Abstract
Abstract. Methane (CH4) emissions from biogenic sources, such as Arctic permafrost wetlands, are associated with large uncertainties because of the high variability of fluxes in both space and time. This variability poses a challenge to monitoring CH4 fluxes with the eddy covariance (EC) technique, because this approach requires stationary signals from spatially homogeneous sources. Episodic outbursts of CH4 emissions, i.e. triggered by spontaneous outgassing of bubbles or venting of methane-rich air from lower levels due to shifts in atmospheric conditions, are particularly challenging to quantify. Such events typically last for only a few minutes, which is much shorter than the common averaging interval for EC (30 min). The steady-state assumption is jeopardised, which potentially leads to a non-negligible bias in the CH4 flux. Based on data from Chersky, NE Siberia, we tested and evaluated a flux calculation method based on wavelet analysis, which, in contrast to regular EC data processing, does not require steady-state conditions and is allowed to obtain fluxes over averaging periods as short as 1 min. Statistics on meteorological conditions before, during, and after the detected events revealed that it is atmospheric mixing that triggered such events rather than CH4 emission from the soil. By investigating individual events in more detail, we identified a potential influence of various mesoscale processes like gravity waves, low-level jets, weather fronts passing the site, and cold-air advection from a nearby mountain ridge as the dominating processes. The occurrence of extreme CH4 flux events over the summer season followed a seasonal course with a maximum in early August, which is strongly correlated with the maximum soil temperature. Overall, our findings demonstrate that wavelet analysis is a powerful method for resolving highly variable flux events on the order of minutes, and can therefore support the evaluation of EC flux data quality under non-steady-state conditions.
Highlights
Methane (CH4) is one of the most important greenhouse gases (Saunois et al, 2016b), but unexpected changes in atmospheric CH4 budgets over the past decade emphasise that many aspects regarding the role of this gas in the global climate system remain unexplained to date (e.g. Saunois et al, 2016a; Nisbet et al, 2016; Schwietzke et al, 2016; Schaefer et al, 2016)
Our findings demonstrate that wavelet analysis is a powerful method for resolving highly variable flux events on the order of minutes, and can support the evaluation of eddy covariance (EC) flux data quality under non-steady-state conditions
Most statistics are based on the number of minutes that were identified as part of an event
Summary
Methane (CH4) is one of the most important greenhouse gases (Saunois et al, 2016b), but unexpected changes in atmospheric CH4 budgets over the past decade emphasise that many aspects regarding the role of this gas in the global climate system remain unexplained to date (e.g. Saunois et al, 2016a; Nisbet et al, 2016; Schwietzke et al, 2016; Schaefer et al, 2016). Current concentration levels are the highest reached in 800 000 years (Masson-Delmotte et al, 2013), and emissions and concentrations are likely to continue increasing, making CH4 the second most important greenhouse gas (after CO2) that is strongly influenced by anthropogenic emissions (Ciais et al, 2013; Saunois et al, 2016b). Schaller et al.: Short-term extreme CH4 fluxes related to non-turbulent mixing igate climate change effects (Saunois et al, 2016a), quantitative and qualitative insights into processes governing CH4 sources and sinks need to be improved in order to better predict its future feedback with a changing climate
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