Contrasting controls on Congo Basin evaporation at the two rainfall peaks

David Crowhurst, Simon Dadson, Jian Peng & Richard Washington

Evaporation is a crucial driver of Congo Basin climate, but the dynamics controlling the seasonality of basin evaporation are not well understood. This study aims to discover why evaporation on the basin-wide average is lower at the November rainfall peak than the March rainfall peak, despite similar rainfall. Using 16-year mean LandFlux-EVAL data, we find that evaporation is lower in November than March in the rainforest and the eastern savannah. The ERA5-Land reanalysis, which effectively reproduces this pattern, shows that transpiration is the main component responsible for lower evaporation in these regions. Using ERA5-Land, we find the following contrasting controls on transpiration, and therefore evaporation, at the two rainfall peaks: (a) In the northern rainforest, there is lower leaf area index (LAI) in November, driven by lower surface downward shortwave radiation (DSR), and lower vapour pressure deficit (VPD) in November, driven by lower sensible heat flux that results from lower net radiation. The combination of lower LAI and VPD explains lower transpiration, and therefore lower evaporation, in November. (b) In the southern rainforest, and in the north-eastern savannah, there is lower LAI in November, driven by lower surface DSR, and this explains lower transpiration, and therefore lower evaporation, in November. (c) In the south-eastern savannah, there is lower LAI in November, driven by lower volumetric water content (VWC), and this explains lower transpiration, and therefore lower evaporation, in November. Collectively, these contrasting controls at the two rainfall peaks explain why the basin-wide average evaporation is lower in November than March.

Understanding mechanisms for trends in Sahelian squall lines: Roles of thermodynamics and shear

Bickle M, Marsham J, Ross A, Rowell D, Parker D, Taylor C.

Squall lines dominate rainfall in the West African Sahel, and evidence suggests they have increased in intensity over recent decades. Stronger wind shear may be a key driver of this trend and could continue to strengthen with climate change. However, global numerical models struggle to capture the role of shear for organised convection, making predictions of changing rainfall intensities in the Sahel uncertain. To investigate the impact of recent and possible future environmental changes, and to isolate thermodynamic effects from shear effects, idealised squall line simulations were initialised with a profile representative of the present day: this profile was then modified using trends from reanalyses and climate projections. Increased shear led to increased storm intensity and rainfall, but the effects of the thermodynamic changes dominated the effects from shear. Simulations initiated with future profiles produced shorter‐lived storms, likely due to increased convective inhibition and the absence of large‐scale convergence or synoptic variability in the idealised model. A theoretical model based on the relative inflow of convectively unstable air and moisture was found to predict bulk characteristics of the storms accurately, including mean rain rates and area‐averaged maximum vertical velocities, explaining the role of shear. However, the model is not a prognostic tool as rainfall is dependent on the storm speed, which remains a free parameter. The study shows the importance of shear to long‐term rainfall trends and highlights the need for climate models to include effects of shear to capture changes in extreme rainfall.

Stochastorm: A stochastic rainfall simulator for convective storms

Wilcox C, Aly C, Vischel T, Panthou G, Blanchet J, Quantin G, Lebel T.

Stochastic rainfall generators aim to reproduce the main statistical features of rainfall at small spatial and temporal scales. The simulated synthetic rainfall series are recognized as suitable for use with impact analysis in water, agricultural, and ecological management. Convection-driven precipitation, dominant in certain regions of the world such as the intertropical belt regions, presents properties that require specific consideration when modeling: (i) strong rainfall intermittency, (ii) high variability of intensities within storms, (iii) strong spatiotemporal correlation of intensities, and (iv) marked seasonality of storm properties. In this article, improvements for an existing stochastic generator of rainfall fields that models convective storms are presented. Notable novelties include (i) the ability to model precipitation event timing, (ii) an improved temporal disaggregation scheme representing the rainfall distribution at subevent scales, and (iii) using covariates to reflect seasonal changes in precipitation occurrence and marginal distribution parameters. Extreme values are explicitly considered in the distribution of storm event intensities. The simulator is calibrated and validated using 28 years of 5-min precipitation data from the 30-rain-gauge AMMA-CATCH network in the Sahelian region of southwest Niger. Both large propagative systems and smaller local convective precipitation are generated. Results show that simulator improvements coherently represent the local climatology. The simulator can generate scenarios for impact studies with accurate representation of convective precipitation characteristics.

Synoptic timescale linkage between midlatitude winter troughs Sahara temperature patterns and northern Congo rainfall: A building block of regional climate variability

Ward N, Fink A, Keane R, Guichard F, Marsham J, Parker D, Taylor C.

