ON MICROMETEOROLOGICAL OBSERVATIONS OF SURFACE-AIR EXCHANGE OVER TALLVEGETATION

It is shown from the mass conservation and the continuity equations th at the net ecosystem exchange (NEE) of a scalar constituent with the a tmosphere should be NEE = integral(0)(zr) partial derivative (c) over bar/partial derivative t dz + (<(w'c')over bar>)(r) + (w) over bar(r)( (c) over bar(r) - 1/z(r) integral(0)(zr) (c) over bar dz) where the fi rst 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 c omponent arising from horizontal flow convergence/divergence or a non- zero mean vertical velocity ((w) over bar(r)) at height z(r). The last term, unaccounted for in previous studies of surface-air exchanges, b ecomes important over tall vegetation and at times when the vertical g radient of the atmospheric constituent ((c) over bar(r) - (1/z(r)) int egral(0)(zr) (c) over bar dz) is large, as is the case with CO2 in for ests at night. Experimental evidence is presented to support the postu lation that the mass how component is in large part responsible for th e large run-to-run variations in eddy fluxes, the lack of energy balan ce closure and the apparent low eddy fluxes at night under stable stra tifications. Three mechanisms causing the non-zero mean vertical veloc ity are discussed. Of these, drainage flow on undulating terrain is th e most important one for long-term flux observations because only a sm all terrain slope is needed to trigger its occurrence. It is suggested from the data obtained at a boreal deciduous forest that without prop er account of the mass flow component, the assessment of annual uptake of CO2 could be biased significantly towards higher values. It is arg ued that quantifying the mass flow component is a major challenge faci ng the micrometeorological community. (C) 1998 Elsevier Science B.V. A ll rights reserved.