Spatiotemporal Variations in Aerosol Optical Depth and Their Relationships with Cloud Properties and Precipitation over Sudan: Insights from Satellite Observations and CMIP6 Model Projections

This study investigates how dust and sulfate aerosols modulate cloud properties and rainfall over Sudan, a key part of the Sahara–Sahel dust belt. Satellite and reanalysis products (MODIS, CHIRPS, MERRA 2, EAC4) are combined with four CMIP6 models to analyze rainy season (JJAS) aerosol optical depth (AOD), cloud water path (CWP), cloud effective radius (Reff), and precipitation for 2003–2014, and to assess future changes under SSP1 2.6, SSP2 4.5, and SSP5 8.5 during 2041–2100. Reanalysis data show that natural mineral dust dominates aerosol loading over Sudan, accounting for approximately 70–85% of total annual mean AOD, with substantial spatial variability across the domain and the highest contributions occurring over the Sahara–Sahel transition zone, whereas sulfate AOD peaks over urban and agricultural regions in central and eastern Sudan. Observations reveal that dust AOD is negatively correlated with CWP and precipitation in northern and central Sudan, while sulfate AOD shows positive correlations with CWP and rainfall in the southeast. All datasets exhibit negative AOD–Reff relationships that are consistent with a Twomey-like signature. However, because AOD is a column-integrated measure that does not directly represent cloud-based cloud condensation nuclei (CCN), these relationships should not be interpreted as direct evidence of the Twomey effect. The models also overestimate the positive AOD–CWP and AOD–precipitation correlations, suggesting that they may simulate stronger aerosol-related cloud persistence and precipitation responses than indicated by the observations. Multi-model projections indicate substantial twenty first century declines in sulfate and total AOD under all SSPs, driven by emission controls, whereas dust AOD shows weaker, climate- and land-use-controlled changes. Together, these results suggest that CMIP6 likely overestimates the sensitivity of Sudan’s hydrological cycle to aerosol perturbations and highlight the need for improved dust parameterizations and high-resolution regional modeling to constrain future water resource risks.