Possible influence of the convection schemes in regional climate model RegCM4.6 for climate services over Central Africa

AbstractIn this study we evaluate the reliability of the Regional Climate Model (RegCM4.6) by performing several sensitivity tests at 50 km horizontal resolution using 10 convection schemes or combination of convection schemes. Emphasis on model output is made for the September–October–November 2017–2019 seasonal period. Part of the Central African region, five sub‐regions, was identified. The simulated temperatures are compared to the daily climatology of near‐surface temperature of the European Centre for Medium‐Range Weather Forecasts Reanalysis fifth‐generation and the National Center for Environmental Prediction datasets, while the simulated precipitations are compared to the precipitation of the Global Precipitation Climatology Project and Climate Hazards Group Infrared Precipitation with Stations datasets. In most analyses, the Grell scheme with Arakawa–Schubert closure assumption tends to have the best index of agreement but its spatial distribution shows underestimation of rainfall. Generally, the Emanuel convection scheme is more suitable to represent rainfall and temperature over Central Africa. Compared to our previous study (Komkoua Mbienda et al., International Journal of Climatology, 2017, 37, 328–342), the present study shows that we have to pay attention to the choice of convective scheme when using any version of RegCM4 released for climate study over Central Africa. This choice is strictly related to the RegCM version released, the study years and the season. It is important to note, however, that the results presented are a preliminary study of the response to the selected convection schemes. The analysis uses a limited sample of climate model simulation (three model years for each convection scheme). Follow‐up work, featuring longer duration climate simulations and a full assessment of statistical significance, is therefore required to confirm the robustness of the analysis presented

Spatio-Temporal Variability of Western Central African Convection from Infrared Observations

The present study has used Meteosat infrared brightness temperature images to investigate the regional and interannual variability of Central African cloudiness. Spatial and temporal variability were investigated using half–hourly data from the Meteosat-7 during June–July–August (JJA) of 1998–2002. The full domain of study (1.5E–17E, 1N–15N) was divided into six regions and statistics in each region were derived. Analysis of the dependence of cloud fraction to the brightness temperature threshold is explored both over land and ocean. Three diurnal cycle regimes (continental, oceanic, and coastal) are depicted according to the amplitude and peak time. Over regions of relatively flat terrain, results indicate enhancement of deep convection in the afternoon followed by a gradual decrease in the night. The diurnal cycle of convection is characterised by afternoon and early evening (around 15:00–18:00 LST) maxima located mainly downwind of the major mountain chains, and a more rapid nighttime decay. In terms of the harmonic amplitude, the diurnal signal shows significant regional contrast with the strongest manifestation over the Adamaoua Plateau and the weakest near the South Cameroon Plateau. This remarkable spatial dependence is clear evidence of orographic and heterogeneous land-surface impacts on convective development. Oceanic region exhibits weak activity of convective cloudiness with a maximum at noon. It is suggested that daytime heating of the land surface and moist environment may play a role in determining the spatial distribution of cloud fraction. This study further demonstrates the importance of the Cameroon coastline concavity and coastal mountains in regulating regional frequencies of convection and their initialization. The strength of the diurnal cycle of convective activity depends on mountain height, mean flow, coastal geometry

Seasonal variations in the diurnal patterns of convection in Cameroon - Nigeria and their neighboring areas

Precipitations in tropical Africa are highly convective. Therefore, knowledge of the diurnal cycle of convective processes enhances understanding and prediction of rainfall patterns. In this study the satellite database is subsetted to look carefully for the regional variability of the diurnal cycle, and geographical patterns of convection in Cameroon-Nigeria and neighbouring areas. Strong spatial variations exist in the seasonal amplitude of diurnal cycle, highlighting that complex orography, land-sea contrast and coastline curvature play an important role in modulating the spatial and diurnal patterns of convection. Results reveal features that can be used to improve convection simulation in models

Projected changes in extreme rainfall and temperature events and possible implications for Cameroon's socio‐economic sectors

Abstract Extreme events like flooding, droughts and heatwave are among the factors causing huge socio‐economic losses to Cameroonians. Investigating the potential response of rainfall and temperature extremes to global warming is therefore critically needed for tailoring and adjusting the country's policies. Recent datasets have been developed for this purpose within the Coordinated Output for Regional Evaluations (CORDEX‐CORE) initiative, at ~25 km grid spacing. These regional climate models were used to dynamically downscaled four global climate models participating in the Coupled Model Intercomparison Project phase 5 (CMIP5), under the optimistic and pessimistic representative concentration pathways (RCPs) 2.6 and 8.5, respectively. These models were employed in this study for characterizing the response of Cameroon's extreme precipitation and temperature events to global warming, using seven indices defined by the Expert Team on Climate Change Detection and Indices. Under global warming, the maximum number of consecutive dry (wet) days' is expected to increase (decrease). However, the annual total rainfall amount is expected to increase, mainly due to the intensification of very wet days and daily rainfall intensity. Furthermore, the temperature‐based indices reveal an increase (decrease) in the total annual hot (cold) days, and overall, changes intensify with increased radiative forcing. The high‐mitigated low‐emission pathway RCP2.6 features attenuated changes, and even sometimes adapts to reverse the sign of changes. Designing reliable policies to limit the risks associated with the above changes is required, as their socio‐economic consequences are likely to include food insecurity, heat‐related illness, population impoverishment, price rises and market instability

