The Vertical Structure of Turbulent Eddies over Vineyards

To effectively manage limited water resources and ensure the sustainability of the California wine industry, robust methods for monitoring both evapotranspiration and vine water stress across the continuum from sub-field to regional scales are needed. Remote sensing is the only viable means of addressing this need. However, due to highly-structured canopies of vineyards, which are characterized by wide rows and a tall canopy with the biomass concentrated in the upper half of the plants, turbulent exchange and transport processes may not be accurately described by model relationships developed over other agricultural and natural landscapes. Therefore, one key step toward enhancing remote sensing models is developing relationships that accurately describe the influence of vineyard canopy architecture on turbulent exchange. In turn, this requires a thorough understanding of the turbulent structure over vineyards. This study seeks to address this need by using data collected during the 2017 growing season as a part of the Grape Remote sensing Atmospheric Profiling and Evapotranspiration eXperiment (GRAPEX), a multi-institution field project ongoing in the Central Valley of California, this study seeks to characterize the vertical structure of turbulence over vineyards under both non-advective and advective conditions. Specifically, a combination of Fourier and wavelet-based approaches were used to analyze time-synchronous wind velocity data collected at four heights (2.50, 3.75, 5.00, and 8.00 m agl) directly assess variations in the spectral distributions, thus turbulent structure, as a function of height. The spectral analyses indicate that the turbulent structure became increasingly decoupled with separation distance and, as a result, the coherence of the turbulence decreases following a well-defined exponential decay relationship under all conditions. They also show there was a strong low-frequency contribution to the turbulent structure that is persistent under both non-advective and advective conditions. While the intrusion of large eddies into the surface layer does not appear alter the turbulent fluxes under non-advective conditions, the entrainment of warm dry air under advective conditions both enhances the latent heat flux generates pseudo-stable conditions which result in greater interaction between the turbulent eddies and the canopy. Although the changes in the vertical structure of the turbulence appears similar under both non-advective and advective conditions, the results of the study demonstrate that the vertical structure of the turbulence follows well-define relationships supported by Monin-Obukhov similarity theory. Perhaps more important in a practical sense, the results highlight the importance of correctly describing advective effects when modeling the surface fluxes over vineyards.