Failed cyclogenesis of a mesoscale convective system near Cabo Verde: The role of the Saharan trade wind layer among other inhibiting factors observed during the CADDIWA field campaign

Abstract

The role of the Saharan Air Layer (SAL) in Cabo Verde cyclogenesis remains uncertain. Here, we investigate the inhibiting factors leading to the failed cyclogenesis of mesoscale convective system (MCS) Pierre Henri observed during two flights of the Clouds-Atmospheric Dynamics-Dust Interactions in West Africa (CADDIWA) campaign. We use CADDIWA data and a convection-permitting simulation run with the Meso-NH model. We show that the African easterly wave in which MCS Pierre Henri is embedded forms a region of cyclonic recirculation that is relatively protected from lateral intrusions, called a marsupial pouch, which keeps the SAL away from the MCS. By contrast, a dusty, dry, and warm air layer between 0.8 and 2 km altitude, called the Saharan trade wind layer (STWL), penetrates into the convective core of MCS Pierre Henri, increasing the convective inhibition (CIN) area and contributing up to 40 % of the CIN area during the MCS' mature phase. The cold pools produced by convection also increase the CIN area and contribute up to 50 % of this area after the MCS' intense phase. Upper-Tropospheric dry air, with relative humidity below 15 % between 7 and 11 km altitude, gradually penetrates into the 150 km circle around the MCS, reaching 18 % of the area during its dissipation phase and preventing the MCS' anvil expansion. The inhibiting roles of the STWL, cold pools, and upper-Tropospheric dry air in leading the cyclogenesis fail to provide new insights into the complex dynamics of cyclogenesis in the Cabo Verde region and challenge the existing model of the SAL.