Process-based analysis of the added value of dynamical downscaling over Central Africa

In this study, nine global climate models (GCMs) and corresponding downscaled runs by means of the regional climate model (RCM) RCA4 are used to investigate added value (AV) in precipitation and its some drivers over Central Africa (CA). By employing a process‐based analysis approach, we intercompare abilities of RCM to those of driving GCMs in representing the total atmospheric moisture flux convergence (TMFC), moisture transport, and African Easterly Jets (AEJs). Results indicate that simulations with highest AVs in the precipitation climatology also show improvements in the representation of the TMFC and AEJs. Degraded precipitation due to the downscaling is associated with deterioration of at least two of three analyzed mechanisms, and sometimes there is inconsistent AVs between precipitation and related drivers. This sustains that a realistic representation of the moisture transport and atmospheric circulation is of great importance for the correct simulation of present (and, consequently, future) precipitation over CA.JRC.E.1-Disaster Risk Managemen

Projected Impact of Increased Global Warming on Heat Stress and Exposed Population Over Africa

This study investigates the impact of increased global warming on heat stress changes and the potential number of people exposed to heat risks over Africa. For this purpose a heat index has been computed based on an ensemble‐mean of high‐resolution regional climate model simulations from the Coordinated Output for Regional Evaluations embedded in the COordinated Regional Climate Downscaling EXperiment, under two Representative Concentration Pathways (RCPs) scenarios (RCP2.6 and RCP8.5), combined with projections of population growth developed based on the Shared Socioeconomic Pathways (SSPs) scenarios (SSP1 and SSP5). Results show that by the late 21st century, the increased global warming is expected to induce a 12‐fold increase in the area extent affected by heat stress of high‐risk level. This would result in an increase of about 10%–30% in the number of days with high‐risk heat conditions, as well as about 6%–20% in their magnitude throughout the seasonal cycle over West, Central, and North‐East Africa. Therefore, and because of the lack of adaptation and mitigation policies, the exacerbation of ambient heat conditions could contribute to the exposure of about 2–8.5 million person‐events to heat stress of high‐risk level over Burkina Faso, Ghana, Niger, and Nigeria. Furthermore, it was found that the interaction effect between the climate change and population growth seems to be the most dominant in explaining the total changes in exposure due to moderate and high heat‐related risks over all subregions of the African continent.Plain Language Summary: This study investigates the impact of increased global warming on heat stress changes and the potential number of persons likely to be exposed to heat risks over Africa. Results show that by the end of the 21st century, the increased global warming is expected to induce a 12‐fold increase in the total area affected by dangerous heat conditions over the continent. This would result in an increase of about 10%–30% in the number of days with these heat conditions, as well as about 6%–20% in their magnitude throughout the seasonal cycle over West, Central and North‐East Africa. Therefore, because of the lack of adaptation and mitigation policies, the exacerbation of ambient heat conditions could contribute to the exposure of about 2–8.5 million person‐events to heat stress of high‐risk level over Burkina Faso, Ghana, Niger, and Nigeria. Since these heat events would be partly driven by interactions effects between climate change and population growth, efficient measures allowing not only to mitigate the increased greenhouse gas emissions, but also the effects of high heat on the human body must be urgently implemented on the affected countries' scale, in order to significantly decrease the vulnerability of their populations to potential heat‐related health problems.Key Points: Increased global warming induces more spatially and temporally widespread extreme heat events over West, Central and North‐East Africa. Populations of some West African countries are projected to be particularly exposed to moderate and high heat conditions. Change in population exposure to dangerous heat categories is mainly driven by the interaction effect between climate and population growth.Deutscher Akademischer Austauschdienst http://dx.doi.org/10.13039/501100001655Projekt DEALhttps://esg-dn1.nsc.liu.se/search/cordex/https://esgf-data.dkrz.de/projects/esgf-dkrz/https://www.isimip.org/gettingstarted/details/31https://sedac.ciesin.columbia.edu/data/set/popdynamics-1-8th-pop-base-year-projection-ssp-2000-2100-rev01/data-downloadhttps://cds.climate.copernicus.eu/cdsapp#!/dataset/reanalysis-era5-single-levels?tab=for

