Abstract
Despite the importance of dust aerosol in the Earth system, state-of-the-art models show a large variety for North African dust emission. This study presents a systematic evaluation of dust emitting-winds in 30 years of the historical model simulation with the UK Met Office Earth-system model HadGEM2-ES for the Coupled Model Intercomparison Project Phase 5. Isolating the effect of winds on dust emission and using an automated detection for nocturnal low-level jets (NLLJs) allow an in-depth evaluation of the model performance for dust emission from a meteorological perspective. The findings highlight that NLLJs are a key driver for dust emission in HadGEM2-ES in terms of occurrence frequency and strength. The annually and spatially averaged occurrence frequency of NLLJs is similar in HadGEM2-ES and ERA-Interim from the European Centre for Medium-Range Weather Forecasts. Compared to ERA-Interim, a stronger pressure ridge over northern Africa in winter and the southward displaced heat low in summer result in differences in location and strength of NLLJs. Particularly the larger geostrophic winds associated with the stronger ridge have a strengthening effect on NLLJs over parts of West Africa in winter. Stronger NLLJs in summer may rather result from an artificially increased mixing coefficient under stable stratification that is weaker in HadGEM2-ES. NLLJs in the Bodélé Depression are affected by stronger synoptic-scale pressure gradients in HadGEM2-ES. Wintertime geostrophic winds can even be so strong that the associated vertical wind shear prevents the formation of NLLJs. These results call for further model improvements in the synoptic-scale dynamics and the physical parametrization of the nocturnal stable boundary layer to better represent dust-emitting processes in the atmospheric model. The new approach could be used for identifying systematic behavior in other models with respect to meteorological processes for dust emission. This would help to improve dust emission simulations and contribute to decreasing the currently large uncertainty in climate change projections with respect to dust aerosol.
Highlights
IntroductionDust aerosol changes the radiation budget directly through scattering and absorption as well as indirectly through altering cloud characteristics and the atmospheric circulation (e.g. Sokolik and Toon 1996; Rosenfeld et al 2001; Lohmann and Feichter 2005; Tompkins et al 2005; Karydis et al 2011; Schmechtig et al 2011)
Mineral dust aerosol from deserts is an important element of the Earth system
The key findings from HadGEM2-ES in the CMIP5 model setup for 1980–2009 are: 1. The order of magnitude in the annual dust emission from HadGEM2-ES is closer to the offline dust emission calculation T-H2ES-O than T-H2ES-O to T-EIO which suggests that dust-emitting winds have a stronger impact on the emission amount than the emission parametrization
Summary
Dust aerosol changes the radiation budget directly through scattering and absorption as well as indirectly through altering cloud characteristics and the atmospheric circulation (e.g. Sokolik and Toon 1996; Rosenfeld et al 2001; Lohmann and Feichter 2005; Tompkins et al 2005; Karydis et al 2011; Schmechtig et al 2011). In addition to these climate effects, suspended dust aerosol decreases the air quality with adverse influences on human health (e.g. Longueville et al 2010; Griffin 2007). Despite the impacts of North African dust aerosol, estimates of both the emitted dust amount (Ginoux et al 2001; Tegen et al 2002; Cakmur et al 2004; Zender et al 2004; Tanaka and Chiba 2006; Schepanski et al 2009; Huneeus et al 2011; Shao et al 2011) and its climatic effects remain amongst the largest uncertainties in the present understanding of the Earth system (e.g. Boucher et al 2013; Mulcahy et al 2014)
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