Fiedler, Stephanie , Schepanski, K., Heinold, B., Knippertz, P. and Tegen, I. (2013) Climatology of nocturnal low-level jets over North Africa and implications for modeling mineral dust emission. Journal of Geophysical Research: Atmospheres, 118 (12). pp. 6100-6121. DOI 10.1002/jgrd.50394.

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Abstract

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.

Key Points:
- Long-term climatology of Nocturnal Low-Level Jets (NLLJ) over North Africa
- Newly developed automated detection algorithm for NLLJs
- NLLJs contribute 15 % to North African dust emission in annual and spatial mean

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