Effects of Land-Use Change and Aerosol Reguiative Properties on the West African Climate System

Abstract

West Africa due to its climatological and geographical conditions is an essential domain for the characterization of atmospheric radiative forcing across the globe. This study investigated the interaction between aerosols and solar radiation over West African climatic zones (Sahara, Sahel, Savannah, and the coast of Guinea) between December 2005 and January 2006. The study also analyzed the dynamics of various aerosol sources and types and their contribution to the forcing effect observed, reporting variations in high load of anthropogenic and dust aerosols in the climatic zones. The study also identifies the distinguishing characteristics of the aerosol radiative property parameters, including the fact that high AOD causes high atmospheric absorption and low surface net radiation fluxes, when the Angstrom exponent is low, a greater amount of radiation is absorbed at the top of the atmosphere, and the radiative properties vary with changing aerosol concentration and distribution in the atmosphere. Additionally, an increase in anthropogenic aerosols leads to an increase in SSA. The study included the regional climate model RegCM 4.7.1 to conduct a sensitivity study, identifying the optimal cumulus convective precipitation scheme, planetary boundary layer (PBL), and land-surface scheme. The resulting simulation demonstrated a strong correlation with observed precipitation patterns and well-simulated temperatures. This study also aimed to explore how radiative mechanisms linked to seasonal dynamics in natural vegetation and abrupt land cover changes caused by human activities can influence the West African climate system. In taller vegetation, such as forests and savannas, albedo increased during the growing period, leading to a decline in net shortwave radiation, while in shorter vegetation, such as grasslands and shrublands, albedo decreased, resulting in an increase in net shortwave radiation. However, the cooling and warming impacts of albedo during the growing period in taller and shorter vegetation, respectively, were significantly outweighed by ET cooling, which had a greater impact on LST seasonality in West Africa. Land cover changes caused an average increase in albedo, with the highest surge observed in the dry period. The albedo changes resulting from all land cover changes combined generated a regional instantaneous shortwave surface radiative forcing of -0.03W/m2. Despite this small regional forcing, the study showed that land cover changes from forest to cropland and savanna to grassland conversions had a more significant contribution to LST cooling than other changes in class. However, these conversion classes displayed the largest observed LST warming of up to 1.2oK and 0.4oK, respectively.