Coupled carbon, water and heat fluxes over the global land surface

Simulating changes to the Earth's energy, water and carbon cycles is a key goal of climate and earth system models. However we need to know the regional fluxes and transports of these quantities much more accurately from observations to provide strong constraints for models such as those used at the Met Office for climate predictions, and to inform developments across the wider global modelling community. This is now recognized by IPCC who will have separate chapters on the energy, water and carbon cycles in the next Assessment Report.

We have developed an energy-water cycle coupled inverse method in the department, Thomas et al (2019), which uses many independently observed satellite datasets and their errors to develop closed heat and water budgets on a global scale following an earlier NASA Energy and Water cycle Study (NEWS) L'Ecuyer et al (2015), Rodell et al (2015), see www.nasa-news.org. We have extended the NEWS study with better results over the oceans by improving the errors used for the satellite derived fluxes, and by using additional ocean transport estimates based on ship measurements. Further work supported by the National Centre for Earth Observations has extended the model to study interannual variability from 2001-11, showing a better seasonal cycle of continental warming and a better land water cycle constrained by precipitation, runoff data and water storage estimates from GRACE gravity data. Interannual variability over Africa is one current focus.

This PhD project will focus on improving the land surface processes. On land, soil moisture and vegetation properties largely determine how much energy the surface can store on seasonal timescales, and hence the resultant land surface temperatures (LST), which are now well measured from satellites. Water, sunlight and temperature also determine photosynthesis and biomass growth, taking up CO2 from the Earth's atmosphere. Biomass growth and CO2 uptake can also be monitored from satellite measurements providing additional datasets that can be used with our inverse method. The aim is to couple the land carbon sink to the energy and water cycles, and to test the resulting model in some key regions of interest; possibilities include Africa and China. The PhD student will therefore use new satellite observations as constraints to improve surface flux estimates. The inverse method will be extended to include carbon budgets alongside the water and energy budgets to produce a truly coupled Earth system cycling analysis with many new applications, including testing Earth and Climate circulation models.

The student will explore energy-water-carbon flux exchanges with the atmosphere, and storage over land using local observations from Fluxnet measurement towers, and then seek larger scale relationships using satellite data. Parameterizations and simulations with the JULES land surface model will be used to explore relationships and to help in developing flux uncertainty estimates. The ultimate aims will be (i) to allow EO land surface temperature measurements to constrain energy fluxes and water storage within the inverse method, and (ii) to extend the inverse method to include a carbon budget, where the land surface component is constrained by plant photosynthesis/growth measurements from satellite data. The student will explore the sensitivity of the inverse method to these additional constraints. Additional carbon budget observational data e.g. atmospheric measurements of CO2 from the NASA OCO-2 satellite, may be brought in at a later stage.