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Submitted to: Agricultural and Forest Meteorology
Publication Type: Peer Reviewed Journal Publication Acceptance Date: 8/28/2013 Publication Date: 1/15/2014 Publication URL: http://handle.nal.usda.gov/10113/58320 Citation: Anderson, R.G., Wang, D. 2014. Energy budget closure observed in paired Eddy Covariance towers with increased and continuous daily turbulence. Agricultural and Forest Meteorology. 184:204-209. Interpretive Summary: Eddy Covariance (EC) is a major technique to observe evapotranspiration, photosynthesis, and respiration by making high frequency observations of atmospheric turbulence. It has major advantages over other methods because it directly observes a larger area than other techniques and it does not modify the environment it measures. One major challenge for this technique has been energy budget closure, where the direct observations of turbulent atmospheric energy transport are usually less than the observed solar and thermal fluxes into and out of the land surface. This lack of closure is a challenge because it seemingly violates law of conservation of energy; the pathway for the missing energy is uncertain. Although multiple pathways for missing energy have been identified, their relative importance is often unknown, which affects the accuracy and precision of the EC observations. In this study, we used data from two EC towers in different windiness conditions to show that the energy budget closure is obtained on a daily basis for days with continuous atmospheric turbulence. Apparent energy imbalance is still observed for 30 minute measurements, but, because they average out over a daily period, we surmise these differences are due to temporary heat storage in plant biomass and shallow soil. Closure of the energy budget with continuous turbulence illustrates the importance of other energy loss pathways, such as cold air drainage, during nighttime periods with little turbulence. The results are also sensitive to the threshold value used to determine continuous turbulence. Our study shows the importance of using daily averaging to correct EC data for energy imbalance as well a need for a robust, objective threshold determination for turbulence which is often used to quality control EC data. This study may help future researchers use EC more accurately for agricultural observation, model parameterization, and model validation. 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. |
