The Coastal Transition of a West African Mesoscale Convective System

Abstract

Mesoscale convective systems (MCSs) are critical sources of precipitation in tropical West Africa, yet their behavior changes dramatically as they transition from land to the eastern Atlantic. This study investigates the mechanisms governing this coastal transition by analyzing an MCS observed on 14 September 2022, using aircraft data from NASA’s Convective Processes Experiment–Cabo Verde (CPEX-CV) field campaign and convection-permitting model simulations. The observed MCS featured two distinct squall lines with divergent fates: a southern line that exited the coast at night and persisted, and a northern line that exited during the day and quickly decayed. Our analysis reveals that shear-cold pool interactions influenced this evolution. While the coastal region provided an environment of high mid-tropospheric moisture and strong vertical wind shear, the open ocean was characterized by drier air and weaker shear. Consequently, as the squall lines moved offshore, the optimal balance required for longevity was lost. The southern line encountered more optimal shear-cold pool conditions than the northern line, partly explaining their diverging evolution after crossing the coast. Numerical sensitivity experiments further highlight the critical role of the diurnal cycle in this process: daytime conditions support optimal shear-cold pool interactions over land, whereas nighttime conditions exhibit persistent convergence zones that can prolong the lifetime of offshore propagating systems over water. This research elucidates the complex interplay between environmental shear, cold pools, and the diurnal cycle in modulating MCS behavior across the West African coast.