Abstract
[1] This study presents the first climatology for the dust emission amount associated with Nocturnal Low-Level Jets (NLLJs) in North Africa. These wind speed maxima near the top of the nocturnal boundary layer can generate near-surface peak winds due to shear-driven turbulence in the course of the night and the NLLJ breakdown during the following morning. The associated increase in the near-surface wind speed is a driver for mineral dust emission. A new detection algorithm for NLLJs is presented and used for a statistical assessment of NLLJs in 32 years of ERA-Interim reanalysis from the European Centre for Medium-Range Weather Forecasts. NLLJs occur in 29% of the nights in the annual and spatial mean. The NLLJ climatology shows a distinct annual cycle with marked regional differences. Maxima of up to 80% NLLJ frequency are found where low-level baroclinicity and orographic channels cause favorable conditions, e.g., over the Bodélé Depression, Chad, for November–February and along the West Saharan and Mauritanian coast for April–September. Downward mixing of NLLJ momentum to the surface causes 15% of mineral dust emission in the annual and spatial mean and can be associated with up to 60% of the total dust amount in specific areas, e.g., the Bodélé Depression and south of the Hoggar-Tibesti Channel. The sharp diurnal cycle underlines the importance of using wind speed information with high temporal resolution as driving fields for dust emission models.Citation: Fiedler, S., K. Schepanski, B. Heinold, P. Knippertz, and I. Tegen (2013), Climatology of nocturnal low-level jets over North Africa and implications for modeling mineral dust emission, J. Geophys. Res. Atmos., 118, 6100-6121, doi:10.1002/jgrd.50394
Highlights
Background onNocturnal Low-Level Jets (NLLJs) [6] Low-level jets (LLJs) are wind speed maxima in the lower troposphere, as defined in the meteorological terminology data base METEOTERM run by the World Meteorological Organization
Based on this research aim and following METEOTERM, NLLJs over North Africa are defined in this study as wind speed maxima in the nocturnal boundary layer (BL) that form above a stably stratified surface layer and have an appreciable vertical wind shear
Since reduced frictional effects at night are expected to enable a nocturnal acceleration of a LLJ, but cases with turbulence beneath the NLLJ need to be included to investigate dust emission associated with NLLJs, the weak stability criterion of 0.001 K m–1 is chosen
Summary
[2] Mineral dust constitutes the largest fraction of atmospheric aerosol by mass and plays an important role in the Earth system. Based on this research aim and following METEOTERM (http:// wmo.multicorpora.net/METEOTERM), NLLJs over North Africa are defined in this study as wind speed maxima in the nocturnal BL that form above a stably stratified surface layer and have an appreciable vertical wind shear This NLLJ definition is a more restrictive form of the relatively general term LLJ. The downward mixing of NLLJ momentum by turbulence increases the near-surface wind speed and potentially leads to mineral dust emission in source areas [Todd et al, 2008; Schepanski et al, 2009; Heinold et al, 2011]. Intermittent turbulence occurs for very stable situations with Richardson numbers larger than 0.3 [Banta et al, 2003]
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