Breaking the tropical convection “dead-lock”: Scale interactions of deep convection and tropical circulation

The tropics cover 40% of Earth’s area, include 36% of its land, and are home to 40% of its people, and its rapidly growing population includes many of the people who are most vulnerable to extreme weather and climate change. Tropical weather is dominated by cumulonimbus storms that not only deliver rainfall, but generate intense local atmospheric heating, which is then communicated to the wider atmosphere. For decades the representation of these moist convective storms has provided a “deadlock” in weather and climate modelling, as it has been impossible to model them directly in a global model due to their small scale. This is one major reason why skill of numerical weather prediction is low in the tropics, and climate change projections of rainfall are very uncertain. Increased computer power is now removing this “dead-lock”: this PhD will use unique new large-domain high-resolution convection-permitting simulations being performed at the Met Office to address fundamental questions about how moist convection interacts with larger scale flows, and the implications for predictions across time scales.

Figure 1: The tropical rain belt over West Africa and the Atlantic (image credit: NOAA NWS National Hurricane Centre).

Air ascends in the inter-tropical convergence zone (or “tropical rain band”), causing clouds and rain, and descends further north or south, causing the great deserts of the world, including the Sahara. Together these form the important large-scale pattern of circulation known as the “Hadley circulation” (see Jackson et al., 2020). The circulation is far from uniform however, with individual cumulonimbus storms providing ascent, and often organised into mesoscale convective systems that can span 100s of km. These are both in turn coupled with larger-scale propagating waves, and interact with continental-scale circulations such as monsoons (Marsham et al., 2013; Jackson et al., 2019). Understanding how moist convection interacts with larger scale flows is made much more challenging by the fact that the cumulonimbus storms that generate ascent and rainfall in the tropics are not explicitly resolved in global models, which have grid-spacings of approximately 10s or 100s of kilometres. This means their effects are normally represented by simplifications known as parametrisations. This project will build on past efforts to run large-domain simulations that have a small enough grid-spacing (approximately 1 to 5 km) to explicitly capture these storms, such as the Cascade project (Marsham et al., 2013) and the IMPALA project of the £20 million Future Climate for Africa (FCFA) programme (Kendon et al., 2019), capitalising on ongoing work at the Met Office to run the first-ever tropics-wide kilometre-scale Unified Model simulations. The explicit representation of convection fundamentally changes the representation of convection and rainfall, and so provides a unique opportunity for new understanding.

Objectives

The project will address the following scientific objectives:

  • How moist convection interacts with tropical waves, how this is affected by the representation of convection, and the implications for predictions.
  • How moist convection influences the water and energy balance of large regions and implications for climate change.
  • Evaluating the role of clouds and convective processes in a changing climate.

These objectives will be tackled using unique new state-of-the-art explicit simulations being performed at the Met Office and existing smaller simulations on which these build, together with observations and reanalyses (a blend of observations and models).

Potential for high impact science

Interactions of moist convection with larger-scale circulations is a limiting factor for predictions across time scales. The coupling of convection with circulation under climate change is highlighted as a grand challenge in the “Clouds, Circulation and Climate Sensitivity” Grand Challenge of the World Climate Research programme (WCRP). Past projects have shown the value of explicitly modelling convection over large domains in allowing new insights to this challenging problem. By capitalising on unique new simulations from the Met Office the project will have new tools to address long-standing challenges of high importance to society.

Training, Supervision and the Research Environment

You will join a large and dynamic team at Leeds, who have a strong record of highly cited publications, with opportunities to visit collaborators overseas, and possibly potential for also gaining fieldwork experience. You will work under the supervision of Dr John Marsham, who is the Met Office Joint Chair at the University of Leeds, and has published 100 papers in 15 years. He manages a large group working across moist atmospheric convection, tropical weather and climate, dust uplift, and associated fields. His past PhD students have strong records of peer-reviewed publication.  Co-supervisor at the Met Office, Dr Jon Petch, is an expert in large-scale convective dynamics and numerical modelling. Co-supervisor Prof Doug Parker has a programme of research studying the interactions between atmospheric convection and larger-scale circulations, making use of field measurements and model data from the mid-latitudes and the tropics. This project will build on our programme of work studying atmospheric convection in the UK, Europe and the global tropics, and will interact with a number of ongoing projects in these areas.

You will be working in a diverse institute (Institute of Climate and Atmospheric Science, ICAS) with dynamical meteorologists, aerosol modellers, chemists, climate scientists, and climate-impact specialists. The wider school (School of Earth and Environment, SEE) includes social scientists, and Leeds hosts the Priestly International Centre for Climate Change, water@leeds and the National Centre for Atmospheric Science, enabling you to maximise inter-disciplinary opportunities, the reach of your research, and your own learning.

You will be encouraged to travel to share your findings and learn from other environments. Leeds is one of the three founding Universities of the now six-strong Met Office Academic Partnership, and you will be expected to visit and work closely with the Met Office, capitalising on the available breadth and depth of expertise. ICAS has a formal partnership the Karlsruhe Institute of Technology (KIT, Germany), facilitating visits there, but there are also opportunities further afield. Projects such as GCRF Africa-SWIFT (https://www.ncas.ac.uk/en/swift-project) and Terra Maris (Indonesia) provide opportunities to engage with researchers across the UK and the tropics, e.g. in project meetings, forecast test beds, summer schools and possibly fieldwork.

Environmental and atmospheric modelling is growing field and this PhD project will provide hands on experience with a world-leading models, and access to appropriate training, for handling the data and with the potential for running models as required for the research, as well as in wider skills (http://www.emeskillstraining.leeds.ac.uk/).

Student profile

You will have a degree in a mathematical, physical or environmental science, and have some, but perhaps limited, familiarity with scientific programming, as the project will involve writing computer codes for data analysis. You will be interested in weather and climate. A background in meteorology is useful but not essential: physical understanding is key and excellent lecture courses in meteorology are available at the University.

You will bring enthusiasm, the ability to learn how to use state-of-the art meteorological models and observational data, and the potential to understand some of the most pressing research questions in meteorology and climate science. A willingness to travel to tropical regions is not essential, but may be useful.

References

Jackson, Lawrence S, Declan L. Finney, Elizabeth J. Kendon, John H. Marsham, Douglas J. Parker, Rachel A. Stratton, Lorenzo Tomassini and Simon Tucker, 2020, The effect of explicit convection on couplings between rainfall, humidity and ascent over Africa under climate change, J. Clim., https://doi.org/10.1175/JCLI-D-19-0322.1

Jackson LS, RJ Keane, DL Finney, JH Marsham, DJ Parker, CA Senior, RA Stratton, 2019,  Regional differences in the response of rainfall to convectively coupled Kelvin waves over tropical Africa, J. Clim., https://doi.org/10.1175/JCLI-D-19-0014.1

Kendon E.J., R. A. Stratton, S. Tucker, J. H. Marsham, S. Berthou, D. P. Rowell and C. A. Senior, 2019, Enhanced future changes in wet and dry extremes over Africa at convection-permitting scale, Nature Comms, 10, 1794, doi: 10.1038/s41467-019-09776-9.

Marsham, J.H., N. Dixon, L. Garcia-Carreras, G.M.S. Lister, D.J. Parker, P. Knippertz and C.E. Birch, 2013,  The role of moist convection in the West African monsoon system – insights from continental-scale convection-permitting simulations, Geophys. Res. Lett., 40, 1843-1849, doi: 10.1002/grl.50347