Energy budget closure observed in paired Eddy Covariance towers with increased and continuous daily turbulence

Authors: Anderson, Raymond - Ray; Wang, Dong

Submitted to: Acta Microscopy
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/28/2013
Publication Date: 9/28/2013
Publication URL: http://handle.nal.usda.gov/10113/58320
Citation: Anderson, R.G., Wang, D. 2013. Energy budget closure observed in paired Eddy Covariance towers with increased and continuous daily turbulence. Acta Microscopy. 184:204-209.

Interpretive Summary: Eddy Covariance is a meteorological technique to measure productivity and water use of general vegetation including agricultural crops. This method is considered advantageous over other measurement techniques because the area that can be measured is large so it’s more representative to actual field conditions. However, there are some uncertainties about the precision and accuracy of the method related to its high-frequency meteorological observations and corrections for times when there is no wind. In this study, we used data from two measurement sites in contrasting high and low wind environments to examine these corrections. Our experimental data supported the theories underlying the corrections for the technique in low or no wind environments and indicated that the corrections did not bias observations of plant and water productivity. The results provided validation of deploying this measurement method in areas with frequent low or no wind such as the San Joaquin Valley of California.

Technical Abstract: The lack of energy closure has been a longstanding issue with Eddy Covariance (EC). Multiple mechanisms have been proposed to explain the discrepancies in energy balance including diurnal energy storage changes, advection of energy, and larger scale turbulent processes that cannot be resolved by field EC. To investigate the energy balance issue, we used a year of data from paired EC towers in irrigated sugarcane in Maui, Hawai’i, USA. The towers were in identical crops and cultivation practices and had similar climate with the notable exception of wind. One tower was in a location where nearby orographic features funneled Trade Winds, resulting in sustained, continuous turbulence. The other was in a leeward location with typical daytime/nighttime patterns of friction velocity (u*). We found significantly improved closure (8.5-10%) at both sites using daily sums of Available Energy in closure regressions as opposed to 30 minute data, illustrating the importance of storage terms. The energy budget closed for both fields when only days with continuous turbulence (all 30 minute u*>critical u*) were considered, with significantly larger uncertainty in the leeward field (±13%) due to the small number of days (n=13) with this condition. Significant energy imbalance appeared in both fields with even 30 minutes of intermittent turbulence in a day, and each field had different turbulence-closure patterns. Closure with continuous turbulence was sensitive to choice of critical u*; an arbitrary u* of 0.1 m s-1 resulted in non-closure. The results show the value of paired EC towers in contrasting turbulence conditions to assess energy budget closure.