Fennec - The Saharan Climate System

Lead Research Organisation: University of Leeds
Department Name: School of Earth and Environment

Abstract

The central Sahara has one of the most extreme climates on Earth. During the northern summer months, a large low pressure system caused by intense sunshine develops over a huge, largely uninhabited expanse of northern Mali, southern Algeria and eastern Mauritania. Temperatures in the high 40s are normal and uplift of dry air through more than 6000m of the atmosphere is routine in what is thought to be the deepest such layer on the planet. This large zone is also where the thickest layer of dust anywhere in the Earth's atmosphere is to be found. Although the central Sahara is extremely remote, it turns out to be vitally important to the world's weather and climate. The large low pressure system drives the West African Monsoon and the dry, dusty air layers are closely related to the tropical cyclones which form over the Atlantic Ocean. Likewise, the dusty air has a strong influence on the way the atmosphere is heated, a process which is poorly understood. It is not surprising that the models we use to predict weather and climate and which are a crucial tool for understanding how the atmosphere works, all have problems in dealing with the central Sahara. Insights into how the climate system works, improving the models and therefore the predictions have all been held back in the case of the Sahara by a lack of measurements of the atmosphere and the processes that make dust and extreme weather. This will always be the case until a team goes to the central Sahara and makes these measurements. A key part of this proposal aims to do just that. We want to set up an array of special instruments, at the surface in two carefully chosen places in the central Sahara, which will monitor the winds, temperatures, dust and so on for an entire year. We will add to this collection for a shorter period of even more intense measurements during the core summer month of June. We plan also to fly a instruments attached to an aeroplane overhead the surface array and across the desert so that we can get an idea of the structure of the atmosphere and how it changes through the day. To find out how dust storms work, we will leave 10 weather stations at places where we think dust storms happen frequently. Satellites play an essential role in measuring weather and climate and are especially useful in remote places. The best available information from satellites will help to quantify how weather and climate works in the Sahara. We also expect to improve the way the satellites are able to make their measurements too. Because models are so important to understanding and predicting weather, we will make heavy use of them in this work. We want to know how well the models work over the Sahara and what can be done to improve them. We are especially interested in seeing whether the models work better if we allow them to deal with small parts of the climate system or whether we can still represent extreme places in the Sahara by ignoring these details in the models.

Publications

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Blechschmidt A (2012) Aircraft-based observations and high-resolution simulations of an Icelandic dust storm in Atmospheric Chemistry and Physics

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Bourgeois E (2018) Characteristics of midlevel clouds over West Africa in Quarterly Journal of the Royal Meteorological Society

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Burton R (2013) The Harmattan over West Africa: nocturnal structure and frontogenesis in Quarterly Journal of the Royal Meteorological Society

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CatonĀ Harrison T (2019) A 14Year Climatology of Saharan Dust Emission Mechanisms Inferred From Automatically Tracked Plumes in Journal of Geophysical Research: Atmospheres

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Cowie S (2014) A climatology of dust emission events from northern Africa using long-term surface observations in Atmospheric Chemistry and Physics Discussions

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Garcia-Carreras L (2015) The Turbulent Structure and Diurnal Growth of the Saharan Atmospheric Boundary Layer in Journal of the Atmospheric Sciences

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Gross A (2015) Variability in Sources and Concentrations of Saharan Dust Phosphorus over the Atlantic Ocean in Environmental Science & Technology Letters

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Hobby M (2013) The Fennec Automatic Weather Station (AWS) Network: Monitoring the Saharan Climate System in Journal of Atmospheric and Oceanic Technology

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Huang Q (2018) Large-eddy simulation of dust-uplift by a haboob density current in Atmospheric Environment

 
Description The Fennec field campaign has provided the first ever comprehensive dataset of climatic conditions over the Sahara. These data are being used to explain the physics of the climatic system in this region, and to evaluate our models of that system.

We have shown that "cold pools" of air moving into the desert at low levels, from thunderstorms on the margins of the desert, have a significant impact on the temperature structure of the region and are not represented in climate models. This helps to explain the biases in those models.

These results have influenced the strategy of climate modelling centres, to include cold-pools in their models.

We have used observations and models to describe the physics of the Saharan atmospheric boundary layer. We have found that there is a fundamentally different physical evolution over the Sahara, which leads to biases in the weather and climate models.

We have also developed new techniques to measure, for the first time from an airborne platform, the emissions of dust from a desert surface.

We have used the surface data, along with satellite data, to show that water vapour is the primary control on the energy balance of the Sahara. This is a surprising result, becuase the Sahara is a very dry place. It is very important in the African climate system. Notably, in comparing water vapour and airborne dust, we find that the water vapour has a much larger contribution to variations in the heating of the Saharan atmosphere than the dust, contrary to the expectations of many scientists.
Exploitation Route The results are being used by the Met Office and other operational forecasting centres to improve their weather and climate prediction models.
Sectors Environment

 
Description Our findings are being used by the UK Met Office to improve their models. This includes a new strategy to include convective "cold pools" in their convection scheme, and a consideration of the need for modified entrainment over the desert.
First Year Of Impact 2011
Sector Environment
Impact Types Economic

 
Description Met Office 
Organisation Meteorological Office UK
Country United Kingdom 
Sector Public 
PI Contribution Our research group analyses atmospheric processes in order to better represent them in the Met Office's forecast models. We also use those forecast models in our research, and evaluate their performance in order to identify the best strategies to improve the models.
Collaborator Contribution The Met Office brings its models and its datasets to the partnership, in addition to the considerable expertise of its staff. The Met Office also represent a conduit to the impact of our research for society, through its provision of operational weather and climate forecasts.
Impact Our research has influenced the Met Office strategy for model development, especially in regard to high-resolution models, and the convective parametrisation scheme. We have jointly influenced international strategy for atmospheric research and measurements.