Influence of stability and seasonal canopy changes on micrometeorology within and above an orchard canopy: The CHATS experiment

Understanding the micrometeorology within and above forest canopies is of great interest for many environmental applications such as weather and climate forecasting as well as for vegetation-atmosphere scalar exchanges. This study investigates the sensitivity of velocity, temperature and humidity fields within and above a deciduous orchard to atmospheric stability and seasonal canopy changes. To that purpose, high spatial resolution micrometeorological measurements from the 30m tower of the Canopy Horizontal Array Turbulence Study (CHATS) are analyzed. CHATS was performed in 2007 in a 10m tall deciduous walnut orchard in California (USA) prior to and following leaf-out. Statistical profiles of micrometeorological fields from first to fourth moments are presented following five stability regimes (free and forced convection, near-neutral, transition to stable and stable) and two seasonal periods (no-leaves and with-leaves). The observations show that leaves not only modify the density of the canopy but also (i) increase the effective canopy height, (ii) modify the location and intensity of heat and humidity sources-sinks, and (iii) limit ground surface cooling at night. The defoliated canopy imparts significant heat flux from the woody biomass to the atmosphere implying that this heat flux should be included in Soil Vegetation Atmosphere Transfer (SVAT) models applied over deciduous canopies. In near-neutral conditions, the canopy flow better resembles a plane mixing-layer flow during the foliated period than during the defoliated period; where during the no-leaves period, the canopy flow appears to be a superposition of a wall boundary-layer flow with a plane mixing-layer flow. Deviations from near-neutral conditions (either unstable or stable), the plane mixing-layer analogy weakens. In unstable conditions, the importance of mixing-layer type coherent structures within the canopy flow decreases as shown by weaker canopy-induced vertical shear of streamwise velocity, lower velocity skewness and kurtosis maxima, and reduced efficiency of turbulence to transport momentum. With increasing atmospheric instability, heat and humidity appear to be transported by thermal plumes. In stable conditions, small and very intermittent ‘shear-driven’ coherent eddy structures may appear at canopy top, with thermal plumes developing in the trunk space during the fully leafed period. Finally, statistical characteristics of scalars (temperature and humidity) within the canopy appear dependent on the location of their sources. Hence, in unstable conditions, scalar sources increase scalar skewness and (to a lesser degree) scalar fluctuation magnitude.