First, we describe a function to model the sensible heat flux during the transition period. This function is then inserted into a simplified version of the turbulent kinetic energy (TKE) budget, which we validate with several eddy covariance datasets. This study shows that the decay of convective turbulence over flat and unobstructed terrain occurs in three different steps: (i) the TKE is relatively constant for several convective eddy turnover time scales; (ii) the TKE decays with a t-2 rate, where t is time after the start of the decay; (iii) an abrupt decay rate of TKE near the sunset is observed indicating a rapid collapse of turbulence. We also study the evening transition period for slope flows developing over alpine terrain under clear skies and weak synoptic conditions. Slope winds travel upslope during the day and downslope at night. Little is known about the transition between these two wind regimes over steep slopes, mostly because they are extremely challenging to monitor. For this reason, in summer 2010, we deployed a suite of meteorological stations on a 25 to 45 degrees slope of the Swiss Alps. The results show that a 'shading front' induced by surrounding topography triggers the evening transition. The impact on the surface temperatures is substantial and in some cases, drops of 10 °C in less than 10 min are found. This is usually followed by an early evening calm period with low turbulence levels and small wind speeds (< 0.5 ms-1). At night, a shallow layer of 'skin' downslope flow forms, with the maximum wind velocity just above the surface (< 1.5 m).

Andreas Christen, Marc Diebold, Chad Higgins, Wolf Hendrik Huwald, Michael Lehning, Holly Jayne Oldroyd, Marc Parlange

Recent field campaigns held at the Val Ferret watershed in 2012 provided turbulent measurements in the atmospheric surface layer with and without snow cover. The turbulent kinetic energy (TKE) over the snow was reduced in comparison to the measurements obtained over bare surface. The “smoothing” of the surface by snow probably has a small role to play in the decrease of the TKE but the importance of the snow cover itself still has to be determined. Recent measurements obtained during the Plaine Morte 2013 field campaign using sonic anemometers are analysed. We discuss how the snowpack impacts the atmospheric turbulence under various snowpack conditions.

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Atmospheric measurements from several field experiments have been combined to develop a better understanding of the turbulence structure of the stable atmospheric boundary layer. Fast response wind velocity and temperature data have been recorded using 3-dimensional sonic anemometers, placed at several heights (≈ 1 m to 4.3 m) above the ground. The measurements were used to calculate the standard deviations of the three components of the wind velocity, temperature, turbulent kinetic energy (TKE) dissipation and temperature variance dissipation. These data were normalized and plotted according to Monin–Obukhov similarity theory. The non-dimensional turbulence statistics have been computed, in part, to investigate the general applicability of the concept of z-less stratification for stable conditions. From the analysis of a data set covering almost five orders of magnitude in the stability parameter ζ = z/L (from near-neutral to very stable atmospheric stability), it was found that this concept does not hold in general. It was only for the non-dimensional standard deviation of temperature and the average dissipation rate of turbulent kinetic energy that zless behaviour has been found. The other variables studied here (non-dimensional standard deviations of u, v, and w velocity components and dissipation of temperature variance) did not follow the concept of z-less stratification for the very stable atmospheric boundary layer. An imbalance between production and dissipation of TKE was found for the near-neutral limit approached from the stable regime, which matches with previous results for near-neutral stability approached from the unstable regime.

2001