Master Thesis
Analysis of Methane Emissions in a Subarctic Permafrost Region using Wavelet Transformation and Conditional Sampling
Carsten Schaller (10/2013-07/2015)
Support: Thomas Foken, Mathias Göckede
In Kooperation mit dem Max-Planck-Institut für Biogeochemie in Jena
Climate change will result in dramatic changes of Arctic ecosystems including permafrost wetlands. An increased warming benefits microorganisms decomposing the permafrost soil organic matter under formation of methane. This greenhouse gas has a global warming potential of 34 times greater than carbon dioxide, but only little is known about the emissions caused by ebullition on ecosystem scale. Ebullition is a process, where bubbles of methane were accumulated in the water-saturated layers of the permafrost, followed by a sudden release within minutes. A direct method to determine exchange rates of gases over ecosystem is eddy covariance, which requires steady state conditions and horizontal homogeneity. Ebullition events might violate both requirements, resulting in an underestimation of the flux. Thus the present study uses wavelet analysis and conditional sampling in order to resolve peaks in the methane flux. The measurements were conducted in a permafrost ecosystem south of Chersky, Sakha (Yakutia) Republic, Russia. Two eddy covariance towers were installed and equipped with a closed-path gas analyser. One of both towers was surrounded by a drainage ditch in order to simulate dry conditions. The Mexican hat wavelet was used to detect the times of ocurring events, while the Morlet wavelet provided information on the frequencies contributing to events. Under best steady state conditions and well developed turbulence a very good agreement of the wavelet results with the reference eddy covariance flux was determined. For conditional sampling, a high sensitivity regarding the correct choice of the mean vertical wind speed led to a non negligible number of outliers. Consequently only the wavelet results were used for peak investigations. Based on the wavelet flux with a timestep of 1 minute a test using the median absolute deviation (MAD) was applied in order to detect peaks. The frequency of peaks showed a well positive correlation with the top soil temperature at the tower and the median flux rate. Surprisingly almost all peaks typically occurred during night time and always simultaneously at both towers. A subset of events, that showed a sharp begin and end was investigated in detail. All those events were obviously triggered by gravity waves, low level jets or weather fronts passing the site, and in half of all cases katabatic winds from a hill ridge nearby were found to be the possibly triggering mechanism. Evidence for ebullition as well as for local, tower-specific triggers was not found. In every case methane was accumulated near the ground over time due to the surface layer being decoupled from the atmosphere in neutral or stable stratification. The triggering mesoscale mechanism mixed the accumulated methane up to the eddy covariance gas analyser resulting in the detected flux peak. Due to advection caused by the triggering mechanisms the classical eddy covariance method failed to resolve the events correctly. This project was hosted at the Max-Planck-Institute for Biogeochemistry, Jena.