Spatial variability of turbulent fluxes in the roughness sublayer of an even-aged pine forest

The spatial variability of turbulent flow statistics in the roughness subla yer (RSL) of a uniform even-aged 14 m (= h) tall loblolly pine forest was i nvestigated experimentally. Using seven existing walkup towers at this stan d, high frequency velocity, temperature, water vapour and carbon dioxide co ncentrations were measured at 15.5 m above the ground surface from October 6 to 10 in 1997. These seven towers were separated by at least 100 m from e ach other. The objective of this study was to examine whether single tower turbulence statistics measurements represent the flow properties of RSL tur bulence above a uniform even-aged managed loblolly pine forest as a best-ca se scenario for natural forested ecosystems. From the intensive space-time series measurements, it was demonstrated that standard deviations of longit udinal and vertical velocities (sigma(u), sigma(w)) and temperature (sigma( T)) are more planar homogeneous than their vertical flux of momentum (u(*)( 2)) and sensible heat (H) counterparts. Also, the measured H is more horizo ntally homogeneous when compared to fluxes of other scalar entities such as CO2 and water vapour. While the spatial variability in fluxes was signific ant (>15%), this unique data set confirmed that single tower measurements r epresent the 'canonical' structure of single-point RSL turbulence statistic s, especially flux-variance relationships. Implications to extending the 'm oving-equilibrium' hypothesis for RSL flows are discussed. The spatial vari ability in all RSL flow variables was not constant in time and varied stron gly with spatially averaged friction velocity u(*), especially when u(*) wa s small. It is shown that flow properties derived from two-point temporal s tatistics such as correlation functions are more sensitive to local variabi lity in leaf area density when compared to single point flow statistics. Sp ecifically, that the local relationship between the reciprocal of the verti cal velocity integral time scale (I-w) and the arrival frequency of organiz ed structures ((u) over bar/h) predicted from a mixing-layer theory exhibit ed dependence on the local leaf area index. The broader implications of the se findings to the measurement and modelling of RSL flows are also discusse d.