Heat and Turbulent fluxes Within and Above a Forested Ridgetop Canopy

The proper representation of turbulent fluxes occurring within and above forest canopies is an essential component of the land-surface models used within numerical weather prediction (NWP) models. As both the horizontal and vertical resolution of NWP models improves, enabling better resolution of terrain and vegetation complexities and their interactions with the overlying atmosphere, proper characterizations of near-surface turbulent exchanges over, forested mountainous terrain, becomes increasingly crucial. In this study, we use heat and turbulent fluxes measured within and above a forested ridgetop canopy near Oak Ridge, Tennessee. These fluxes are obtained from a 60-m micrometeorological tower surrounded by a ~ 25 m mixed-deciduous canopy which is representative of other forested regions in the southeastern United States. During a one-year period (i.e., 1 January 2016 through 31 December 2016), within-canopy turbulence measurements were sampled at 5 m, 10 m, and 15 m above ground level (AGL), and above-canopy turbulence measurements were sampled at 30 m, 35 m, 40 m, and 45 m AGL. We use these measurements to quantify the vertical variability in kinematic heat flux (w'T') and temperature variance (T'T') as a function of time of year to assess how these relationships differ between foliated and non-foliated canopies. We also assess how these relationships vary with the time of day to assess the impact of near-surface atmospheric stability. To quantify the efficacy of these relationships and turbulence structure within- and above the canopy, we compute R_wT and R_wu as w'T'/σ_wσ_T and u'w'/σ_uσ_w, respectively. We also compute other metrics including for example σ_v/σ_u and σ_w/σ_u.

We observe peaks in w'T', T'T', R_wT and R_wu, at the interface between the canopy and overlying atmosphere during the afternoon (which we define as between 1200 and 1600 local time) throughout the year. This finding suggests that maximum turbulent exchanges occur at this interface are similar to exchanges occurring within the entrainment zone between the atmospheric boundary layer and free atmosphere. During the nighttime (which we define as between 0000 and 0400 local time), within-canopy R_wT was generally largest during the warm season (R_wT ~ 0.15) and smallest during the cool season (R_wT ~ -0.05), while within-canopy R_wu was negative, ranging from 0 to ~ 0.2. Above-canopy R_wT was ~ -0.20 across all months, whereas above-canopy R_wu was ± ~ 0.4. Additionally, σ_v/σ_u was ~ 1 across all seasons and canopy heights, whereas σ_w/σ_u exhibited a maximum near the canopy top during both in the afternoon and at night.

Overall, these results provide insight on the turbulent exchange processes within and above forested canopies in complex terrain, and can help to inform improvements to turbulence parameterizations in the land surface models that are an essential component of NWP models.