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

Africa is highly vulnerable to climate change, yet predictions from global models are fundamentally limited by their representation of atmospheric convection. This project will use unprecedented high-resolution “convection permitting” ensembles of African climate change simulations, run by the Met Office, to understand the processes controlling climate change in rainfall over Africa.

The Met Office is an industry partner and the student will receive a Met Office CASE award, which provides additional stipend and research expense funding.

 

Project motivation

Africa is home to many of the people who are most vulnerable to climate change. Yet predictions of climate change in rainfall for Africa, in common with much of the tropics and sub-troics, remain highly uncertain and in many regions we do not know if it will get wetter or drier. Understanding and predicting changes in rainfall is made challenging by the fact that the convective storms that generate ascent and rainfall in the tropics are not explicitly captured in global climate models, due to  coarse model grid-spacings of 50 km or more. Convection in such models must therefore be represented by simplifications known as parameterisations, which are known to lead to systematic model errors. The new generation of convection-permitting models, which have fine enough grid-spacings to explicitly model convection, finally provide a way to address this long-standing challenge. To date such simulations for Africa have been limited to only one model for a single possible realisation of global change, limiting their value¹. This PhD will capitalise on the first “ensemble” set of multiple convection-permitting climate change simulations for Africa, where each simulation is driven by different possible predictions for global change. This will allow unprecedented insights into risks, uncertainties and physical processes controlling climate change in African rainfall.

Figure 1: From Kendon et al (2019) Figure 5 showing how explicit convection (“CP4A”, panel f) gives a greater increase in extreme rainfall than a standard parameterised model (“R25”, panel e). Extreme rainfall here is the 99th percentile of wet values (>0.1 mm h⁻¹) for 3-hourly precipitation – see Kendon et al. (2019) for details.

The £20 million Future Climate for Africa (FCFA) programme recently ran the first convection-permitting simulations for Africa, referred to as “CP4A”^1,2,3. The high resolution of these runs fundamentally changes the representation of convection and rainfall, which affects changes on scales much larger than the convection itself, and so CP4A provided a unique opportunity for new understanding.  CP4A showed that explicitly modelling the circulations in convective storms increases the climate change increase in extreme rainfall (Figure 1), and sometimes dry spells^1,3. It modifies local response to tropical wave modes and affects land-atmosphere coupling. It not only affects couplings with mesoscale circulations, such as sea-breezes, affecting projections, but affects changes in circulations on hundreds of kilometres, including the Hadley circulation (Figure 2)^4,5. With one simulation it has been possible to analyses some mechanisms, but an ensemble will enable new understanding of how robust the sensitivity to explicit convection is, and allow new analysis of processes controlling climate change¹.

Figure 2: From Figure 2 of Jackson et al. (2020), showing how explicit modelling of convection (b, “CP4A”) generates large scale differences in climate change in rainfall when compared with a standard parameterised model (a, “P25”). Results for atmospheric circulation and water vapour fields are similar – see Jackson et al. (2020) for details.

This PhD builds on a long series of collaborations between Leeds and the Met Office on large-domain convection-permitting modelling of African weather and climate, which dates back to the Cascade project⁶ and includes 3 of the 5 FCFA projects¹, GCRF African SWIFT⁷ and other projects. Leeds expertise in this area was recently recognised in our 2021 Queens Anniversary Prize. Alongside the new “CP4A ensemble”, this PhD may also make use of new simulations from pan-tropical simulations from the Met Office, as well as new simulations from the ELVIC multi-modal convection-permitting inter-comparison for the Lake Victoria region⁸.

 

Project objectives

The CP4A simulations were limited to a 10-year future simulation driven at its boundaries by a single model realisation of global climate change.  This PhD project will exploit the new opportunities created by a new ensemble of simulations, each similar to the original CP4A simulations, to address the following scientific objectives:

• Understand the upscale impacts of explicitly capturing convection on larger-scale circulations and climate change as a function of the modelled global change, addressing both mean state and variability.
• Understand how the mechanisms for changing extremes under climate change depend on both the local unperturbed local climate and the local expression of global change provided from the global model.
• Assess both the implications for how we use costly convection-permitting simulations and how to use information from them together with global simulations to inform real-world decisions.

Throughout the PhD, the project will seek to understand physical processes and trace the dependence of modelled climate change on the explicit modelling of convection back to processes that can be evaluated using observational and reanalysis datasets.

Convection-permitting climate modelling is a rapidly developing research area, but there has been limited research on the tropics, including on Africa. This PhD therefore not only has great scope for new scientific understanding, but its outputs should also be valuable for informing future strategies for modelling climate change, and for supporting use of models to inform decisions. The growth in convection-permitting modelling globally means that there will be several opportunities to interact with other groups in Leeds, the UK and overseas, who are working on related projects (including through the Met Office Academic Partnership at Leeds, NCAS, and our partnership with KIT).

 

References and further reading

¹Senior, C.A., Marsham, J.H. et al, 2021, Convection-Permitting Regional Climate Change Simulations for Understanding Future Climate and Informing Decision-Making in Africa, Bull. Amer. Meteorol. Soc., https://doi.org/10.1175/BAMS-D-20-0020.1

²Stratton, R.A. et al., 2018, A Pan-African Convection-Permitting Regional Climate Simulation with the Met Office Unified Model: CP4-Africa, J. Clim., https://doi.org/10.1175/JCLI-D-17-0503.1

³Kendon, E.J., R.A., Stratton, S. Tucker, J.H. Marsham, S. Berthou, D.P. Rowell, C.A. Senior, 2019, Enhanced future changes in wet and dry extremes over Africa at convection-permitting scale, Nat. Comms., https://doi.org/10.1038/s41467-019-09776-9.

⁴Finney, D.L., J.H. Marsham, D.P. Rowell, E.J.  Kendon, S.O. Tucker, R.A. Stratton and L.S. Jackson, 2020, Effects of Explicit Convection on Future Projections of Mesoscale Circulations, Rainfall, and Rainfall Extremes over Eastern Africa, J. Clim., https://doi.org/10.1175/JCLI-D-19-0328.1

⁵Jackson^, L.S., D.L. Finney¹, E.J. Kendon, J.H. Marsham³, D.J. Parker, R.A. Stratton, L. Tomassini, and S. 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

⁶Marsham, J.H., N.S. 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.,  https://doi.org/10.1002/grl.50347

⁷Woodhams, B., C.E. Birch, J.H. Marsham, C.L. Bain, N.M. Roberts, D.F.A. Boyd, 2018, What Is the Added Value of a Convection-Permitting Model for Forecasting Extreme Rainfall over Tropical East Africa?, Mon. Wea. Rev., 146 (9). pp. 2757-2780. ISSN 0027-0644.

⁸Lipzig, N.P.M.v et al., 2022, Representation of precipitation and top-of-atmosphere radiation in a multi-model convection-permitting ensemble for the Lake Victoria Basin (East-Africa), Clim. Dyn., https://doi.org/10.1007/s00382-022-06541-5