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dc.contributor.authorHeinold, Bernd
dc.contributor.authorKnippertz, Peter
dc.contributor.authorBeare, Robert J.
dc.date.accessioned2016-02-11T15:29:23Z
dc.date.issued2015-08-19
dc.description.abstractNocturnal low-level jets (LLJs) are maxima in the wind profile, which often form above the stable nocturnal boundary layer. Over the Sahara, the world's largest source of mineral dust, this phenomenon is of particular importance to the emission and transport of desert aerosol. We present the first ever detailed large-eddy simulations of dust-generating LLJs. Using sensitivity studies with the UK Met Office large-eddy model (LEM), two key controls of the nocturnal LLJ are investigated: surface roughness and the Coriolis force. Functional relationships derived from the LEM results help to identify optimal latitude-roughness configurations for a maximum LLJ enhancement. Ideal conditions are found in regions between 20 and 27°N with roughness lengths >0.0001 m providing long oscillation periods and large jet amplitudes. Typical LLJ enhancements reach up to 3.5 m s<sup>-1</sup> for geostrophic winds of 10 m s<sup>-1</sup>. The findings are largely consistent with results from a theoretical LLJ model applied for comparison. The results demonstrate the importance of latitude and roughness in creating regional patterns of LLJ influence. Combining the functional relationships with high-resolution roughness data over northern Africa gives good agreement with the location of morning dust uplift in satellite observations. It is shown that shear-induced mixing plays an important role for the LLJ evolution and surface gustiness. With decreasing latitude the LLJ oscillation period is longer and, thus, shear-induced mixing is weaker, allowing a more stable nocturnal stratification to develop. This causes a later and more abrupt LLJ breakdown in the morning with stronger gusts, which can compensate for the slower LLJ evolution that leads to a weaker jet maximum. The findings presented here can serve as the first step towards a parametrization to improve the representation of the effects of nocturnal LLJs on dust emission in coarser-resolution models.en_GB
dc.description.sponsorshipEuropean Research Councilen_GB
dc.identifier.citationVol. 141 (690), pp. 1740 - 1752en_GB
dc.identifier.doi10.1002/qj.2475
dc.identifier.grantnumber257543en_GB
dc.identifier.urihttp://hdl.handle.net/10871/19736
dc.language.isoenen_GB
dc.publisherWiley / Royal Meteorological Societyen_GB
dc.rights© 2014 The Authors. Quarterly Journal of the Royal Meteorological Society published by John Wiley & Sons Ltd on behalf of the Royal Meteorological Society. This is an open access article under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs License (http://creativecommons.org/licenses/by-nc-nd/4.0/), which permits use and distribution in any medium, provided the original work is properly cited, the use is non-commercial and no modifications or adaptations are made.en_GB
dc.subjectnocturnal low-level jeten_GB
dc.subjectboundary-layer transitionen_GB
dc.subjectlarge-eddy simulationen_GB
dc.subjectdust emissionen_GB
dc.subjectSaharaen_GB
dc.subjectgustsen_GB
dc.subjectshear-induced mixingen_GB
dc.titleIdealized large-eddy simulations of nocturnal low-level jets over subtropical desert regions and implications for dust-generating windsen_GB
dc.typeArticleen_GB
dc.date.available2016-02-11T15:29:23Z
dc.identifier.issn0035-9009
dc.identifier.eissn1477-870X
dc.identifier.journalQuarterly Journal of the Royal Meteorological Societyen_GB


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