A coherent synoptic sequence, mostly over North Africa, is identified whereby an upper‐level midlatitude trough (in November‐March) excites several days of quasi‐stationary near‐surface warming across the Sahara, leading to rainfall events over northern Congo (NC), and perturbed weather more widely. Ahead of NC rainfall events, composite sequences first identify troughs for several days near Iberia, followed by relatively quick transfer to the Central Mediterranean (CMed). Iberia and CMed daily trough‐strength indices reveal that both lead to warming and NC rainfall. Iberia trough linkages develop through West Africa and take longer to reach NC, while CMed linkages reach NC faster (2‐3 days), with impact extent focused mostly south and east of CMed. Building up to the rainfall events, initial warming over the central Sahara migrates southeastward close to NC, ultimately with typical magnitude of about 1‐2°C at 10‐15°N. Such anomalies are statistically predictive for NC daily rainfall and associated nearby atmospheric features: anomalous low‐level southerly wind and increased moisture; anomalous low‐level westerly wind and vertical easterly shear to 600 hPa; increased mid‐level moisture (600 hPa), which along with low‐level moisture, connects northward into midlatitudes. A secondary route identified by which Iberia troughs can impact NC rainfall is through direct atmospheric teleconnection with precipitation to the west of NC, and subsequent migration of that convection eastward into NC. The eastern side of NC generally shows a small lag on western parts, and links more strongly to CMed troughs. Taken together, the lagged synoptic expression of Iberia and CMed troughs is widespread over several days, including much of North Africa (to equatorial latitudes), southwestern Asia, eastern Africa and the western Indian Ocean. Overall, these results can contribute to situational awareness for weather forecasters across the zones influenced by the troughs, while also providing a framework for climate timescale analyses.

Understanding Intermodel Variability in Future Projections of a Sahelian Storm Proxy and Southern Saharan Warming

Rowell D, Fitzpatrick R, Jackson L, Redmond G.

Projected changes in the intensity of severe rain events over the North African Sahel—falling from large mesoscale convective systems—cannot be directly assessed from global climate models due to their inadequate resolution and parameterization of convection. Instead, the large-scale atmospheric drivers of these storms must be analyzed. Here we study changes in meridional lower-tropospheric temperature gradient across the Sahel (ΔT_Grad), which affect storm development via zonal vertical wind shear and Saharan air layer characteristics. Projected changes in ΔT_Grad vary substantially among models, adversely affecting planning decisions that need to be resilient to adverse risks, such as increased flooding. This study seeks to understand the causes of these projection uncertainties and finds three key drivers. The first is intermodel variability in remote warming, which has strongest impact on the eastern Sahel, decaying toward the west. Second, and most important, a warming–advection–circulation feedback in a narrow band along the southern Sahara varies in strength between models. Third, variations in southern Saharan evaporative anomalies weakly affect ΔT_Grad, although for an outlier model these are sufficiently substantive to reduce warming here to below that of the global mean. Together these uncertain mechanisms lead to uncertain southern Saharan/northern Sahelian warming, causing the bulk of large intermodel variations in ΔT_Grad. In the southern Sahel, a local negative feedback limits the contribution to uncertainties in ΔT_Grad. This new knowledge of ΔT_Grad projection uncertainties provides understanding that can be used, in combination with further research, to constrain projections of severe Sahelian storm activity.

Seasonality and Trends of Drivers of Mesoscale Convective Systems in Southern West Africa

Klein C, Nkrumah F, Taylora C, Adefisan E.

Mesoscale convective systems (MCSs) are the major source of extreme rainfall over land in the tropics and are expected to intensify with global warming. In the Sahel, changes in surface temperature gradients and associated changes in wind shear have been found to be important for MCS intensification in recent decades. Here we extend that analysis to southern West Africa (SWA) by combining 34 years of cloud-top temperatures with rainfall and reanalysis data. We identify clear trends in intense MCSs since 1983 and their associated atmospheric drivers. We also find a marked annual cycle in the drivers, linked to changes in the convective regime during the progression of the West African monsoon. Before the peak of the first rainy season, we identify a shear regime where increased temperature gradients play a crucial role for MCS intensity trends. From June onward, SWA moves into a less unstable, moist regime during which MCS trends are mainly linked to frequency increase and may be more influenced by total column water vapor. However, during both seasons we find that MCSs with the most intense convection occur in an environment with stronger wind shear, increased low-level humidity, and drier midlevels. Comparing the sensitivity of MCS intensity and peak rainfall to low-level moisture and wind shear conditions preceding events, we find a dominant role for wind shear. We conclude that MCS trends are directly linked to a strengthening of two distinct convective regimes that cause the seasonal change of SWA MCS characteristics. However, the convective environment that ultimately produces the most intense MCSs remains the same.

