Detection of temperature ramps and flow structures at a deciduous forest site

Abstract

Objective methods for the identification of coherent turbulent structures observed in the air layers within and above a forest are described. The techniques are based on the detection of scalar ramps, such as those frequently appearing in temperature traces, and on the detection of ejection/sweep flow structures. Coherent structures are revealed most distinctly by characteristic ramp patterns in scalar traces. A relatively simple multipoint technique to pick up the abrupt decrease of temperature terminating each ramp was successful, as was the variable-interval time-averaging (VITA) method with respect to detection at a single level. Identification of coherent ejection/sweep flow structures was accomplished by examination of the components of the fluctuating velocity, along the major axis of the elliptical joint probability contours of the longitudinal and vertical turbulent velocities. Integration of the absolute value of this rotated velocity over each period encompassing a negative excursion followed by a positive one (approximating an ejection/sweep combination), yielded a detection signal that matched well that from the temperature ramps. Detection of large accelerations in the rotated velocity, indicating rapid transitions from ejection to sweep, was less successful because such velocity changes were not easily distinguished from background turbulence.