The seasonal cycle of cloud radiative effects over Congo Basin based on CERES observation and comparison to CMIP6 models

This study investigates the seasonal variability of the cloud radiative effects (CREs) over Congo Basin (CB) using 15-year observations from Clouds and the Earth's Radiant Energy System (CERES) Energy Budget and Filled (EBAF) Ed4.1 level 3b dataset involving CERES and Moderate Resolution Imaging Spectroradiometer (MODIS) instruments on board Terra and Aqua satellites. The relationships between CREs and cloud properties such as total cloud fraction (TCF), cloud top height (CTH), cloud top temperature (CTT) and cloud optical thickness (COT) are checked. An evaluation of Coupled Model Intercomparison Project (CMIP) Phase 6 in capturing the seasonal cycle of CREs as well as the magnitudes of the CREs along the seasonal cycle is also performed. This study shows a net cloud cooling effect of −8.4 W/m2 and − 43.9 W/m2 respectively at the top of the atmosphere (TOA) and at the surface, leading to a net warming effect of 35.67 W/m2 in the atmosphere. This value implies a large energy source over the Central Africa (CA) atmospheric column. The associated relationships between CREs and cloud properties show that the shortwave CRE is more sensitive to TCF and optical thickness whereas its longwave counterparts is more sensitive to CTH, CTT and COT at the TOA and in the atmosphere. All of the four CMIP6 models used in this study can capture the spatial pattern of CREs as well as their seasonal cycle but misrepresent intensity of CREs. Results also show that a better-simulated TCF considerably reduces the intensity of the annual mean underestimation in both longwave and shortwave CRE for some CMIP6 models, but not for models with overestimated shortwave CRE

The ERA5's diurnal cycle of low-level clouds over Western Central Africa during June–September: Dynamic and thermodynamic processes

International audienceThis paper analyzes the diurnal cycle of low-level cloud cover (LLC) and its atmospheric drivers over Western Central Africa (WCA) during the cloudiest season (June-September). Moderate Resolution Imaging Spectroradiometer (MODIS) observations, Extended Edited Clouds Reports Archive (EECRA) and the fifth generation of reanalysis of the European Centre for Medium Range Weather Forecasts (ERA5) are used. LLC peaks between 04 LT and 07 LT and tends to be less extensive during the afternoon. Strong low-level (below 1000 m) southwesterly flow in the evening supplies the region with humidity from the ocean and leads to cloud formation. Relative humidity (RH) tendencies show that temperature contributes to 100% of RH changes during the night: the strong cooling observed after sunset increases RH in WCA by about 8%/h in the layer below 1000 m. Strong cooling rates of about − 1.4 K/h occur indeed from sunset until 22 LT, then decrease then after to reach a value of about − 0.3 K/h at 07 LT. Cloud formation is mostly related to horizontal moisture flux, strong convergence in the lower layer and turbulent upward mixing of moisture. The reinforcement of the radiative cooling at the cloud top helps to maintain the cloud deck once it has formed. During daytime, latent and sensible heat flux from the surface, due to downward shortwave radiation, contributes to increase the vertical turbulence and deepen the boundary layer and thus, combined with decreasing RH in low levels ~1000 m mostly due to the diurnal warming, act to partly destroy the cloud deck and decrease the cloud fraction

Projected Impact of Increased Global Warming on Heat Stress and Exposed Population Over Africa

Abstract This study investigates the impact of increased global warming on heat stress changes and the potential number of people exposed to heat risks over Africa. For this purpose a heat index has been computed based on an ensemble‐mean of high‐resolution regional climate model simulations from the Coordinated Output for Regional Evaluations embedded in the COordinated Regional Climate Downscaling EXperiment, under two Representative Concentration Pathways (RCPs) scenarios (RCP2.6 and RCP8.5), combined with projections of population growth developed based on the Shared Socioeconomic Pathways (SSPs) scenarios (SSP1 and SSP5). Results show that by the late 21st century, the increased global warming is expected to induce a 12‐fold increase in the area extent affected by heat stress of high‐risk level. This would result in an increase of about 10%–30% in the number of days with high‐risk heat conditions, as well as about 6%–20% in their magnitude throughout the seasonal cycle over West, Central, and North‐East Africa. Therefore, and because of the lack of adaptation and mitigation policies, the exacerbation of ambient heat conditions could contribute to the exposure of about 2–8.5 million person‐events to heat stress of high‐risk level over Burkina Faso, Ghana, Niger, and Nigeria. Furthermore, it was found that the interaction effect between the climate change and population growth seems to be the most dominant in explaining the total changes in exposure due to moderate and high heat‐related risks over all subregions of the African continent