On the vertical variability of turbulent heat fluxes within and above a deciduous forest

Abstract

Characterizing near-surface turbulent exchanges over forested, mountainous terrain is critical for a comprehensive understanding of micrometeorological and boundary-layer processes, including exchanges of mass, momentum, and energy. However, observations over mountainous terrain are relatively sparse compared with observations over flat terrain. We used turbulent fluxes obtained from three heights within and five heights above a 25-m tall mixed-deciduous canopy in eastern Tennessee in the Southeast United States. Analyses of measured vertical profiles of temperature variance ($\overline{{T^{\prime }}^2}$), kinematic heat flux ($\overline{w^{\prime }{T}^{\prime }}$), and normalized correlation coefficients obtained via regression analyses between the vertical wind and temperature ($R_{wT}$, which is an indicator of the efficacy of turbulent heat transfer) revealed how these quantities varied between foliated and non-foliated canopies and for different wind speed, wind direction, and atmospheric stability regimes. Results indicated that $\overline{{T^{\prime }}^2}$, $\overline{w^{\prime }{T}^{\prime }}$, and $R_{wT}$ peaked at the canopy top at the beginning and end of the growing season. Overall, larger values of $\overline{w^{\prime }{T}^{\prime }}$ corresponded with smaller wind speeds, whereas the relationship was less consistent between $\overline{{T^{\prime }}^2}$ and horizontal wind. The magnitudes of $\overline{{T^{\prime }}^2}$ and $\overline{w^{\prime }{T}^{\prime }}$ were slightly larger under northwesterly flows as compared with other wind directions. Furthermore, $\overline{{T^{\prime }}^2}$and $\overline{w^{\prime }{T}^{\prime }}$ were largest under the most unstable atmospheric regimes, and the connection between $\overline{w^{\prime }{T}^{\prime }}$ and static stability was strongest at the canopy top. In closing, this study is the first of its kind to investigate how vertical profiles of $\overline{{T^{\prime }}^2}$ and $\overline{w^{\prime }{T}^{\prime }}$ vary with season and ambient meteorological conditions. The findings motivate the need for future studies to leverage long-term micrometeorological measurements of the vertical variability in turbulence structures across different lower atmospheric stability regimes in order to improve the surface-layer parameterizations used within weather forecasting models.