An analysis of turbulent sensible heat fluxes within a heterogeneous black spruce boreal forest in Alaska

Abstract

Turbulent sensible heat fluxes within the heterogeneous canopy of a black spruce boreal forest in Interior Alaska are evaluated at three different scales in order to assess their spatial variability, and to determine the feasibility of upscaling locally measured flux values to the landscape scale for modeling applications and climate studies. The first evaluation is performed locally at a single micrometeorological tower in an area of the boreal forest with a mean canopy height of 4.7 m. The data were taken across winter, spring and summer of 2012 from two sonic anemometers, one below the canopy at 3 m above ground, and one above the canopy at 12 m above ground. A multiresolution analysis is used to isolate coherent structures from the turbulent temperature time series at both instruments. When mean global statistics of coherent structures are analyzed at the two levels independently, results show an average of 8 structures per period, a mean duration of 85 s, and a mean sensible heat flux contribution of 48%. A spectral version of the Stokes parameters is applied to the turbulent horizontal wind components to show that 31% of the coherent turbulent structures detected at 12 m, and 13% at 3 m, may be complicated by canopy waves due to the prevalence of stable flows at this high latitude location. A most remarkable finding is that less than 25% of the coherent structures detected at these two heights occur synchronously, which speaks robustly to the lack of flow interaction within only 9 vertical meters of the forest, and to the complexity of the vertical aggregation of sensible heat therein. The second evaluation quantifies differences in turbulent sensible heat fluxes horizontally between two micrometeorological towers 600 m apart, one in a denser canopy (DC) and the other in a sparser canopy (SC), but under approximately similar atmospheric boundary layer conditions. Results show that SC is ~ 3°C cooler and more stably stratified than DC during nighttime. This suggests that changes in the height and density of the canopy impact local temperature and stability regimes. Most importantly, the sensible heat flux at DC is greater during midday periods, with that difference exceeding 30% of the measured flux and over 30 W m⁻² in magnitude more than 60% of the time. This difference is the result of higher mechanical mixing due to the increased density of roughness elements at DC. Furthermore, the vertical distribution of turbulent heat fluxes verifies a maximum above the canopy crown when compared with the levels below and well above the canopy. These spatial variations of sensible heat flux result from the complex scale aggregation of energy fluxes over a heterogeneous canopy, and suggest that locally measured fluxes will likely differ from large-scale area averaged values. The third evaluation compares locally measured sensible heat fluxes from a sonic anemometer atop a 24 m micrometeorological tower to those derived from a large aperture scintillometer (LAS) whose beam is centered near the tower at an average height of 36 m above ground, and over a path length of 1423 m. This analysis focuses on unstable daytime periods from June, July and August of 2013. The daytime is defined as 0700-2000 Alaska Standard Time, since local sensible heat flux values derived from the sonic anemometer (HEC) are robust (above 50 W m⁻²) during this time, and since this time also agrees with the minima in the mean diurnal pattern of Cn² from the LAS. For daytime periods with robust sensible heat flux values, HEC and the large-scale flux from the LAS (HLAS) correlate with R² = 0.68, while HEC captures about 82% of HLAS on average. The magnitude of HEC and HLAS are both strongly sensitive to incoming solar radiation, with HLAS having a better correlation and regression slope, suggesting that the local measurements are adjusting also to surface and/or flow conditions above the heterogeneous canopy. Evaluation of the magnitude of the ratio of HEC/HLAS for days with varying amounts of solar radiation suggests that while radiation affects the magnitude of HEC and HLAS independently, it does not affect their ratio. For daytime periods with lower fluxes (HEC between 10 and 50 W m⁻²), HEC captures about 69% of HLAS on average. However, local and large-scale fluxes during this low flux regime correlate poorly with incoming solar radiation (R² = 0.42 for HLAS and R² = 0.15 for HEC), and with one another (R² = 0.27), suggesting that local heterogeneities are not well-integrated into the large-scale flux. Therefore, low flux periods should be considered separately for the purposes of upscaling local to landscape scale flux values in the boreal forest. For the high flux regime, a finer resolution of upscaling can be provided based on the mean diurnal pattern of HEC/HLAS and the Obukhov length (L). Namely, as the boundary layer becomes less unstable in late afternoon, HEC/HLAS increases, supporting that the eddy covariance technique can capture more of the large-scale flux when the boundary layer is more shear-driven (less buoyancy driven).