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Abstract The turbulent static pressure spectrum as a function of longitudinal wavenumber in the roughness sublayer of forested canopies is of interest to a plethora of problems such as pressure transport in the turbulent kinetic energy budget, pressure pumping from snow or forest floor, and coupling between flow within and above canopies. Long term static pressure measurements above a sub‐arctic forested canopy for near‐neutral conditions during the winter and spring were collected and analyzed for three snow cover conditions: trees and ground covered with snow, trees are snow free but the ground is covered with snow, and snow free cover. In all three cases, it is shown that obeys the attached eddy hypothesis at low wavenumbers —with and Kolmogorov scaling in the inertial subrange at higher wavenumbers—with , where is the friction velocity at the canopy top, is the mean turbulent kinetic energy dissipation rate, is the distance from the snow top, and is the boundary layer depth. The implications of these two scaling laws to the normalized root‐mean squared pressure and its newly proposed logarithmic scaling with normalized wall‐normal distance are discussed for snow covered and snow free vegetation conditions. The work here also shows that the in the appears more extensive and robust than its longitudinal velocity counterpart.
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Probing eddy size and its effective mixing length in stably stratified roughness sublayer flows
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.
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- Award ID(s):
- 2028633
- PAR ID:
- 10454784
- Date Published:
- Journal Name:
- Quarterly Journal of the Royal Meteorological Society
- Volume:
- 148
- Issue:
- 749
- ISSN:
- 0035-9009
- Page Range / eLocation ID:
- 3756 to 3773
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1002/qj.4386
