TURBULENT PROCESSES AND FLUX EXCHANGE WITHIN FOREST CANOPIES OVER COMPLEX TERRAIN
Russell, Eric Scott
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This research made use of eddy covariance measurements and data collected in complex forest terrain to address questions regarding turbulence structure and scalar transport under weak wind and complex conditions. Two data sites were used to investigate the dependencies of mean turbulence under different conditions. The results show that canopy depth and wind speed influence degree of turbulence. In addition, there was influence from wind direction overnight at one site as overnight was more directly influenced by the sloped terrain. Similarities between daytime and nighttime turbulence and wind speed magnitude for daytime and nighttime did exist though the range of turbulence was lower overnight. The base turbulence structure was built around canopy depth and modulated by stability and wind direction. The level of intermittency of the turbulence was also determined through an intermittent fraction calculation with a case study to investigate the effects on the underlying structure. Aside from two very turbulent nights, the level of intermittency was relatively consistent at each height investigated. Connection between different time scales aided in the generation of higher turbulence periods and increased mixing. Investigation into the scalar fluxes was completed at the Priest River Experimental Forest site. The underlying terrain was linked to the scalar variation through its influences on the turbulence profile as it affects the scalar mixing. The evolution of temperature and water vapor created distinct periods of stratification that influenced how scalars were mixed generating distinct time periods throughout the day. The morning transition effects were more prominent on CO2, and the evening transition effects more prominent on H2O while daytime and nighttime were dominated by turbulent mixing and advective effects, respectively. The evening and overnight in-canopy mixing would not be captured by above canopy measurements directly. The mixing was limited by the increased stratification so the effects near the ground were not translated to higher in the canopy even though wind speed and turbulence were not dissimilar.