Probing eddy size and its effective mixing length in stably stratified roughness sublayer flows

• Published in: Quarterly journal of the Royal Meteorological Society 2022-10, Vol.148 (749), p.3756-3773
• Main Authors: Peltola, Olli, Aurela, Mika, Launiainen, Samuli, Katul, Gabriel
• Format: Article
• Language: English
• Subjects: Adiabatic; Canopies; Canopy; DTS; Eddy covariance; ENVIRONMENTAL SCIENCES; Field tests; Forest floor; Forests; Heat exchange; Height; Length scale; Meteorology & Atmospheric Sciences; Mixing length; Momentum; Plant cover; Potential temperature; Roughness; Roughness sublayer; Stable stratification; Stratification; Temperature profile; Temperature profiles; Topology; Turbulence; Velocity gradients
• ISSN: 0035-9009; 1477-870X
• DOI: 10.1002/qj.4386

Stably stratified roughness sublayer flows are ubiquitous yet remain difficult to represent in models and to interpret using field experiments. Here, continuous high‐frequency potential temperature profiles from the forest floor up to 6.5 times the canopy height observed with distributed temperature sensing (DTS) are used to link eddy topology to roughness sublayer stability correction functions and coupling between air layers within and above the canopy. The experiments are conducted at two forest stands classified as hydrodynamically sparse and dense. Near‐continuous profiles of eddy sizes (length scales) and effective mixing lengths for heat are derived from the observed profiles using a novel conditional sampling approach. The approach utilizes potential temperature isoline fluctuations from a statically stable background state. The transport of potential temperature by an observed eddy is assumed to be conserved (adiabatic movement) and we assume that irreversible heat exchange between the eddy and the surrounding background occurs along the (vertical) periphery of the eddy. This assumption is analogous to Prandtl's mixing‐length concept, where momentum is transported rapidly vertically and then equilibrated with the local mean velocity gradient. A distinct dependence of the derived length scales on background stratification, height above ground, and canopy characteristics emerges from the observed profiles. Implications of these findings for (1) the failure of Monin–Obukhov similarity in the roughness sublayer and (2) above‐canopy flow coupling to the forest floor are examined. The findings have practical applications in terms of analysing similar DTS data sets with the proposed approach, modelling roughness sublayer flows, and interpreting nocturnal eddy covariance measurements above tall forested canopies. Nocturnal air flows above and within forests are highly complex, due to trees blocking air flow and stably stratified air layers inhibiting vertical movement. These complexities cause severe challenges for micrometeorological models and measurements alike. Here, continuous high‐frequency potential temperature profiles observed with distributed temperature sensing are used to study these flows. Eddy topology is derived from the measurements and used to estimate roughness sublayer stability correction functions and coupling between air layers within and above the canopy.