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Dust in the climate system: Saharan and Sahelian dust emission

Dr John Marsham (SEE), Dr Graham Mann (SEE), Dr Wolfgang Buermann (SEE)

Contact email: J.Marsham@leeds.ac.uk

This PhD project will build on past studies to use models and observational datasets to understand how these factors combine to control inter-annual variability in dust over the Sahara and Sahel, and observed long-term trends in dustiness.  It is arguable that unless a model can capture seasonal and inter-annual variability in dust it cannot be relied on for projecting changes in dust. This project will therefore directly contribute to our understanding of the role of mineral dust in climate change.

Airborne mineral dust is a key component of the Earth system, interacting with incoming solar and outgoing infrared radiation, nucleating ice clouds, darkening snow and ice when deposited on surfaces and delivering essential nutrients to both the oceanic and land-based biosphere. Our understanding of the meteorology of dust emission has increased substantially over the last decade and dust is increasingly being represented predicted within weather, climate and Earth-System models (Knippertz and Todd, 2012). Our understanding of the roles of dust on climate time-scales are very limited, however, and recent studies have questioned the value of dust in climate models (Evan et al., 2014); the Intergovernmental Panel on Climate Change (IPCC) highlights the uncertainty in human-induced changes to dust, with “low confidence” in dust projections. Our understanding of observed trends is limited (e.g. Ridley et al., 2014), although recently there has been some intriguing evidence published in Nature that some models have skill (Evan et al., 2016). This project will capitalise on recently gained understanding of mineral dust emission, and recent model developments, to investigate controls on dust emission across time-scales.

Figure 1: A haboob dust storm over the West African Sahel. These events are important for the seasonal dust cycle, and are missing from global models, but the “Desert Storms” project has now developed a parameterisation, that is being integrated into the Met Office Unified Model in the IMPALA project, providing a new research opportunity.

Objectives

The project will improve the understanding of the controls of dust emission from the Sahara and Sahel, the resultant variability in emissions on seasonal to decadal timescales, and the representation of this in models used for projections.  We anticipate four key steps in the work:

  1. Collate remote-sensing datasets, model datasets/analyses and long-term in-situ data to understand seasonal cycle in winds, dust uplift mechanisms and dustiness, developing an understanding of model biases.

  2. Use these datasets to study inter-annual variability and multi-year trends in dust uplift and dustiness. This will allow both evaluation of models and new quantification of the drivers of variability on these time-scales.

  3. Investigate the roles of changing vegetation, roughness and winds using remote-sensing data and new simulations.

  4. Investigate the role on long time-scales of both haboobs and rare meteorological events (e.g. Cowie et al., 2015).

Background

Dust uplift is a highly non-linear function of near-surface wind-speed and is sensitive to soil type, moisture and vegetation. Recent projects in which Leeds has taken a leading role (e.g. AMMA, Fennec and Desert Storms) have greatly improved our understanding of the processes controlling dust uplift in the Sahara and Sahel, the world’s largest dust sources (e.g. Marsham et al., 2013). These have demonstrated the key role of cold-pool outflows from deep moist convection (‘haboobs’, Figure 1) in generating strong winds (Marsham et al., 2011; Heinold et al., 2013) and hypothesised the role of changing vegetation in controlling surface roughness, rather than simply bare-soil area (Figure 2). Desert Storms has provided a parameterisation for haboobs for global models that is being introduced into the UM (Pantillion et al., 2015), providing new model capability.

Figure 2: It is hypothesised that increased vegetation over the Sahel has increased roughness, decreased wind-speeds and decreased dust emission. The Figure (from Cowie et al., 2013) shows a downward trend in winds (black solid line), observed dust generating winds (DUP, red solid line) and observed frequency of dust emission (blue solid line). The trend in winds is not captured by ERA analyses (dashed lines) which use a fixed seasonal cycle in roughness.  

The work is timely in that will allow interactions with several ongoing projects and consortia on African climate at Leeds and builds on short time-scale process-understanding from past highly-successful projects to address the longer time-scale processes key to our understanding of dust in the climate system.

Potential for high impact outcomes

Aerosols are a key component of the earth’s climate system and mineral dust is the largest component of the atmosphere’s aerosol load by mass. Despite this, our understanding of the controls of mineral dust on seasonal-to-decadal time-scales is low, as is our understanding of the role of anthropogenic mineral dust in climate change. The research is therefore directly relevant to climate projections and model development and has great potential for high impact outcomes. We anticipate several important papers in international journals. There are good routes for communication of these outcomes through existing links with the Met Office and ECMWF. Leeds is very active within the £20M “Future Climate for Africa” programme and this and other existing links will be able to provide opportunities for direct communication with West African meteorological services.

