Vertical Variations in the Turbulent Structure of the Surface Boundary Layer over Vineyards under Unstable Atmospheric Conditions

Due to their highly-structured canopy, turbulent characteristics within and above vineyards, may not conform to those typically exhibited by other agricultural and natural ecosystems. Using data collected as a part of the Grape Remote sensing and Atmospheric Profiling and Evapotranspiration Experiment (GRAPEX), an ongoing multi-institution field project conducted in the Central Valley of California, this study evaluated the spectral characteristics of 10 one-hour periods spanning the 2014 growing season to investigate vertical variations in the turbulent structure over vineyards. Focusing on unstable daytime conditions when the surface boundary layer was well developed, the study compared the turbulent structure at three above-canopy heights: 2.5 m, 3.75 m, and 8 m, agl using both traditional and wavelet-based spectral analysis. Fourier-based spectral analysis of the wind velocity components indicates a strong tendency for the spectral peak to broaden and shift toward lower frequencies as the measurement height increases. Also, beginning with the highest-frequency eddies, the turbulent structure at differing heights become increasingly decoupled as the separation distance between the measurements increases indicating the eddies contributing to the measurements at one height act independently of similarly sized eddies measured at another height. As a result, the overall correlation between the turbulent flows measured at differing heights decreases exponential with increasing separation distance. Additionally, wavelet analyses showed the intensity of the turbulent mixing fluctuated significantly over each of the hourly measurement periods with intermittent periods of strong turbulent activity interspersed between intervals of relative calm. While these effects were evident for all of the hourly periods considered, the magnitude of the effects appears to vary in response to the direction of the wind relative to the orientation of the vineyard rows.