This study presents the meso-β/γ scale dynamical features involved in an extreme African dust outbreak, which occurred during 20-21 February 2016 over the Iberian Peninsula (IP), the southwest corner of Europe. During this episode, nearly 90% of the air quality stations in Spain exceeded the European Union’s PM10 daily limit. We used observations and performed nested-grid 2km simulations with the Weather Research and Forecasting model using coupled Chemistry (WRF-Chem) to understand the development of this dust outbreak. The surface observations and the false-color RGB dust product from the Spinning Enhanced Visible and Infrared Imager (SEVIRI) revealed that the dust storm was initiated on the southeastern flank of the Saharan Atlas Mountains at two distinct phases of dust emissions. The first dust plume crossed the Saharan Atlas during midday on the 20th, the second one followed in the afternoon of the 21st. The first dust plume was advected towards the Western IP, while the second one towards the Eastern IP. The WRF-Chem simulation results indicated that the phase I dust emission was associated with strong barrier jet (BJ) formation on the southeastern foothills of the Saharan Atlas Mountains. The BJ strengthened just after sunrise on the 20th and emitted a massive amount of dust resulting in the first strong dust storm. In phase II, a long-lived westward propagating mesoscale gravity wave (MGW) was triggered near the northeastern edge of the Tinrhert Plateau in eastern Algeria. When this westward propagating long-lived MGW crossed the Tademaït Plateau, multiple hydraulic jumps were formed on its lee side. The strong winds accompanying these multiple hydraulic jumps emitted and mixed dust aerosols upwards which enabled the second strong dust plume to reach the IP. The lifted dust extended over 2-3 km in altitude in the growing daytime planetary boundary layer (PBL) and was advected poleward by the southerly/southeasterly wind at 700hPa. Our results underline the importance of resolving meso-scale processes to understand dust storm dynamics in detail, which are difficult to represent in coarse-resolution (aerosol-) climate models.

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.

On 13 November 2017, a strong continental-scale Saharan dust outbreak was observed in satellite imagery over Mindelo, Cape Verde, located about 650 km off the coast of Senegal in West Africa. Horizontal visibility was reduced to 1100 m leading to major disruptions of the local air traffic. Dust mobilization was already observed over the foothills of the Saharan Atlas Mountains at 0600 UTC on 10 November 2019 but did not appear clearly in SEVIRI pink dust images in the subsequent days. In this study, we examined the multi-scale dynamical processes associated with this particular dust storm using ECMWF ERA-Interim reanalysis data, newly performed very-high resolution WRF-CHEM simulations with horizontal grids of 18 km and less, NAAPS aerosol forecast output, ship-based observation dataset from the North Atlantic Expedition MSM 68/2, as well as surface observations, and upper-air soundings from weather stations in North Africa. Our analyses of this storm highlights the following meteorological processes: (1) the event was associated with a typical Harmattan surge, i.e., the post-frontal strengthening of the northerly winds behind an eastward moving cyclone, (2) a series of earlier Rossby Wave Breaking events (RWBs) made the environment favorable for the Harmattan surge, (3) the dust storm was composed of two distinct dust surges, (4) the dust aerosol from the first surge was later mixed with the dust from the second surge while simultaneously propagating south-westward and later westward, (5) the PBL became adiabatic along the leading edge of the leftover cold front between the southern/southeastern flank of the Atlas Mountains and western/northwestern flank of the Hoggar Mountains, and (6) vertical dust mixing then occurred due to very strong surface heating associated with the development of a deep daytime PBL in the region behind the cold front. The results of this research demonstrate that very-high spatial resolution WRF-CHEM model can resolve the dynamical processes and realistically simulate large-scale North African dust storm.