The vertical change of coefficient b, used in the relaxed eddy accumulation method for flux measurement above and within a forest canopy

Abstract

The relaxed eddy accumulation method (REAM), a conditional sampling technique, provides a potential alternative to the traditional eddy correlation method for measuring turbulent flux of chemical trace gases in the lower atmosphere, because it does not require a high sampling rate that is not feasible with most chemical analyzers. Recent studies suggest that the turbulent flux ( w′c′ ) is proportional to the standard deviation of vertical velocity ( σ w ) and the difference ( Δc w ) between mean concentrations conditionally sampled during updrafts and downdrafts. The relationship of w′ c′ = b c σ w Δc w is used as the basis for the REAM. This relationship was tested with turbulence measurements made at six heights in and above a deciduous forest with emphasis on the height dependence of the coefficient b in the forest region. Results, based on the analysis of only sensible heat and water vapor fluxes with the REAM, show that the w′c′−σ wΔ w linear relation may hold for different atmospheric stabilities and canopy leaf densities. The values of the coefficients b T and b q for both temperature and water vapor density, however, change with height, possibly because of the change in the efficiency of conditional concentration sampling during updrafts and downdrafts associated with the turbulence structure at different heights. At about 2.5 times the canopy height, b T is approximately equal to 0.58, which is close to the value of 0.6 found over a smooth surface. However, both b T and b q decrease as the canopy is approached and fall to about 0.51 at the canopy top. Using b c = 0.6 could cause the flux within the forest roughness sublayer to be overestimated as much as 18%. The possible role of organized turbulence structures of flow and the scalar fields existing in the lower atmosphere near the forest was examined.