Convection velocity and spatial turbulence statistics in the trunk space of a forest

Turbulence within vegetation canopies is an important control in various processes ranging from microscale dispersion to global climate modeling where we need accurate information on exchange and dispersion within those three-dimensional land-atmosphere interfaces. The increasing popularity and availability of turbulence sensors has helped to promote field studies with more than one sensor operated simultaneously. However, most field campaigns so far measured turbulent fluctuations above, and less often within the canopy layer in terms of vertical profiles (towers). Our information from field experiments is restricted to one-point statistics, i.e. any spatial interpretation of data is severely complicated or even impossible due to the inapplicability of Taylor's frozen turbulence hypothesis within canopies.

The field experiment HX06 was designed to quantify rarely measured two-point statistics in the canopy layer. The term 'two-point' refers to the fact that we do not make use of Taylor's frozen turbulence hypothesis but use truly horizontally separated sensors. Of particular interest in this experiment was the magnitude of the convection velocity uc and any dissimilarities in the spatial statistics of different variables (wind, temperature, water vapor and CO₂).

A total of 21 ultrasonic anemometer-thermometers were arrayed in a uniform and moderately dense Scots Pine forest at Hartheim Research Station (University of Freiburg, Germany, 47º 56' 04" N, 7º 36' 02" E, stand density 800 trees ha^-1). Twelve sensors were horizontally arrayed with spacings between 4 and 192 m along two cardinal axes in the trunk space. Those sensors were installed at a uniform height of z = 2 m above ground (z/h = 0.13, where h is canopy height of h = 14.5 m). Six sensors along the first axis were paired with open-path gas-analyzers (Licor 7500). Additionally, seven sonics were vertically arrayed at a tower reaching up to 27 m (z/h = 1.86). The set-up was run continuously at 20Hz for 30 days (April 13 to May 13, 2006). Using carefully selected runs with mean wind along the two cardinal axes, two-point correlations for different time lags and pairs of sensor separations were calculated in lateral and longitudinal direction and related to the above canopy wind and turbulence profiles under different conditions.

Results show that dominating turbulent structures - as expected - are transported much faster than the average Eulerian velocity u_z in the trunk space. The speed of transport of the dominating structures – the convection velocity u_c – is determined at this depth to be u_c = 1.75 u_h = 7.7 u_z where u_h is the average wind speed at the top of the canopy. The explanation for this discrepancy between convection and Eulerian velocity – an hence the failure of Taylor's hypothesis in the canopy layer – will be discussed in the context of the plan mixing layer analogy that implies that the canopy dominating turbulence structures originate from the higher velocity stream above the canopy and penetrate with increased variance down into the canopy airspace.