An investigation of the conditional sampling method used to estimate fluxes of active, reactive, and passive scalars

Abstract

The conditional sampling flux measurement technique was evaluated for four scalars (temperature, water vapor, ozone, and carbon dioxide) by comparison with direct eddy correlation measurements at two sites. The empirical constant β relating the turbulent flux to the accumulated concentration difference between updrafts and downdrafts was computed from 10-Hz turbulence measurements. Comparison between the simulated relaxed eddy accumulation flux formulation and the eddy correlation measurements allowed the direct determination of β for all four scalars. The β models previously proposed overpredicted the measured β by about 8%-10%. It was found that a mean β = 0.58 reproduced the eddy correlation measurements independent of the scalar type being analyzed, roughness and atmospheric stability conditions, in agreement with previous studies. The role of energy-containing eddy motion in the deviations between the measured and predicted β was considered using orthonormal wavelet expansion in conjunction with a wavelet shrinkage approach. It was demonstrated that the energy-containing large eddy motion contributed to a reduction in β when compared to the predicted β. Finally, the deadband vertical velocity effects were also considered and found to reduce β exponentially, in agreement with other studies.