Theoretical Examination of Rainfall Intensification Mechanisms Under Global Warming in the Humid Tropics of Southern Nigeria

The thermodynamic intensification of extreme precipitation under global warming, governed by the Clausius-Clapeyron relation, predicts increasing moisture availability and extreme rainfall intensity as temperatures rise, but the locally realized rate of intensification in Nigeria's humid tropics deviates significantly from theoretical expectations and demands mechanistic investigation. This study theoretically and empirically examines rainfall intensification mechanisms over southern Nigeria's humid tropical zone under observed warming using a combination of long-term instrumental records, convection-permitting model simulations, and atmospheric thermodynamic diagnostics. Daily rainfall extremes from 22 NiMet stations in the South West, South South, and South East zones for the period 1980 to 2022 are analyzed alongside ERA5 thermodynamic variables. Convection-permitting WRF simulations at 2-kilometre resolution under pseudo-global warming perturbations quantify the dynamical and thermodynamical contributions to changing precipitation extremes. Results demonstrate that sub-daily extreme rainfall has intensified at rates of 7.1 to 9.3 percent per degree Celsius of warming in coastal South West and South South states, exceeding the Clausius-Clapeyron rate of approximately 7 percent per degree and suggesting positive dynamical amplification. In contrast, inland South East stations show intensification rates of only 4.2 to 5.8 percent per degree, reflecting competing dynamical suppression from increased lower tropospheric stability. An original scaling framework distinguishing thermodynamic and dynamic intensification contributions specific to Nigerian humid tropical conditions is developed. Keywords: rainfall intensification, Clausius-Clapeyron, humid tropics, extreme precipitation, convection-permitting model.