Predicting airflow above canopies in complex terrain
Submitter
Chamecki, Marcelo — University of California, Los Angeles
Area of research
General Circulation and Single Column Models/Parameterizations
Journal Reference
Science
Mathematically describing air flow in the vicinity of tall canopies is required in a plethora of climate- and atmospheric-related applications. The swirling motion associated with air flow and how it interacts with the canopy structure has resisted complete theoretical treatment, which is the subject of this work.
Impact
The work offers a new formation of how the canopy alters air flow near the canopy-atmosphere interface, which is then field tested in Amazonia. The discovery constitutes a new link between the kinetic energy in gusty motion and the diffusive capacity of air to transport gases and aerosols between the biosphere and the atmosphere.
Summary
The law of the wall has shaped the description of the mean velocity of fluids above solid walls since its inception in the 1930s and it continues to be used today in climate and atmospheric models. However, the flow near canopies differs substantially from the flow over rough solid surfaces because the vegetation structure distorts the flow statistics from predictions offered by the law of the wall. The work here derives a mathematical formula describing these distortions from the law of the wall based on the kinetic energy associated with the multiple scales of motion describing eddies or the swirling motion. The derived formulation, which was tested at two sites across the Amazon forest, shares resemblance with the so-called fluctuation dissipation theorem in statistical mechanics that links the energy in the micro-states (eddies here) to the movement at the macro-scales (a bulk diffusion coefficient here). The difference between the derivation offered here and the fluctuation dissipation theorem is that the work here allows for large separation between the scales at which vegetation impacts the swirling motion and molecular processes converting this energy into heat.
Figure 1. Schematic displaying the effects of the forest canopy and the underlying gentle topography on the mean wind speed profile within and above the forest.