Energy balance and partitioning and vertical profiles of turbulence characteristics during initial growth of a banana plantation in a screenhouse

This study focused on boundary layer characteristics and evapotranspiration in a cultivated screenhouse. The temporal course of latent and sensible heat fluxes, as well as vertical profiles of turbulence characteristics, below and above the screen were determined as the crop developed. The study was in a young banana plantation screenhouse, 5.5 m high, from June 17 (DOY 169) to August 8, 2016 (221), i.e. 53 days. Two masts measured simultaneously. A manually operated lifting tower mast near the eastern down-wind edge of the screenhouse operated either continuously at a fixed height or measured vertical profiles between 2.5 and 10.2 m height during the same day. The second 'reference' mast was 20 m north of the tower. Continuous measurements included latent and sensible heat fluxes above the plants and below the screen. During the experiment plant height increased from 1.7 to 4.1 m, LAI increased from 0.3 to 1.6 and the Bowen ratio, defined as the ratio between sensible and latent heat fluxes measured below the screen, decreased from 1.9 to 0.3. Energy balance closure, expressed as the slope of the relationship between half-hourly consumed and available energy for this period was 76% (R2 = 0.92); however, after summing up the fluxes over the whole period, the closure increased to 86%. Energy balance closure decreased with plant growth, presumably due to the more stable boundary layer associated with larger plants. During plant growth, ET per unit ground area increased linearly with LAI, but when calculated per unit leaf area it decreased, presumably due to increased mutual shading of leaves. Friction velocity below the screen was about 50% of that above the screen, illustrating the effect of the screen in absorbing momentum flux. Spectral energy slopes above the screen were close to the theoretical value typical of the inertial sub-range of steady state boundary layers, −5/3. Below the screen and within the canopy deviations from this theoretical value increased and the slope of spectra decay was higher than −5/3, indicating a higher rate of transfer of turbulent kinetic energy across scales. Integral length scale of turbulent vortices within and just above the canopy scaled with plant dimensions and between-plant distances, respectively; above the screen the integral length scale increased with height, as expected for fully developed turbulent flows over flat surfaces.