Vertical variations in the turbulent structure over vineyards

Abstract:

Due to their highly-structured canopy, turbulent characteristics within and above vineyards, may not conform to those exhibited by other agricultural and natural ecosystems. As a result, the current generation of land surface models may not adequately describe the turbulent exchange of heat and moisture between the atmosphere and the surface over vineyards. Using data collected during 2014 as a part of the Grape Remote Sensing Atmospheric Profiling and Evapotranspiration Experiment (GRAPEX), an ongoing multi-agency field campaign conducted in the Central Valley of California, this study sought to characterize the variations in the turbulent structure over vineyards. Focusing on unstable daytime conditions, the study compared the turbulent structure at three above-canopy heights: 2.5 m, 3.75 m, and 8 m, agl. Both wavelet and Fourier-based spectral analysis of the wind velocity components indicates a strong tendency for the spectral peak to broaden and shift to 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 distance between the measurements increases. In other terms, eddies contributing to a measurement at one height act independently of similarly-sized eddies at another height. As a result, the overall correlation between the turbulent flows measured at differing heights decreases exponential with increasing separation distance. While this effect was seen for all of the periods analyzed, the magnitude of the effect does appear to vary in response to the direction of the wind relative to the vineyard rows.