Training and the Research Environment

You will join a very successful team of scientists at Leeds. You will work under the supervision of Dr John Marsham in the dynamics and clouds research group, with co-supervisors who each bring complementary expertise: Mann (aerosol modelling) and Wolfgang Buermann (remote sensing of vegetation). You will also work with researchers led by Peter Knippertz at the Karlsruhe Institute of Technology (KIT, Germany). ICAS has a formal partnership with KIT and there will be the opportunity for regular visits during the PhD.

There will be opportunities for interactions with researchers at Leeds working on dust in West Africa (SWAMMA and IMPALA projects) and African climate change (IMPALA, AMMA2050 and HyCRISTAL projects), as well as others working on tropical meteorology, vegetation-atmosphere coupling and aerosols. Many of these projects involve the UK Met Office and it is planned that the student will discuss plans with and present their results to scientists there. It is anticipated that the student will make regular (annual) visits to KIT. Leeds takes part in and leads a range of meteorological field campaigns across the world: involvement in these will require more extensive travelling. 

This project will provide a high level of specialist scientific training in: (i) use of in-situ and remotely sensed data; (ii) interpretation and running of state-of-the-art global aerosol models, (iii) combing models and observations to gain new insights into earth’s climate system and the implications for projections.

The student will have access to a broad spectrum of training workshops put on by the Faculty that include an extensive range of training workshops in numerical modelling, through to managing your degree, to preparing for your viva (http://www.emeskillstraining.leeds.ac.uk/).

Student profile

The student should have a degree in a mathematical or physical science, and have some familiarity with scientific programming, as the project will involve both writing computer codes for data analysis, and running meteorological models. A background in meteorology is useful but not essential.

References

Cowie, S, P Knippertz and JH Marsham(2013), Are vegetation-related roughness changes the cause of the recent decrease in dust emission from the Sahel?, Geophys. Res. Lett., 40, 1868-1872, doi: 10.1002/grl.50273.

Cowie, SM, P Knippertz and JH Marsham (2015), The importance of rare, high-wind events for dust uplift in northern Africa, accepted to Geophys. Res. Lett..

Evan, AT, C Flamant, S Fiedler, and O Doherty (2014), An analysis of aeolian dust in climate models, Geophys. Res. Lett., 41, 5996–6001, doi:10.1002/2014GL060545.

Evan, AT, C Flamant, M Gaetani and f Guichard (2016),  The past, present and future of African dust, Nature, 531, 493–495, doi:10.1038/nature17149.

Heinold, B, P Knippertz, JH Marsham, S Fiedler, N Dixon, K Schepanski, B Laurent and I Tegen (2013), The Role of Deep Convection and Low-Level Jets for Dust Emission in Summertime West Africa, J. Geophys. Res. Atmos., 118, 1-16, doi: 10.1002/jgrd.50402.

Knippertz, P, and MC Todd (2012), Mineral dust aerosols over the Sahara: Meteorological controls on emission and transport and implications for modeling, Rev. Geophys., 50, RG1007, doi: 10.1029/2011RG000362.

Marsham, JH, M Hobby, CJT Allen, JR Banks, M Bart, BJ Brooks, C Cavazos-Guerra, S Engelstaedter, M Gascoyne, AR Lima, JV Martins, JB McQuaid, A O'Leary, B Ouchene, A Ouladichir, DJ Parker, A Saci, M Salah-Ferroudj, MC Todd and R Washington (2013), Meteorology and dust in the central Sahara: Observations from Fennec supersite-1 during the June 2011 Intensive Observation Period, J. Geophys. Res. Atmos., 118, 1-21, doi: 10.1002/jgrd.50211.

Marsham, JH, P Knippertz, N Dixon, DJ Parker, GMS Lister (2011), The importance of the representation of deep convection for modeled dust-generating winds over West Africa during summer, Geophys. Res. Lett., 38, L16803, doi:10.1029/2011GL048368.

Pantillon, F, P Knippertz, JH Marsham and C Birch (2015), A parameterization of convective dust storms for models with mass-flux convection schemes, J. Atmos. Sci., 72, 2545–2561, doi:10.1175/JAS-D-14-0341.1.

Ridley, D.A., C. L. Heald and J. M. Prospero (2014), What controls the recent changes in African mineral dust aerosol across the Atlantic? Atmos. Chem. Phys., 14, 5735-5747, doi:10.5194/acp-14-5735-2014

Related undergraduate subjects:

  • Chemistry
  • Computer science
  • Geology
  • Geophysics
  • Physics