Complex terrain poses significant problems to eddy covariance measurements above forest canopies. Improving eddy covariance measurements over complex terrain requires a better understanding of how complex terrain influences spatial and temporal variability in turbulent flows above and within forest canopies. This will lead to improvements in measurements of the exchange of momentum, heat, and scalars between the atmosphere and vegetation, so that reliable interpretations/assessments of the surface energy balance, water cycle, and carbon budget over complex terrain can be made over various temporal and spatial scales. Currently, there is no clear understanding of how the simultaneous action of complex terrain, dynamic and thermodynamic conditions of inflows, and plant canopies modulate turbulence structures and thus transport of momentum, heat, water vapor, and carbon dioxide. In this research, turbulence in the forest canopy-atmosphere interface over a complex terrain will be examined by conducting analyses of the data measured in a European EGER experiment (ExchanGE processes in mountainous Regions) integrated with large-eddy simulations (LES). The Washington State University (WSU) research team along with six other international groups participated in the EGER experiment that was conducted in June and July of 2011 at the FLUXNET site in Weidenbrunnen Waldstein (DE-Bay), located in North-Eastern Bavaria, Germany. Each group contributed different instruments and research activities to map, to the fullest extent possible, three-dimensional turbulence structures at the site. In addition to data analysis, a multi-layer canopy module will be incorporated into the Weather Research and Forecasting Model (WRF) - LES (WRF-LES) to explore spatial variations and temporal evolutions of mean/turbulent flows and quantify relative contributions of different mechanisms to momentum/heat/H2O/CO2 transfer. Collectively, this research will examine:
1) How does the interaction of 'real' topography-induced pressure perturbations and canopy alter turbulence structures, including coherent structures and high-order turbulent statistics such as velocity variances, turbulent stresses/fluxes, pressure variance, and production/loss of TKE above and within the canopy, as compared with the structures observed over idealized hills? 2) How do different atmospheric stability conditions alter spatial and temporal variations in the main features of mean/turbulent flows within and above the canopy, as compared with previous results under neutral atmospheric conditions? The main features of turbulent flows include shear layer, inflexion point, TKE, second-order statistics, skewness and kurtosis of u and w profiles, wake region and wake depth (lee side only), and recirculation (lee side only). 3) How do mean/turbulent flows as a result of the simultaneous actions of topography, canopy, and stability, lead to spatial and temporal variations in CO2 fluxes, horizontal and vertical advections, flux divergence, and CO2 sources and sinks? What are the dynamic mechanisms for these spatial and temporal variations in CO2 fluxes and the implications for tower measurements of CO2 fluxes?
Intellectual Merit: Overall, the study will provide an improved understanding of mean and turbulent flows and exchange of momentum, heat, and scalars (e.g., CO2) between the canopy and the atmosphere over mountainous regions. Applications include: 1) simulation of carbon cycling in complex terrain, 2) wind energy predictions in complex terrain, and 3) pollutant dispersion in complex environments.
Broader Impacts: Results from this work will improve our overall ability to quantify carbon, water, and energy flows in complex terrain and thus improve our understanding of important components of global carbon science. The results will be beneficial to carbon cycle science and the FLUXNET community in helping constrain the carbon budget and upscale CO2 fluxes from tower to landscape scale and even to regional scale over complex terrain. The updated WRF-LES modeling system with a multi-layer canopy module will be beneficial to a variety of research communities in studying canopy flows and PBL flows over complex terrain; wind energy applications in terms of identifying potential locations for wind turbines; forest management in identifying locations of high risks of tree damage in windy conditions; and forest fire behaviors in quantifying fire propagation; The research will contribute directly to the educational research training of Ph.D. students at WSU. The results will be disseminated to a broader audience through the FLUXNET community, conferences, and seminars, and will be used in courses and workshops related to WSU's undergraduate and graduate curriculum as well as the summer REU program which is focused on atmospheric chemistry, air quality, and climate change.
