Atmospheric Dynamics of a Saharan Dust Outbreak Over Mindelo, Cape Verde Islands, Preceded by Rossby Wave Breaking: Multiscale Observational Analyses and Simulations

Abstract

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 multiscale observations and the Weather Research and Forecasting model Coupled with Chemistry simulations. The analyses indicate that the dust storm was initiated on the lee side of the Saharan Atlas Mountains (SAM) in Algeria on 10 November 2017. This dust storm was associated with a double Rossby wave break linked through nonlinear wave reflection. Two successive Rossby wave breaks 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-like cold front which formed due to the downslope transport of cold airflow over the SAM and then triggered undular bores on the lee side. Each bore perturbed the dust loading and then the subsequent diurnal heating generated differential planetary boundary layer turbulence kinetic energy strengthening the dust frontogenesis. Dust became confined behind the cold surge and interacted with the daytime Saharan planetary boundary layer 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.

Measurements of a Dusty Density Current in the Western Sonoran Desert

“…Next, we identified cold pools as a mechanism for producing high wind speeds and dust emission and transport in the region. Cold pools generated by orographic precipitation (e.g., Gläser et al., 2012; Knippertz et al., 2007) or frontal passage (Dhital et al., 2020) have been shown to generate strong winds and dust storms in Northern Africa, and thus this work shows that those same processes are also important in the small and closed Salton Basin. We also corroborated several results from numerical modeling studies of dust within a density current (Huang et al., 2018; Solomos et al., 2012), including that dust mixing outside the density current primarily takes place within the current head, that the peak in dust emission occurs after head passage, and that variations in height in the dust‐pristine sky interface are due to K‐H instability and billows.…”

“…However, a series of images from the RoundShot camera looking west from camera's location (Figure 2) do not show obvious regions of widespread virga (Figure S1 in Supporting Information S1), which could be due to the 10-min resolution of the imagery. Given the available evidence, we cannot conclusively rule out the possibility that the density current feature described in the subsequent section was generated in association with cold post-frontal downslope flow (Dhital et al, 2020;Karyampudi et al, 1995).…”

“…Dust aerosols originate from arid and semi‐arid regions characterized by extremely low rainfall and substantially accumulated alluvial sediments, are advected around the globe, and alter the climate system via direct and indirect effects (Choobari et al., 2014). It is well known that dust emission primarily occurs from discrete point sources (Schepanski et al., 2009), and oftentimes the high wind speeds required for dust emission are associated with orographically forced flows (e.g., Dhital et al., 2020; Evan et al., 2016; Jiang et al., 2011; Kalenderski & Stenchikov, 2016; Washington et al., 2006), or convective downdrafts, which are oftentimes referred to as haboobs (e.g., Chen & Fryrear, 2002; Eagar et al., 2017; Miller et al., 2008), and which are common dust producing mechanisms globally (Knippertz, 2014), potentially responsible for 20% of all dust emission events in North Africa (Pantillon et al., 2016). Spanning these two categories of high wind speed events are convectively driven downdrafts that are forced by lee‐side–or spillover–orographic precipitation (Gläser et al., 2012; Knippertz et al., 2007; Reinfried et al., 2009), although such processes are broadly considered to be in the category of density currents.…”

Stratospheric impacts on dust transport and air pollution in West Africa and the Eastern Mediterranean

“…Case studies of North African dust storms with strong impact over southern Europe found a common large-scale upper-level precursor (a double Rossby wave breaking process) developing 5–10 days prior to dust storm formation. Using this precursor signal as an indicator might provide early warnings of high-impact North African dust outbreaks over southern Europe at lead times of 5–10 days 20 , 21 . In general, the predictive lead time is limited to several days.…”

Climatology of Rossby Wave Breaking Over the Subtropical Indian Region

“…Since RWB modifies dynamic conditions of the atmosphere, it gives rise to several weather events including not only the EREs (Antokhina et al, 2021;Martius and Rivière, 2016;Moore et al, 2019;Vellore et al, 2016;Vries., 2021), but also cold/heat waves (Ali et al, 2022;Song & Wu, 2021), anomalous changes in tropospheric ozone concentrations (Isotta et al, 2008;Zhang et al, 2022), and dust transport (Dhital et al, 2020;Nielsen-Gammon, 2001;Orza et al, 2020). Recently, Moore et al (2019) documented a strong correlation between the ERE and RWB in the United States which was found to be associated with strong warm air advection, tropical moisture export, and a warm conveyor belt supported by baroclinically forced ascent.…”