Non-Closure of Surface Energy Balance Linked to the Influence of Large Turbulent Eddies

There is increasing evidence from field experiments and simulation studies that large turbulent eddies stand out as one principal contributor leading to underestimated sensible and latent heat fluxes and thus the non-closure of the surface energy balance. However, the underlying mechanistic processes as to how large turbulent eddies influence non-closure remain unexplored extensively. It is demonstrated here that asymmetric turbulent flux transport of heat and water vapor by sweeps and ejections of large eddies under unstable atmospheric stability conditions reduce fluxes. Such asymmetry causes positive gradients in the third-order moments in the turbulent flux budget equations, primarily attributed to substantially reduced flux contributions by sweeps and sustained large flux contributions by ejections. Small-scale surface heterogeneity in heating generates ejecting eddies with larger air temperature variance than sweeping eddies, causing asymmetric flux transport in the atmospheric surface layer. Changes in asymmetry with increasing instability are congruent with observed increases in the surface energy balance non-closure. To assess the contributions of asymmetric flux transport by large eddies to the non-closure requires two eddy covariance systems on the tower to measure the gradients of the turbulent heat flux and other third-order moments.