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

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 −1 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 −1 , 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 −1 , below‐core wind shear ≥ 0.6 m s −1 /30 m, and wind shear between the core and 500 m above the core ≤−1.8 m s −1 . 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 −1 and 6 m s −1 , respectively). The Met Office Africa Limited Area Model underestimates Harmattan LLJ core winds by only 0.2 m s −1 . Monsoon LLJ core winds, however, are underestimated by 8.5 m s −1 . Surface winds at 0900 are underestimated in both cases by up to 6 m s −1 .