Stability dependence of the relaxed eddy accumulation coefficient for various scalar quantities

Abstract

The coefficient b of the relaxed eddy accumulation (REA) technique was investigated by simulation studies using extensive high-resolution eddy correlation data sets of three different scalar quantities: the air temperature and the concentrations of CO2 and H2O. Measurements were performed in June-July 1995 over a senescent cereal field in the Rhineland-Palatinate region of southwestern Germany. The wide range of stability conditions during the experiment made it possible to describe the coefficient b as a function of the dimensionless stability parameter z/L within the framework of the Monin-Obukhov similarity theory. In good agreement with previous publications, a constant value of about 0.56 was obtained for unstable and near-neutral conditions if no dead band applied. For stable conditions, however, a significant increase with stability was observed which could be well described by a logarithmic functional relationship between b and z/L. By additional dead-band simulation studies the relationship was extended to variable dead-band widths. The observed dependency on z/L is interpreted in relation to the bivariate joint frequency distribution of the vertical wind speed and the scalar quantity.