How large turbulent eddies influence non‐closure of the surface energy balance is an active research topic that cannot be uncovered by the mean continuity equation in isolation. 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.
The nonclosure of surface energy balance remains an outstanding problem in eddy covariance (EC) measurements of land‐surface fluxes of heat, water vapor, and CO2. Here data collected from an EC tower over a semiarid sagebrush ecosystem indicate that under unstable atmospheric conditions, the nonclosure becomes increased with increasing instability, consistent with many other studies. It is demonstrated that the increased nonclosure is not caused by the inadequate sampling of large‐scale turbulent motions using a 30‐min averaging interval, though the scales of turbulent motions dominating sensible and latent heat fluxes become enlarged with increasing instability. Quadrant analysis is then used to reveal that the flux contributions from ejections remain nearly constant with increasing instability, whereas the flux contributions from sweeps are reduced and their time fractions increase. Our results imply that the increased nonclosure of surface energy balance is associated with changes in turbulent structures including their dominant time scales and flux contributions of ejections and sweeps as the atmospheric instability increases, which require further studies using vertically distributed observations and/or large eddy simulations.
more » « less- NSF-PAR ID:
- 10448116
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Atmospheres
- Volume:
- 125
- Issue:
- 14
- ISSN:
- 2169-897X
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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