Humid heat extremes over Africa

This project will investigate humid heat extremes over Africa, a climate extreme that is not well understood, yet poses a significant risk to humankind.

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

Background and Rationale

Humid heat is a serious risk to human health, reducing the body’s ability to expel heat through sweating. The human impact of heat extremes will increase under climate change, particularly in tropical ‘hot spots’, such as equatorial Africa (Fig.1), which is highly populated, already very hot and humid and vulnerable to climate variability. Whilst there is a fairly well established body of research on dry bulb temperature extremes, there is very limited understanding of the meteorological drivers of humid heat extremes, particularly the role of moisture transport, rainfall, and evaporation of moisture from the Earth’s surface. The ability of climate models to represent these processes in the context of humid heat extremes is largely unquantified. This is despite an urgency to adapt to and mitigate the impacts of globally increasing heat extremes.

Fig. 1 Number of days per year in 2090-2100 exceeding the threshold of temperature and humidity beyond which climatic conditions become deadly, for a high greenhouse gas emissions scenario (RCP8.5). Taken from Mora et al. (2017).

Project details and objectives

Emerging work suggests that humid heatwave formation is the result of a complex interaction between moist processes involving cloud, rainfall, evaporation and moisture advection (Birch et al. 2022) alongside a lack of mixing in the lower atmosphere, which prevents the ventilation of heat (Raymond et al. 2021). Over West Africa, humid extremes occur at the start of the monsoon season, following the seasonal increase in humidity, but at a time when the dry bulb temperature remains high. This is contrary to dry bulb extremes, which occur during the pre-monsoon season when humidity is low but dry bulb temperatures are at their peak. Humidity tends to contribute more to humid heat extremes in the arid of Africa regions, whereas dry bulb temperature is also a key contributor in the already very humid equatorial tropics. There is a critical need to regionally assess the extent to which humid heat is modulated by circulation extremes versus surface fluxes and evaluate these drivers in climate models.

Fig. 2 Frequency of present day humid heat heatwaves in ERA5 reanalysis (similar to observations), a climate model with parameterised convection (P25) and a convective-scale climate model (CP4).

It essential that climate and weather forecast models are able to realistically represent humid heat extremes in order to provide accurate predictions on weather, sub-seasonal, seasonal and climate timescales. Climate projections are almost ubiquitously provided by relatively coarse resolution regional or global climate models, which require a parameterisation scheme to represent convective processes. Such models are known to poorly represent the key moist processes that can cause or ventilate severe moist heat stress events, which over Africa, translates into an underestimation of present day humid heatwave frequency (Fig. 2), and their future change (Birch et al. 2022). Convective-scale regional climate models can be run at a sufficiently high resolution to allow the convective parameterisation to be turned off and to represent the largest convective motions explicitly. Convective-scale models are thus better able to represent processes important for humid heat extremes such as intense rainfall and dry spells, the atmospheric water cycle and soil moisture-precipitation feedbacks. They provide an opportunity to advance process-based understanding of humid heat extremes, but have yet to be fully exploited in this way.

In this project the student will initially use existing 4.5km pan-African convective scale climate simulations, later augmented by the new ensemble simulations once available, to better understand present day humid heat extremes and assess how they may change in the future. An ensemble is a group of climate model simulations, each with a slightly different configuration, which produces a number of different climate scenarios to quantify the uncertainty in future projections.

Objectives:

• Use satellite retrievals (e.g. Bouniol et al. 2021) of the Earth’s energy and radiative budgets, cloud, rainfall, temperature and water vapour to quantify the drivers of humid heat extremes over different regions of Africa for observed case studies and on average, over many past events
• Evaluate the representation of humid heat extremes and their drivers in the pan-African convective-scale climate ensembles and in more traditional global climate model simulations (CMIP6)
• Use the pan-African convective-scale climate ensembles to assess the uncertainty in future projections of humid heat extremes

Project outputs

This project will make use of the first ever convective-scale ensemble climate simulations over the African continent. Humid heat is a climate extreme that is not well understood, yet poses a significant risk to humankind. There is an opportunity of novel, high impact research to be conducted in this project, with a potential for results to be published in high impact journals.

References

Birch CE, L. Jackson, D. Finney, J. M. Marsham, R. A. Stratton, S. Tucker, et al. (2022) Future changes in African heatwaves and their drivers at the convective scale. J Climate, in press.

Bouniol, D., Guichard, F., Barbier, J., Couvreux, F., & Roehrig, R. (2021). Sahelian heat wave characterization from observational data sets. Journal of Geophysical Research: Atmospheres, 126, e2020JD034465.

Mora C, Dousset B, Caldwell IR, Powell FE, Geronimo RC, Bielecki Coral R, et al. (2017) Global risk of deadly heat. Nature Climate Change, 7(7): 501-506.

Raymond C, Matthews T, Horton RM, Fischer EM, Fueglistaler S, Ivanovich C, et al. (2021) On the Controlling Factors for Globally Extreme Humid Heat, GRL, 48(23): e2021GL096082.

Senior, C. A. et al. (2021) Convection permitting regional climate change simulations for understanding future climate and informing decision making in Africa, Bull. Am. Meteorol. Soc., https://doi.org/10.1175/BAMS-D-20-0020.1.