Momentum and scalar transport within a vegetation canopy following atmospheric stability and seasonal canopy changes: the CHATS experiment

摘要

Momentum and scalar (heat and water vapor) transfer between a walnutcanopy and the overlying atmosphere are investigated for two seasonalperiods (before and after leaf-out), and for five thermal stabilityregimes (free and forced convection, near-neutral condition,transition to stable, and stable). Quadrant and octant analyses ofmomentum and scalar fluxes followed by space-time autocorrelations ofobservations from the Canopy Horizontal Array Turbulence Study's(CHATS) thirty meter tower help characterize the motions exchangingmomentum, heat, and moisture between the canopy layers and aloft.During sufficiently windy conditions, i.e. in forced convection,near-neutral and transition to stable regimes, momentum and scalarsare generally transported by sweep and ejection motions associatedwith the well-known canopy-top "shear-driven" coherent eddystructures. During extreme stability conditions (both unstable andstable), the role of these "shear-driven" structures intransporting scalars decreases, inducing notable dissimilarity betweenmomentum and scalar transport.In unstable conditions, "shear-driven" coherent structures areprogressively replaced by "buo-yantly-driven" structures, knownas thermal plumes; which appear very efficient at transportingscalars, especially upward thermal plumes above the canopy. Withinthe canopy, downward thermal plumes become more efficient attransporting scalars than upward thermal plumes if scalar sourcesare located in the upper canopy. We explain these features bysuggesting that: (i) downward plumes within the canopy correspondto large downward plumes coming from above, and (ii) upward plumeswithin the canopy are local small plumes induced by canopy heatsources where passive scalars are first injected if there sourcesare at the same location as heat sources. Above the canopy, thesesmall upward thermal plumes aggregate to form larger scale upwardthermal plumes. Furthermore, scalar quantities carried by downwardplumes are not modified when penetrating the canopy and crossingupper scalar sources. Consequently, scalars appear to be preferentiallyinjected into upward thermal plumes as opposed to in downward thermalplumes.In stable conditions, intermittent downward and upward motionsprobably related to elevated shear layers are responsible forcanopy-top heat and water vapor transport through the initiationof turbulent instabilities, but this transport remains small. Duringthe foliated period, lower-canopy heat and water vapor transportoccurs through thermal plumes associated with a subcanopy unstablelayer.