This page outlines modelling work being done at the University of Leeds in support of MOYA.

To simulate atmospheric transport and chemical processes the TOMCAT model is used, more details of which can be found here. The model is setup in a simplfied chemistry mode to simulate atmsopheric concentrations of CH4 and δ13CH4 used in the MOYA project. The δ13CH4 tracer is a recent addition to the model, the development of which is based upon previous work (Rigby et al. 2012).

Forward modelling with TOMCAT has previously been used to identify trends in atmospheric CH4 (e.g. Wilson et al. 2016, McNorton et al. 2016a and McNorton et al. 2016b). Model simulations are typically compared with observational datasets, in the figure on the right TOMCAT simulated CH4 is evaluated against GOSAT satellite measurements. The simulations shown use different wetland emissions and all three show reasonable seasonal and interannual agreement with the satellite observations.

The group has recently introduced a simple chemistry scheme to simulate δ13CH4 based on assumed emission signatures, preliminary comparisons have been performed with surface site measurements, shown in the figure below. The top panel shows the model captures both the observed pause in CH4 growth between 1999 and 2006 and the resumed growth since 2007. The growth rate since 2007 is overestimated in the model simulation at both sites.

Monthly column CH4 concentrations from GOSAT (black) and 3 TOMCAT simulations. McNorton et al. 2016

The bottom panel shows reasonable δ13CH4 model agreement with obersvations up until the lightening, which has been occuring since 2007. Model δ13CH4 continues to increase; whilst the observed values decrease. This suggests a deficiency in the model caused by either a poor representation of atmospheric transport and chemistry, or inaccuracies in the estimated source/sinks.

CH4 and δ13CH4 from observations (black) and TOMCAT (green) at Alert, Canada and Cape Grim, Australia. Observation data courtesy of NOAA ESRL GMD.

As further evaluation of model capabilities to simulate atmospheric δ13CH4, specifcally at high latitudes, comparisons can be made with recently available ACE satellite data. Early comparisons (shown below) suggest TOMCAT can simulate the vertical gradient in δ13CH4; however δ13CH4 is underestimated in the polar high-latitudes.

Seasonal δ13CH4 values for ACE satellite data (left) and TOMCAT model data (right). ACE data courtesy of the Canadian Space Agency

Inverse modelling of both CH4 and δ13CH4 will be performed using the TOMCAT model and a simple one-box model to attribute changes in sources/sinks to observed trends in atmospheric concentrations. This work is on-going and results are not yet available.

Moya @ Leeds Website 2017