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University of Oxford (2021)

Characteristics of Saharan dust emission mechanisms in boreal summer : a satellite and modelling approach

Caton Harrison, Thomas

Titre : Characteristics of Saharan dust emission mechanisms in boreal summer : a satellite and modelling approach

Auteur : Caton Harrison, Thomas

Université de soutenance : University of Oxford

Grade : Doctor of Philosophy (PhD) 2021

Mineral dust is one of the most abundant atmospheric aerosols and has a wide range of impacts on climate, meteorology, health and biogeochemistry. North African sources are responsible for about half of the total atmospheric burden, peaking as an emission source in the boreal summer months of June, July and August. In-situ data from the Fennec observation campaign of 2011 and 2012 indicates that cold pool outflows (CPOs) from convective downdrafts are the primary meteorological driver of these summertime emissions, followed by nocturnal low-level jet (LLJ) breakdown, both mesoscale phenomena which numerical models struggle to represent faithfully. Very few surface observations are available to characterise these emission mechanisms, however, and the extent to which Fennec observations are representative remains unclear. Satellite data from the Spinning Enhanced Visible and Infrared Imager (SEVIRI) offers both the volume of data and resolution to observe emission mechanisms on the long term.

In this thesis, two sets of automated algorithms are applied to 14 years of summertime SEVIRI data, the first of which categorises automatically identified and tracked dust by emission mechanism and the second of which tracks and characterises dust-laden cold pool outflows. CPOs account for 82% of the total observed dust and 88% at the point of emission in the central and western Sahara during boreal summer, which is the highest estimate yet of their contribution. Whereas CPO dust is widespread and the majority of dust source regions are primarily CPO-driven, LLJs dominate a small number of hotspots such as the Tidihelt Depression. CPOs are far-reaching, with 22.5% travelling over 300 km. They also follow a clear diurnal cycle which favours emissions in the late afternoon and evening. Unlike dust emission, CPO frequency peaks in August in southern Algeria.

In the final component of the thesis, satellite observations are used to support a Met Office Unified Model experiment diagnosing the role of orography in driving dust-emitting LLJs over central Algeria. Fluvial drainage from mountains is thought to contribute erodible sediment to western Saharan dust sources, but their effect upon emission mechanisms there is untested. Removing the Hoggar mountains reduces LLJ emission frequency by approximately 30% as an elevated heating anomaly helps sustain the strong pressure gradient driving low-level winds across the region in summer.

This thesis offers a meteorological perspective on satellite dust source maps, showing the contribution of erosivity to known summertime emission hotspots on climatic timescales. The fact that most central and western Saharan dust sources are predominantly activated by CPOs poses a challenge for numerical modelling of the dust cycle given the inadequate representation of downdrafts in models with convective parameterization.


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