Large eddies modulating flux convergence and divergence in a disturbed unstable atmospheric surface layer

The effects of large eddies on turbulence structures and flux transport were studied using data collected over a flat cotton field during the Energy Balance Experiment 2000 in the San Joaquin Valley of California in August 2000. Flux convergence (FC; larger fluxes at 8.7 m than 2.7 m) and divergence (FD) in latent heat flux (LE) were observed in a disturbed, unstable atmospheric surface layer, and their magnitudes largely departed from the prediction of Monin-Obukhov similarity theory. From our wavelet analysis, it was identified that large eddies affected turbulence structures, scalar distribution, and flux transport differently at 8.7 m and 2.7 m under the FC and FD conditions. Using the ensemble empirical mode decomposition, time series data were decomposed into large eddies and small-scale background turbulence, the time-domain characteristics of large eddies were examined, and the flux contribution by large eddies was also determined quantitatively. The results suggest that large eddies over the frequency range of 0.002 Hz < f < 0.02 Hz (predominantly 300-400 m) enhanced the vertical velocity spectra more significantly at 8.7 m than 2.7 m, leading to an increased magnitude of the cospectra and thus LE at 8.7 m. In the FD case, however, these large eddies were not present and even suppressed in the vertical velocity spectra at 8.7 m. Consequently, the cospectra divergence over the low-frequency ranges primarily caused the LE divergence. This work implies that large eddies may either improve or degrade the surface energy balance closure by increasing or decreasing turbulent fluxes, respectively.