Non-local perturbations modulating flux divergence and convergence in a disturbed atmospheric surface layer

Non-local perturbations including large coherent eddies and horizontal advection have great influence on turbulence structure and fluxes in the atmospheric surface layer (ASL). We analyzed data collected over a flood-irrigated flat cotton field with heterogeneous soil moisture during the Energy Balance Experiment (EBEX) in the San Joaquin Valley, California in August 2000. Our results indicated that flux divergence and convergence was frequently observed under dry-to-wet contrasts in soil moisture. As identified from turbulence spectra/cospectra and coherent structures, it was found that flux divergence/convergence was largely attributed to non-local perturbations of large coherent eddies and horizontal advection. The turbulence data at 2.7 m and 8.7 m were interpreted and compared under two atmospheric stratifications (i.e., the unstable ASL and stable ASL beneath the convective boundary layer) to illustrate the influence of large eddies and horizontal advection on the difference in turbulent fluxes between these two levels. It was identified that multiple sizes of large eddies created different disturbance signatures in the low- to mid- frequency parts of turbulence spectra/cospectra at different levels and under the two stratifications. By comparing the characteristics of coherent structures (i.e., the ramp intensity for velocity components, temperature, and specific humidity), we found that non-local perturbations, like large eddies and horizontal advection, modulated turbulence flow and altered turbulent transfer differently at the two levels and under different stratifications. Our results also suggest that large eddies and horizontal advection in response to the dry-to-wet contrast in soil moisture had greater influences on latent heat flux at the high level (8.7 m) than the lower level (2.7 m), leading to significant divergence and convergence in latent heat flux. Therefore, non-local perturbations including large eddies and horizontal advection as a result of soil moisture heterogeneity may be one key factor responsible for flux divergence and convergence in the ASL.