Abstract
The Canopy Horizontal Array Turbulence Study (CHATS) took place in spring 2007 and is the third in the series of Horizontal Array Turbulence Study (HATS) experiments. The HATS experiments have been instrumental in testing and developing subfilterscale (SFS) models for large-eddy simulation (LES) of planetary boundary layer (PBL) turbulence. The CHATS campaign took place in a deciduous walnut orchard near Dixon, California, and was designed to examine the impacts of vegetation on SFS turbulence. Measurements were collected both prior to and following leafout to capture the impact of leaves on the turbulence, stratification, and scalar source/sink distribution. CHATS utilized crosswind arrays of fast-response instrumentation to investigate the impact of the canopy-imposed distribution of momentum extraction and scalar sources on SFS transport of momentum, energy, and three scalars. To directly test and link with PBL parameterizations of canopy-modified turbulent exchange, CHATS also included a 30-m profile tower instrumented with turbulence instrumentation, fast and slow chemical sensors, aerosol samplers, and radiation instrumentation. A highresolution scanning backscatter lidar characterized the turbulence structure above and within the canopy; a scanning Doppler lidar, mini sodar/radio acoustic sounding system (RASS), and a new helicopter-observing platform provided details of the PBL-scale flow. Ultimately, the CHATS dataset will lead to improved parameterizations of energy and scalar transport to and from vegetation, which are a critical component of global and regional land, atmosphere, and chemical models. This manuscript presents an overview of the experiment, documents the regime sampled, and highlights some preliminary key findings.
Highlights
Observations from a California walnut orchard—before and after leaves emerged— should help advance understanding, simulation capabilities, and modeling of coupled vegetation–atmosphere–land surface interactions
Recent studies postulate that these eddies are produced by larger planetary boundary layer (PBL)-scale turbulent motions triggering an instability associated with the inflection in the velocity profile induced by plant canopy drag (e.g., Raupach et al 1996; Finnigan et al 2009)
The distribution of canopy sources/ sinks depends on the amount and state of the canopy foliage, which varies throughout the seasonal cycle for deciduous trees: from bare limbs in winter to rapid growth in spring to maturity in summer
Summary
THE CANOPY HORIZONTAL ARRAY TURBULENCE STUDY by Edward G. Vegetation covers nearly 30% of Earth’s land surface and influences climate through the exchanges of energy, water, carbon dioxide, and other chemical species with the atmosphere (Bonan 2008). It has become clear that the bulk of the exchange between the canopy layers and aloft occurs through turbulent eddies that are of similar scale to the canopy itself rather than to the scale of the individual canopy elements (e.g., Gao et al 1989). Recent studies postulate that these eddies are produced by larger PBL-scale turbulent motions triggering an instability associated with the inflection in the velocity profile induced by plant canopy drag (e.g., Raupach et al 1996; Finnigan et al 2009). The distribution of canopy sources/ sinks depends on the amount and state of the canopy foliage, which varies throughout the seasonal cycle for deciduous trees: from bare limbs in winter (no photosynthesis and an open canopy) to rapid growth in spring (increasing photosynthesis and canopy density) to maturity in summer
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