On micrometeorological observations of surface-air exchange over tall vegetation

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 beNEE=∫zr0∂c̄∂tdz+(w′c′)r+w̄rc̄r−1zr∫zr0c̄dzwhere the first term on RHS is the storage below the height of observation (zr), 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 (w̄r) at height zr. 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 (c̄r−(1/zr)∫zr0c̄dz) is large, as is the case with CO2 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 CO2 could be biased significantly towards higher values. It is argued that quantifying the mass flow component is a major challenge facing the micrometeorological community.