On micrometeorological observations of surface-air exchange over tall vegetation

Abstract

It is shown from the mass conservation and the continuity equations that the net ecosystem exchange (NEE) of a scalar constituent with the atmosphere should be NEE=∫ z r 0 ∂ c ̄ ∂t dz+( w′c′) r+ w ̄ r c ̄ r− 1 z r ∫ z r 0 c ̄ dz where the first term on RHS is the storage below the height of observation ( z r ), the second term is the eddy flux, and the third term is a mass flow component arising from horizontal flow convergence/divergence or a non-zero mean vertical velocity ( w ̄ r ) at height z r . The last term, unaccounted for in previous studies of surface-air exchanges, becomes important over tall vegetation and at times when the vertical gradient of the atmospheric constituent ( c ̄ r−(1/z r)∫ z r 0 c ̄ dz) is large, as is the case with CO 2 in forests at night. Experimental evidence is presented to support the postulation that the mass flow component is in large part responsible for the large run-to-run variations in eddy fluxes, the lack of energy balance closure and the apparent low eddy fluxes at night under stable stratifications. Three mechanisms causing the non-zero mean vertical velocity are discussed. Of these, drainage flow on undulating terrain is the most important one for long-term flux observations because only a small terrain slope is needed to trigger its occurrence. It is suggested from the data obtained at a boreal deciduous forest that without proper account of the mass flow component, the assessment of annual uptake of CO 2 could be biased significantly towards higher values. It is argued that quantifying the mass flow component is a major challenge facing the micrometeorological community.