Flux Convergence and Divergence Linked to Asymmetric Transport by Large Turbulent Eddies in the Unstable Atmospheric Surface Layer

Abstract

It is well-established that large eddies significantly influence the turbulent transport of heat and scalars in the atmospheric surface layer. However, the mechanistic understanding of how large eddies originating from both the ground (updrafts) and aloft (downdrafts) regulate flux convergence (FC) and divergence (FD) remains relatively unexplored. Based on turbulence data measured at 12 levels, spanning from 1.2 to 60.5 m above the ground, we observe a notable increase in the variability of sensible heat flux magnitudes with height. Our results show that FC and FD of sensible heat are primarily linked to variations in the respective transport efficiencies (${\gamma }_{w\theta }$) at different heights. Using the cross-wavelet transform, we find that in FC cases, the regions with high wavelet coherence expand with height, resulting in higher ${\gamma }_{w\theta }$ at higher levels compared to low ones. Conversely, in FD cases, the regions with high wavelet coherence decrease with height, leading to lower ${\gamma }_{w\theta }$ at higher levels. Large eddies with length scales of approximately 120–500 m have a significant impact on amplifying or attenuating ${\gamma }_{w\theta }$ at higher levels compared to lower levels. Using conditional sampling to extract the updrafts and downdrafts of large eddies, distinct patterns are observed in the characteristics of updrafts and downdrafts between FC and FD groups especially in their flux contribution and transport efficiencies. This work emphasizes the significant contribution of asymmetric turbulent transport by updrafts and downdrafts to the discrepancy between the observed turbulent fluxes and those predicted by the Monin-Obukhov similarity theory.