The low-level jet dust emission mechanism in the central Sahara: Observations from Bordj-Badji Mokhtar during the June 2011 Fennec Intensive Observation Period

Abstract

This paper presents the first detailed analysis of low-level jets (LLJs) in the central Sahara from ground-based observations at Bordj-Badji Mokhtar, Algeria, and addresses their operation as a dust emission mechanism. On LLJ mornings, composite wind speeds in the core (300 m aboveground level) reach 13.5 m s À1 at 0400. Surface temperatures increase from 0545 (30 min after sunrise), and jet decay begins around 0600. Ten meter winds lag those in the core by 5 h; peak 10 m wind speed, 7.5 m s À1 , occurs at 0900. Only the deepest and strongest LLJs lead to dust emission. At 0600, these five LLJs have core wind speeds ≥16 m s À1 , below-core wind shear ≥ 0.6 m s À1 /30 m, and wind shear between the core and 500 m above the core ≤À1.8 m s À1 . On these occasions, momentum mixes down from the LLJ after surface heating, leading to emission. On nondusty LLJ mornings, the convective boundary layer is 100 m shallower, and the LLJ is too weak to provide enough momentum to be mixed down for emission. LLJs are most frequently embedded in the monsoon flow or in the Harmattan; there is a clear association with the Saharan Heat Low. ERA-Interim reanalysis underestimates both Harmattan and monsoon LLJ core winds (by 4 m s À1 and 6 m s À1 , respectively). The Met Office Africa Limited Area Model underestimates Harmattan LLJ core winds by only 0.2 m s À1 . Monsoon LLJ core winds, however, are underestimated by 8.5 m s À1 . Surface winds at 0900 are underestimated in both cases by up to 6 m s À1 .

Dust detection from ground‐based observations in the summer global dust maximum: Results from Fennec 2011 and 2012 and implications for modeling and field observations

“…Allen and Washington [] show that the presence of a LLJ is not a guarantee of dust emission since momentum does not always mix down to the surface. Indeed, in dusty regions it is possible that daytime turbulent mixing in the boundary layer actually decreases, because the dust reduces the sunlight reaching the surface [ Miller et al ., ; Pérez et al ., ].…”

“…As part of Fennec, eight automatic weather stations were deployed in the central Sahara [ Hobby et al ., ] and two supersites were established, in Bordj‐Badji Mokhtar, Algeria [ Marsham et al ., ], and Zouerat, Mauritania [ Todd et al ., ]. Observations from these supersites have provided new insights into Saharan dust, including among others the links between dust, cloud, and the surface energy balance [ Marsham et al ., ]; the relative importance of dust production mechanisms [ Allen et al ., ; Marsham et al ., ]; the operation of the low‐level jet dust emission mechanism [ Allen and Washington , ]; and the role of the Atlantic Inflow on the diurnal cycle of the boundary layer [ Todd et al ., ].…”

Taklimakan Desert nocturnal low-level jet: climatology and dust activity

“…Recent studies (Allen and Washington, 2014;Heinold et al, 2015;Knippertz, 2008;Schepanski et al, 2009) have indicated that the breakdown of the NLLJ over Africa can induce the downward mixing of momentum during the evolution of the boundary layer in midmorning and cause enhancement of near-surface wind speed. Here, we firstly compared the midmorning surface wind speed distribution coincident with the appearance of NLLJ with that when no NLLJ was detected.…”

“…The strong surface wind will blow dust particles up from the desert surface, and the same turbulent mixing will also loft these dust particles to the upper level of the boundary layer, promoting horizontal transport. Therefore, the NLLJ may play an important role in the both dust emission and transport (Allen and Washington, 2014;Heinold et al, 2015;Knippertz, 2008;Tegen et al, 2013;Todd et al, 2008).…”

“…Much work has been done to address the features of the NLLJ, demonstrating a link between NLLJs and dust suspension and their contribution to dust emission over northern Africa (Allen and Washington, 2014;Fiedler et al, 2013;Schepanski et al, 2009;Washington and Todd, 2005). Since the TD has a hyper-arid environment and relatively flat terrain, strong radiative cooling during the night in this region can stabilize the near-surface layer and at least partly decouple the air from the surface layer friction, which will provide a favorable condition for NLLJ formation.…”