Understanding climate change in African rainfall via a new generation of high-resolution models

Africa’s rapidly growing population includes many of the people who are most vulnerable to climate change. Climate projections for Africa, and in fact elsewhere in the tropics, are highly uncertain and in many regions we do not know if it will get wetter or drier. It is known that the representation of the convective clouds that deliver rainfall in the tropics is a major source of error in climate models. The new generation of “convection-permitting” models finally provide a route for addressing this long-standing problem, and a new opportunity to answer fundamental questions on the role of upscale impacts of convection on regional climate change.

This project will use the first ever ensemble of future African climate simulations run at a convection-permitting scale to generate new understanding of the processes controlling climate change in Africa, how this is represented in state-of-the-art models and the implications for climate projections and uncertainty.

You will join a large, dynamic and collaborative group at Leeds studying tropical meteorology and convective storms, whose work was recently recognised in a 2021 Queen’s Anniversary Prize. You will need a strong background in physical science and computational approaches, and a willingness to collaborate with and visit the Met Office. You will be based within ICAS at the School of Earth and Environment. Other opportunities include collaboration with overseas co-supervisors (NSF NCAR, USA). There may also be opportunities for travel to Africa.

Motivation

Rainfall in the tropics and sub-tropics is dominated by convective storms (Figure 1). It is these storms, or their absence, that provides most high-impact weather. Furthermore, these storms form a key component of tropical atmospheric circulation. Rainfall corresponds to the net condensation of water vapour, and this condensation is a dominant source of heat in the tropical atmosphere. This heating is communicated to the wider atmosphere and affects atmospheric circulations on scales far greater than the convection itself. Global climate models are run at spatial resolutions that do not capture convective storms, and so the storms are parameterised as sub-grid processes in such models. These parameterisations are known to introduce errors in not only local rainfall, but also large-scale weather systems, and indeed entire monsoon systems.

Figure 1: NASA image of cumulonimbus over Africa. Such storms release huge amounts of energy into the atmosphere, but are sub-grid in global models, limiting predictions. “Convection-permitting models” explicitly model such storms, which not only affects modelled rainfall, but regional climate projections.

Convection-permitting simulations allow an explicit representation of convection and a step-change in performance for not only modelled rainfall, but also couplings between the convection and larger scales. The Met Office is now running the first-ever ensemble of convection-permitting climate change simulations for Africa, with each member of the ensemble driven by a different parent global climate model. Leeds played a leading role in the £20 million Future Climate for Africa (FCFA) programme, which ran the first convection-permitting future climate simulations for Africa (Senior et al., 2021). Referred to as “CP4A”, these had a small enough grid-spacing (approximately 4km) to explicitly capture convective storms, but the simulations were limited by the fact that they were only for a single ten-year future period, with the boundary-conditions of the regional model provided by a single realisation of global climate change from a single global model. The new ensemble simulations provide a unique new opportunity for developing our understanding of how climate change will affect weather in Africa, how convection affects larger scales, and the limitations of the current generation of global climate models.

Objectives
1 Determine how upscale impacts of explicitly capturing convection in a regional model depend on the driving global model, for both current and future climate.

2 Understand the upscale impacts of explicitly capturing convection on larger-scale circulations and climate change over Africa, including how this depends on the couplings between convection and larger scale systems, from sea breezes to the entire Hadley circulation.

3 Understand how changes seen in models relate to observed processes, and how this can be used to constrain the diverse predictions from models.

The objectives will be tackled using the unique new ensemble of convection-permitting Unified Model simulations run by the Met Office, alongside satellite and other observational datasets and reanalyses (a blend of observations and models). Depending on the interests of the candidate, other objectives can be developed as the project progresses, such as how upscale impacts depend on internal atmospheric processes versus coupled processes, or a focus on extremes.

 

Further Reading

Overview of first convection-permitting simulations for Africa, “CP4A”: Senior, C. A., Marsham, J. H., Berthou, S., Burgin, L. E., Folwell, S. S., Kendon, E. J., et al. (2021). Convection permitting regional climate change simulations for understanding future climate and informing decision making in Africa. Bulletin of the American Meteorological Society, 1–46. https://doi.org/10.1175/bams-d-20-0020.1

Analysis of effects of explicit convection in the Hadley circulation using CP4A: Jackson LS et al (2020) The effect of explicit convection on couplings between rainfall humidity, and ascent over Africa under climate change. J Clim 33:8315–8337. https://doi.org/10.1175/Jcli-D-19-0322.1

First paper using CP4A simulations of future climate to show greater changes in extremes in a convection-permitting model: Kendon, E.J., Stratton, R.A., Tucker, S. et al. Enhanced future changes in wet and dry extremes over Africa at convection-permitting scale. Nat Commun 10, 1794 (2019). https://doi.org/10.1038/s41467-019-09776-9

First analysis of upscale impacts of convection on the West African monsoon, using first regional convection-permitting simulations run over many days: Marsham, J. H., Dixon, N. S., Garcia-Carreras, L., Lister, G. M. S., Parker, D. J., Knippertz, P., and Birch, C. E.: The role of moist convection in the West African monsoon system: Insights from continental-scale convection-permitting simulations, Geophys. Res. Lett., 40, 1843–1849, https://doi.org/10.1002/grl.50347, 2013.