Refining projections of future temperature change in West Africa

Macadam I, Rowell D, Steptoe H

Future warming in West Africa will have a detrimental effect on the communities living there. To support assessments of climate change impacts, we propose a method for refining regional temperature projections and demonstrate its application to West Africa for the mid-21st century. Our focus is on characterising uncertainty more comprehensively by considering projections of global warming. We calculate a transformation between a frequency distribution of global warming values derived from the Coupled Model Intercomparison Project phase 5 (CMIP5) models and a broader published probability distribution of global warming developed by the Met Office. The latter draws on perturbed parameter ensembles of simpler climate models to account for uncertainties related to the atmosphere, ocean, carbon cycle and aerosol processes that are not well characterised by the CMIP5 ensemble. Noting that West African warming is highly correlated with global warming in the CMIP5 ensemble, and that a significant portion of the uncertainty in projected West African warming arises from the uncertainty in global warming, we then apply the same transformation to CMIP5-derived distributions for warming in different regions of West Africa. The resultant regional warming distributions have longer tails than distributions estimated directly from the CMIP5 ensemble. Our results imply that CMIP5-based assessments of temperature-sensitive applications may underestimate the probability of large (and small) impacts. Our method could be used to refine temperature projections for other regions of the world in which regional temperature changes are highly correlated with global mean temperature changes.

Mobilizing Climate Information for Decision-Making in Africa: Contrasting User-Centered and Knowledge-Centered Approaches

Blane Harvey, Ying-Syuan Huang, Julio Araujo, Katharine Vincent, Jean-Pierre Roux, Estelle Rouhaud and Emma Visman

This study examined ways in which climate information was mobilized for use under Future Climate for Africa (FCFA), an applied research program to improve the use of climate information to support medium-term (5–40 years) policies and planning in sub-Saharan Africa. Past research has underscored the interdependent relationship between user engagement and knowledge mobilization in effective climate knowledge uptake. The study used a document analysis of 46 program ou tputs and semi-structured interviews with 13 FCFA researchers to contrast user-centered and knowledge-centered approaches to effectively mobilize climate information uptake for use. A total of 20 knowledge mobilization tools and approaches were identified across the program and analyzed. This analysis reveals a complex interplay between user engagement and knowledge mobilization processes, including the strategic or flexible use and re-use of knowledge products as the user engagement process evolved. These findings have important implications for future programmatic design and planning in promoting engagement and mobilization approaches that can contribute to long-term policy and decision-making.

Refocusing the climate services lens: Introducing a framework for co-designing “transdisciplinary knowledge integration processes” to build climate resilience

Elizabeth Daniels, Sukaina Bharwani, Åsa Gerger Swartling, Gregor Vulturius, Karen Brandon

This paper seeks to reconceptualize climate services in light of the prevailing inability of existing climate information to spur needed policy and action. We propose refocusing the climate services lens by moving away from a narrow, supply-driven emphasis on products. Instead, we advocate moving towards a process-centric approach defined by transdisciplinary collaboration that purposefully seeks to bring about fundamental, long-term benefits. Such benefits include increased human and institutional capacity, and the creation of relationships that are essential components of science-informed decision-making for climate adaptation and beyond. Work underpinning this paper consists of a review of existing climate services guidance, and analyses of a survey of climate services stakeholders, and a climate information co-production process case study in Lusaka, Zambia. We identify elements needed to support complex, real-world decision-making that many existing climate services fail to sufficiently consider. We respond by introducing a framework (Tandem), which consists of structured elements and practical, guiding questions informed by empirical analysis. To lay the foundation for both science-informed policy and policy-informed science, the Tandem framework puts forward guidance to achieve three goals: 1) to improve the ways in which all participants work together to purposefully design transdisciplinary knowledge integration processes (co-exploration and co-production processes that bring together different knowledge types across the science-society interface); 2) to co-explore decision-relevant needs for the co-production of integrated climate information (i.e., decision-relevant climate and non-climate information); and, 3) to increase individual and institutional capacities, collaboration, communication and networks that can translate this information into climate-resilient decision-making and action.

Alternative inclusive approaches for improving climate information services and decision-making in Harare, Zimbabwe

Chipo Plaxedes Mubaya, Mzime Regina Ndebele-Murisaa, Rudo Mamombe

City-scale climate information is needed to inform policy, decision-making and climate action in the context of development agendas like Sustainable Development Goals (SDGs), Sendai Framework of Disaster Risk Reduction, New Urban Agenda and the Paris Agreement. Traditional approaches in climate services are often top-down and inadequate in meeting the needs of stakeholders in urban areas like African cities. To enhance the provision of relevant climate information, it is important to understand the context of the urban environment in question and decision-making processes. Researchers from the FRACTAL project used participatory, transdisciplinary co-production approaches to understand the context of Harare and to investigate perceptions and values that underpin decision-making in the water sector. These were tied to historical trends in climate and hydrological flows as well as narratives of future climate and hydrological scenarios to draw timely, relevant information for policy, and practice implementation and for building resilience in the city. Lessons learnt, which are applicable to similar African cities, show the importance of understanding decision-making in providing a context to build on for distilling, improving and mainstreaming climate information services and that engagement processes are useful in building sustained working relationships and trust.