The Role of Coarse Mineral Dust Particles in the Climate System

Mineral dust particles are uplifted from desert regions by strong surface winds and lofted into the atmosphere, where they can be transported over thousands of kilometres around the world. The Sahara is the world's largest source of dust, and from here dust is frequently transported westwards across the Atlantic Ocean, as far as the Amazon rainforest and the Caribbean. Several times a year weather patterns transport Saharan dust towards Northern Europe, where dust is often observed when deposited on cars.

While being transported by the atmosphere, dust exerts important effects on the climate system and humans. For example, dust particles can change the properties of ice in clouds, altering cloud extent and lifetime. When deposited to the ocean, dust particles provide a source of nutrients to oceanic phytoplankton, which in themselves can modulate atmospheric carbon dioxide levels. Dust particles also provide a supply of nutrients to the Amazon rainforest. Dust storms raise particulate matter levels which have an effect on health close to dust sources, such as in North Africa, but also thousands of kilometres away such as in the Caribbean. Dust also poses a hazard for aviation, and can decrease the effectiveness of solar energy farms by obscuring the sun.

Dust particles interact with both sunlight and infrared radiation which are components of the Earth's energy balance. For example, dust particles reflect sunlight back to space, and re-emit infrared radiation back towards the Earth's surface. These interactions perturb the Earth's energy balance. The specific magnitude and sign of this perturbation are dependent on many complex factors, which include the size of the dust particles in question. As a result, dust particles affect regional atmospheric circulation and precipitation. For example Saharan dust affects West African Monsoon precipitation, upon which millions of people depend, and can also affect Atlantic hurricane development.

Since mineral dust exerts such wide-ranging impacts, it is of great importance to be able to accurately represent dust in weather and climate models. Despite this, current models are unable to adequately represent dust processes. Much of this is due to inadequate representations of the size of dust particles. Recent NERC-funded aircraft measurements over the central Sahara have revealed the presence of much larger 'giant' dust particles than previously considered, which remain in the atmosphere for several days, influencing the radiation balance. Other field measurements have confirmed that 'coarse' particles are also present in dust transported far away from dust sources. Climate models do not represent these coarse or giant particles, despite their importance for the energy balance and subsequent circulation changes. Satellite measurements of dust must make assumptions of dust properties, and these also do not include a representation of coarse and giant dust particles.

This fellowship aims to provide dust properties from recent and future aircraft measurements, tailored towards both climate models and satellite retrieval algorithms, with full incorporation of coarse and giant particles. The impact of these coarse particles on satellite retrievals will be assessed, with a view to improving satellite retrievals of dust, which in themselves are used for climate model validation. The impact of the large particles on the radiation balance will be fully quantified to determine what radiative effect climate models omit by not representing coarse particles. This will be done by using specialist aircraft in-situ and radiometric observations from previous and future measurement campaigns, scattering and radiation calculations and satellite and ground-based observations, from local to global scales. Overall this will lead to an improvement of satellite measurements of dust and a quantification of the radiative importance of coarse and giant dust particles applicable to climate models.

This fellowship aims to quantify the optical and radiative properties of coarse and giant dust particles, which are not currently represented in numerical weather prediction (NWP) and climate models, or in optical models of aerosols used in satellite retrievals. Therefore NWP and climate models omit radiative effects and subsequent circulation changes due to coarse dust particles, bringing into question their ability to simulate aspects of the climate system impacted by dust. Satellite retrievals may be in error due to this omission, affecting their use in validation of NWP and climate models, and in data assimilation into NWP models.

The research proposed in this fellowship application is relevant to a number of non-academic beneficiaries:

-Policy Makers and General Public

Via climate modelling: Policy makers require climate prediction information from climate modelling centres and groups such as the International Panel for Climate Change in order to make informed policy decisions. These policy decisions also impact the general public. Improved climate representations of mineral dust will improve simulation of aspects of climate sensitive to dust, such as regional African circulation and monsoon precipitation, on which millions of people depend, and how these will change in the future.

Via weather and dust outbreak forecasting: Many weather forecast models now include dust forecast on short timescales (several days). Improved representation of dust in these models will result in improved dust forecasts, which are required to forecast high particulate matter levels which may be harmful to health, aviation hazards and hazards to military equipment, and degradation to solar energy farms by coating solar panels in dust and obscuring the sun.

-Government agencies operating satellites

Government agencies operating satellites will benefit from this research, which will provide satellite-specific dust properties based on new state-of-the-art measurements of dust over the remote Sahara and of transported dust, to be evaluated in the retrieval algorithms, with scope to improve them, thereby increasing their value.

-Non-Governmental Organisations (NGOs)

NGOs such as charities based in Africa will benefit from this research, since health, equipment and aviation are affected by dust storms. Aid to local communities and activity planning may therefore be affected: better dust forecasting will be of benefit to these groups.