Atmospheric Dynamics of a Saharan Dust Outbreak over Mindelo, Cape Verde Islands: Multi-scale Observational Analyses and Simulations

We investigate the synoptic precursors to the Harmattan wind and dust frontogenesis during the high impact Saharan dust outbreak over the Cape Verde Islands on 13 November 2017. We employ multi-scale observations including ship data and Weather Research and Forecasting model Coupled with Chemistry simulations. The analyses indicate that the dust storm was initiated on the leeside of the Saharan Atlas Mountains (SAM) in Algeria on the 10. This dust storm was associated with a double Rossby Wave Break (RWB) linked through non-linear wave reflection. Two successive RWB contributed to the wave amplification over the Eastern North Atlantic Ocean which transported large magnitude potential vorticity air into the North African continent. The resulting coupled pressure surge was associated with cold air advected equatorward over the SAM which organized the strong near-surface wind that ablated the dust. The simulation results indicate that the dust front was initially related to a density current which formed due to the cold airflow over the SAM. The density current then triggered undular bores on the leeside. Each bore perturbed the dust loading and then the subsequent diurnal heating generated differential planetary boundary layer (PBL) turbulence kinetic energy strengthening the dust frontogenesis. Dust became confined behind the cold surge and interacted with the daytime Saharan PBL leading to increased dust loading while the dust front propagated equatorward. Two distinct dust plumes arrived successively at low-levels at Mindelo, Cape Verde Islands; (1) from the coasts of Mauritania and Senegal and (2) from the SAM southern flank.