Wang, W., K.J. Davis, B.D. Cook, C. Yi, D.M. Ricciuto, M.P. Butler, and P.S. Bakwin, submitted. Influences of advection and horizontal flux divergence on net ecosystem-atmosphere exchange of CO2 as observed from two towers, Agricultural and Forest Meteorology.

The influences of horizontal advection, vertical advection, and horizontal flux divergence on measurements of net ecosystem-atmosphere exchange of CO2 (NEE) have been investigated using the data from two towers separated by about 20km in a forested region of Northern America: a multilevel flux observation tower (WLEF) and a one-level flux tower (Willow Creek). Significant horizontal advection of CO2 is inferred from the measurements at night in the growing season. Analyses indicate that ignoring the contribution of horizontal advection and vertical advection to NEE measurements can lead to an overestimate and underestimate of nighttime NEE by about 15% and 23%, respectively. The influences of horizontal and vertical advection on the measurements are usually negligible in the day. The estimated contribution of horizontal flux divergence between the two towers is negligible. Errors in measurements of the mean vertical velocity and CO2 mixing ratio are also discussed. Combination of those errors results in a rather large relative error in the calculation of the vertical advection term by multiplying the measured mean vertical velocity and the vertical gradient of CO2 mixing ratio. The results show that it is inappropriate, at least for short time scales, to use the measured mean vertical velocity to assess the contribution of vertical advection to NEE measurements.