Characterization of Atmospheric Conditions During the Saharan Air Layer Intrusion Events Over Nigeria

The Saharan Air Layer (SAL) is a warm, dry, and dust-laden mass of air that periodically intrudes into the lower to mid-troposphere over West Africa, suppressing rainfall and reducing surface solar radiation. This study characterizes the atmospheric conditions associated with SAL intrusion events over Nigeria using satellite and reanalysis data for the period 2019 to 2023. MODIS Aerosol Optical Depth composites, ERA5 vertical potential temperature profiles, and specific humidity cross-sections were analyzed to identify and document SAL signatures. In-situ radiosonde ascent data from NiMet stations in Kano and Lagos provided vertical profile validation at two contrasting sites. Results confirm that SAL events are most frequent and intense between June and August, when the SAL interacts with the Atlantic monsoon layer, creating the characteristic mid-tropospheric dry warm layer at 850 to 700 hPa. Quantitative analysis of precipitation records from NiMet stations confirms a statistically significant 22 percent reduction in weekly rainfall totals during confirmed SAL weeks compared to non-SAL weeks. The study also documents substantial solar radiation attenuation under the SAL layer, with surface shortwave radiation reductions of up to 35 percent at IITA Ibadan. Findings contribute to understanding of intraseasonal rainfall suppression mechanisms and have implications for operational rainfall forecasting in Nigeria during the summer monsoon season. Keywords: Saharan Air Layer, aerosol, Nigeria, rainfall suppression, West African monsoon.