Atmospheric Controls on Mineral Dust Emission From the Etosha Pan, Namibia: Observations From the CLARIFY‐2016 Field Campaign

Abstract

Relatively little attention has been given to dust emission from southern Africa, despite the identification of major source areas across the arid southwest of the subcontinent (Prospero et al., 2002;Vickery

Quantifying Mechanisms of Aeolian Dust Emission: Field Measurements at Etosha Pan, Namibia

“…In cases where dust plumes tracked over the lidar during the monitoring campaign, data provide evidence supporting the operation of a LLJ‐driven dust emission mechanism during dry winter months, supporting the findings of Clements and Washington (2021). Figure 8 presents data on horizontal wind speed and aerosol backscatter from the lidar together with SEVIRI satellite data over a period of 3 days (8th, 9th, and 10th July 2015) for a series of medium intensity dust events.…”

“…Data from the same field campaign also noted the existence of a frequent night-time temperature inversion and the associated development of a low-level jet (LLJ; Zunckel, Hong, et al, 1996). The existence of a nocturnal LLJ at Etosha Pan was recently confirmed by Clements and Washington (2021) using Doppler lidar. In this latter study the measured LLJ displayed characteristics of a strong, easterly wind (core windspeeds of around 12 m/s) strengthening to a maximum between 0600 and 0800 local time at a height of between 150 and 300 m. After sunrise, Clements and Washington (2021) noted that the breakdown of the LLJ resulted in strong surface winds between 0900 and 1100 local time which were associated with dust emission events observed during the months of August and September.…”

“…The development of near‐surface low‐level jets associated with dust emissions has been investigated across the Sahara (Allen et al., 2013; Bergametti et al., 2018; Fiedler et al., 2013; Kaly et al., 2015; Marsham et al., 2013; Schepanski et al., 2009), in the Bodélé depression, Chad (Todd et al., 2007; Washington et al., 2006; Washington & Todd, 2005), and in the Taklimakan, China (Ge et al., 2016). There are also observations of the frequent occurrence of LLJs with an easterly component over Etosha Pan during winter months (Clements & Washington, 2021; Zunckel, Held, et al., 1996) when strong radiative cooling at night promotes the decoupling of air aloft from the surface and nocturnal stratification of the atmosphere (Allen & Washington, 2014; Blackader, 1957). Inertial oscillation leads to a nocturnal acceleration of the wind which is then mixed down to the surface following strong surface heating.…”

“…On each of these days a jet of fast‐moving wind in excess of 9.0 m/s was established around 300–500 m above the ground in the early morning between 0600 and 0900 hr (Figure 8a). After sunrise (at 0630 hr), the onset of radiative heating is seen to drive vertical mixing of the wind profile (Allen & Washington, 2014; Clements & Washington, 2021), conveying the fast‐moving winds at altitude toward the surface as the elevated LLJ breaks down and becomes absorbed into the synoptically‐forced gradient wind (Zunckel, Held, et al., 1996). Between 0900 and 1000 hr these high‐velocity horizontal winds are seen to have reached the pan surface resulting in the erosion of surface sediments, driving peaks in aerosol backscatter as measured by the lidar 1–2 hr later (Figure 8b).…”

“…In both regions, the development and subsequent breakdown of LLJs are recognised as a principal mechanism for dust raising. It is notable that Clements and Washington (2021) identify the morning development of LLJs on >90% of days during their study at Etosha Pan, with strong LLJ structures associated with all six of their detected dust emission events.…”