Environmental Moisture Influence on African Mesoscale Convective Systems

Mesoscale convective systems (MCSs) contribute to a large portion of the precipitation in the midlatitudes and the tropics. In Western Africa, rainfall is mainly due to MCSs, which have been found to be modulated by synoptic features such as the West African Monsoon (WAM), the African Easterly Jet (AEJ), and African Easterly Waves (AEWs). The spatial and temporal non-homogeneous moisture distribution and data scarcity over the tropics limits the understanding of these tropical convective systems. For this reason, in this study, three moisture sensitivity experiments are explored using the Model for Prediction Across Scales-Atmosphere (MPAS-A) at high-resolution over the West Africa and Eastern Atlantic region. The initial and lateral boundary relative humidity conditions are taken from ERA5 where pressure level data is altered to add and remove moisture (20% and 50%, respectively). To observe the MCSs that develop in the three experiments (control, moist, and dry), after an initial subjective tracking, an automated tracker, Tracking Algorithm for Mesoscale Convective Systems (TAMS), is approached to objectively identify, track, and classify the simulated MCSs. By evaluating the atmospheric mean states over the region and the MCSs statistics from TAMS, it is found that more moisture does not necessarily lead to a higher number of MCSs; however, those MCSs that develop in the moister environment tend be larger in area and with longer-lasting extreme cases. The MCSs in the moist experiment are observed to be mainly influenced by a WAM that is stronger compared to the control WAM. In this study, the influence of the AEJ and vertical wind shear, alongside rainfall amounts, trajectories, and propagation speeds of selected MCSs across the experiments are being examined as well. The findings can provide an insight on how the atmosphere could transform with added water vapor in a changing climate.