Turbulent transport processes that occur within canopies are extremely complex and have not been accurately represented in past models, especially for ecosystems with hilly or mountainous terrain. Better understanding of these transport processes is important for understanding the introduction of pollutants into the atmosphere, and the transfer of water and carbon dioxide between soil and vegetation, and the atmosphere. Prior research has identified three typical canopy-flow patterns: drainage flows; "chimney phenomenon"; and oscillations. These three regimes of canopy flows represent different mechanisms of ecosystem-atmosphere exchanges over complex terrain. The overall goal of this project is to develop analytical approaches for understanding the transition (instability) conditions between these three different regimes of canopy flows. To address this goal, specific objectives are to:
(1) derive analytical criteria for transition conditions between different regimes of terrain-induced canopy flows by a nonlinear dynamics approach from simplified hydrothermal equations;
(2) verify the analytical results using computational fluid dynamics; and
(3) validate the analytical and numerical results using field observations.
Intellectual Merit: Understanding canopy flow over complex terrain is crucial for understanding the global carbon dioxide budget. The annual budget of carbon dioxide relies on the delicate balance between daytime summer carbon dioxide uptake by canopies, and nighttime and winter ecosystem release due to respiration. The nighttime upward transport is strongly affected by respiration when drainage flow is a dominant transporting mechanism, even over reasonably flat terrain. Many eddy flux tower sites are located in complex terrain where topographic advection errors can be of the same order as the upward eddy flux itself during calm nights. Errors that accumulate during eddy flux measurements in the stable nighttime atmosphere have confounded researchers and slowed the widespread use of flux measurements. The instability conditions can be used at flux sites for understanding these advection problems.
Broader Impacts: The stability studies of canopy flows over complex terrain will be beneficial not only for the eddy flux research community in solving the advection problem, but also for assimilating canopy layer observations into atmospheric, hydrological, ecological, and air quality models. The theoretical analysis will be incorporated into university-teaching curriculum and text books. The Principal Investigator will offer a graduate lecture/lab course in Academic Year 2011-2012 with the title "Canopy Flow Theory and Simulations" in the School of Earth and Environmental Sciences at Queens College of CUNY. A post-doctoral Research Associate will be trained as a theoretical scientist in using nonlinear dynamics to understand the instability of slope flows.
