Characteristics of Canopy Turbulence during the Transition from Convective to Stable Stratification

Increasing atmospheric stability is known to suppress turbulent mixing and often leads to a decoupling of above and within canopy flows. This situation is frequently associated with the development of drainage flows. Understanding the dynamics and characteristics of drainage flows is of particular importance when interpreting nocturnal exchanges of CO2 and other trace gas fluxes.

During a special observational period we measured profiles of temperature and wind components at 9 heights on a 70m tower in and above a 40m high open Eucalyptus forest. The measurements were carried out with Type T thermocouples and 2D Windsonics and 1Hz data were stored. A second 11m high tower was situated on a nearby slope and sub-canopy turbulence was measured simultaneously with 3D Sonics at 7 heights. All raw 20Hz data were stored.

This data set is used to demonstrate how the different transport processes of momentum and heat at leaf scale lead to the development of Richardson number profiles that induce decoupling of the canopy and boundary layer flows. We show how turbulence properties change in the transition from the convective to the stable boundary layer by describing changes in the dominant length scales and changes in the efficiency and intermittency of turbulent exchange.