Abstract
Abstract. Global and regional models have large systematic errors in their modelled dust fields over West Africa. It is well established that cold-pool outflows from moist convection (haboobs) can raise over 50 % of the dust over parts of the Sahara and Sahel in summer, but parameterised moist convection tends to give a very poor representation of this in models. Here, we test the hypothesis that an explicit representation of convection in the Met Office Unified Model (UM) improves haboob winds and so may reduce errors in modelled dust fields. The results show that despite varying both grid spacing and the representation of convection there are only minor changes in dust aerosol optical depth (AOD) and dust mass loading fields between simulations. In all simulations there is an AOD deficit over the observed central Saharan dust maximum and a high bias in AOD along the west coast: both features are consistent with many climate (CMIP5) models. Cold-pool outflows are present in the explicit simulations and do raise dust. Consistent with this, there is an improved diurnal cycle in dust-generating winds with a seasonal peak in evening winds at locations with moist convection that is absent in simulations with parameterised convection. However, the explicit convection does not change the AOD field in the UM significantly for several reasons. Firstly, the increased windiness in the evening from haboobs is approximately balanced by a reduction in morning winds associated with the breakdown of the nocturnal low-level jet (LLJ). Secondly, although explicit convection increases the frequency of the strongest winds, they are still weaker than observed, especially close to the observed summertime Saharan dust maximum: this results from the fact that, although large mesoscale convective systems (and resultant cold pools) are generated, they have a lower frequency than observed and haboob winds are too weak. Finally, major impacts of the haboobs on winds occur over the Sahel, where, although dust uplift is known to occur in reality, uplift in the simulations is limited by a seasonally constant bare-soil fraction in the model, together with soil moisture and clay fractions which are too restrictive of dust emission in seasonally varying vegetated regions. For future studies, the results demonstrate (1) the improvements in behaviour produced by the explicit representation of convection, (2) the value of simultaneously evaluating both dust and winds and (3) the need to develop parameterisations of the land surface alongside those of dust-generating winds.
Highlights
During the summer season the Sahara is the world’s largest source of mineral dust (Ginoux et al, 2012; Prospero et al, 2002) and representations of dust are known to improve numerical weather prediction (NWP) models (Haywood et al, 2005; Tompkins et al, 2005; Rodwell and Jung, 2008), the accuracy of dust forecasts remains limited (Chaboureau et al, 2016; Huneeus et al, 2016; Terradellas et al, 2016)
In the SWAMMA experiments, extinction per unit mass is greatest for particle size division 2 (0.1–0.3 μm mean radius), dust mass is maximum in division 4 (1–3 μm mean radius), and total extinction for dust is dominated by particles in size division 3 (0.3–1 μm mean radius)
The dust loadings in the SWAMMA experiments (5–6 Tg May to September seasonal mean for the whole domain) are at the low end of, but not outside, the range reported by other modelling studies; Huneeus et al (2011) reviewed 15 global models within the AeroCom project and found global loadings ranged between 7 and 30 Tg, of which ∼ 70 % has been estimated to be attributable to the Sahara (Luo et al, 2003)
Summary
During the summer season the Sahara is the world’s largest source of mineral dust (Ginoux et al, 2012; Prospero et al, 2002) and representations of dust are known to improve numerical weather prediction (NWP) models (Haywood et al, 2005; Tompkins et al, 2005; Rodwell and Jung, 2008), the accuracy of dust forecasts remains limited (Chaboureau et al, 2016; Huneeus et al, 2016; Terradellas et al, 2016). Simulated cold pools are likely to differ from real-world examples in terms of size, duration and wind speed Another approach has been the development and application of a haboob parameterisation, in which additional low-level winds are added that are linked to mass fluxes from the convection scheme (Pantillon et al, 2015, 2016). Chaboureau et al (2016) compared near-surface winds and prognostic dust from in-line simulations with both explicit and parameterised convection They show some success in increasing the occurrence of strong winds in the evening (haboobs) when explicitly representing convection, and in improving the dust AOD biases relative to observations by increasing AOD values in the southern Sahara and northern Sahel.
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