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621.
An Attempt to Close the Daytime Surface Energy Balance Using Spatially-Averaged Flux Measurements 总被引:1,自引:0,他引:1
Matthias Mauder Raymond L. Desjardins Elizabeth Pattey Devon Worth 《Boundary-Layer Meteorology》2010,136(2):175-191
Single-tower eddy-covariance measurements represent the complete surface flux of a scalar only under idealized conditions.
Therefore, we often find an underestimation of energy fluxes expressed as a lack of energy balance closure at many sites.
In this study, a multi-tower approach to measure atmospheric energy fluxes based on spatial averaging is evaluated and possible
mechanisms causing a lack of energy balance closure are analysed, focussing on daytime data only. It is shown that the multi-tower
technique is also unable to measure the entire flux for our site, likely because the assumption of horizontal homogeneity
is violated. Heterogeneity-induced and buoyancy-driven quasi-stationary circulations are probably the dominant processes causing
the underestimation of energy fluxes. A dependence of the energy balance residual on stability is found, with residuals close
to zero for stable stratification, a maximum under unstable to near-neutral conditions and still relatively large residuals
for stronger instability. Assuming the processes transporting energy and CO2 are similar, the implications on long-term CO2 flux measurements are analysed. Accordingly, the resulting selective systematic error of cumulative net ecosystem exchange
estimates for agricultural regions such as ours can be of the order of more than 100%, since mainly the fluxes during periods
of net CO2 uptake are underestimated while periods of net CO2 release are much less affected by this bias. Further investigations about this issue are highly warranted. 相似文献
622.
Extension of the Averaging Time in Eddy-Covariance Measurements and Its Effect on the Energy Balance Closure 总被引:1,自引:0,他引:1
Doojdao Charuchittipan Wolfgang Babel Matthias Mauder Jens-Peter Leps Thomas Foken 《Boundary-Layer Meteorology》2014,152(3):303-327
The modified ogive analysis and the block ensemble average were employed to investigate the impact of the averaging time extension on the energy balance closure over six land-use types. The modified ogive analysis, which requires a steady-state condition, can extend the averaging time up to a few hours and suggests that an averaging time of 30 min is still overall sufficient for eddy-covariance measurements over low vegetation. The block ensemble average, which does not require a steady-state condition, can extend the averaging time to several days. However, it can improve the energy balance closure for some sites during specific periods, when secondary circulations exist in the vicinity of the sensor. These near-surface secondary circulations mainly transport sensible heat, and when near-ground warm air is transported upward, the sensible heat flux observed by the block ensemble average will increase at longer averaging times. These findings suggest an alternative energy balance correction for a ground-based eddy-covariance measurement, in which the attribution of the residual depends on the ratio of sensible heat flux to the buoyancy flux. The fraction of the residual attributed to the sensible heat flux by this energy balance correction is larger than in the energy balance correction that preserves the Bowen ratio. 相似文献