Evolution of turbulence in the vertical under near-calm conditions within a forest canopy over complex terrain

Understanding sub-canopy turbulence structures and micrometeorological processes is very important in quantifying emissions of trace gases and their exchanges in the forest-atmosphere interface. Here we investigate the vertical evolution of turbulent statistics in complex terrain under near-calm conditions during which time local thermally-driven slope flow dominated. We focus on the changes in the sub-canopy microclimate conditions over the study period and diurnally—affected turbulent structures and micrometeorological processes. Data were collected at the Priest River Experimental Forest within a pine forest canopy on a sloped hillside. Two towers were operated during the period of June 20, 2012 to August 30, 2012. The upslope tower had sensors at three heights, 8 m, 4.8 m, and 2.1 m. The second tower was located 14.3 m downslope and had sensors at a height of 1.7 m. The data analyzed here is a subset of data taken from the entire period: July 4, 2012 to July 13, 2012. Conditions over the site at this time were near-calm and no synoptic-scale systems moved through the study site. There was a difference between the daytime and nighttime flow pattern when the data were averaged diurnally. Due to the thermally driven nature of the flow and turbulence, when the vertical potential temperature gradient reversed sign, the wind direction and turbulent fluxes followed. A lag was observed between the time when the potential temperature gradient reversed sign and the time of the flow pattern reversal. During this transitional time, the flow changed to the new regime to match with the night/stable or day/unstable conditions. This dichotomy was seen over the course of the entire study period. The turbulence statistics were observed to change from higher values typically associated with daytime instability to lower values during the overnight stable period delineated by the change of sign in the vertical potential temperature gradient. The observed data reflected the proximity to various roughness elements as well as distance from the ground. Friction velocity values decreased from 0.1352 m/s and 0.1318 m/s under unstable conditions to 0.075 m/s and 0.0655 m/s during stable conditions for the 8 m and 2.1 m heights, respectively. During the unstable period, the fluxes at 8 m were larger (>2 times) compared to 2.1 m but during the stable period, the fluxes and turbulent quantities were similar at the two heights.