AMMA Further Analysis: Convective life-cycles over African continental surfaces

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

Abstract

The population of West Africa rely on rainfall for their survival, since rainfall in most areas is the most important limit on agricultural production. Rainfall is also important in the availability of water for human consumption and for hydroelectric power generation, while the seasonal rainfall influences certain diseases such as malaria. Currently, our ability to predict rainfall in this region is extremely poor. This project aims to improve our understanding of the rain-bringing storms of West Africa, and improve our ability to forecast them. Through a knowledge-exchange programme with weather forecasters from the UK and West Africa, we will feed the new scientific understanding into daily and longer-term predictions. In the Sahel region during the monsoon season, most of the rain is delivered by large thunderstorm clouds, known as cumulonimbus, which can extend 20 kilometres upwards into the atmosphere. These cumulonimbus clouds band together into large, organised storms, several hundred km in extent, that produce intense rain, leading to patches of very wet soil. Often, after a storm, patches of wet soil lie adjacent to soil which remains extremely dry. In turn, the soil moisture patterns lie on top of a mixed distribution of vegetation, from forest to agriculture, savanna and desert. We know that such patterns in the land surface can interact with storms on the following days, and it is important to understand these processes for accurate weather forecasting. In the past, a major problem in quantifying this interaction between the land surface and cloud has been a lack of useful measurements from this remote region. Following the successful NERC-funded AMMA (African Monsoon Multidisciplinary Analysis) study, we now have unprecedented observational data for this region of the continental tropics. Using this dataset, we will analyse cases in which storms interact with the land surface, and therefore improve weather prediction models. The West African Monsoon is an airflow which brings humid air from the Atlantic Ocean into the continent: this humidity feeds the rain-bringing storms. However, moisture is also transported out of the low level, humid monsoon layer by smaller clouds, known as cumulus congestus. The rate at which moisture is mixed upwards by congestus clouds influences the water budget of the entire West African region, so we need to assess their effects if we are to get the forecast right. We will make use of chemical tracers measured during AMMA, to understand and evaluate the rates of congestus mixing, in reality and in models. We will use the UK Met Office weather and climate prediction model - the 'Unified Model' (UM). The UM is excellent for this kind of study because it can be used to describe a wide range of phenomena in the atmosphere, from small clouds a kilometre or so across, or the whole global atmosphere. Advances made in a sister project (Cascade) mean that now we have a version of the UM which can resolve clouds right across the continent, and assess the interactions between these clouds. Such interactions are just as important as the land surface state in setting off new storms -a big thunderstorm sets of patterns of waves in the atmosphere which move thousands of km across the continent and may commonly set off a new storm at a remote location. By resolving these processes in the Cascade version of the UM, we aim to understand what is needed to get them right in a weather prediction version of the UM. The UM is also used as a climate prediction model: currently, climate predictions for West Africa disagree on projections of future rainfall, which hinders effective mitigation strategies that could help in planning for local and international governments. The knowledge gained from this project will help not only in the day-to-day prediction of storm events but also in prediction of future rainfall trends.

Publications

10 25 50
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Abdou K (2010) The diurnal cycle of lower boundary-layer wind in the West African monsoon in Quarterly Journal of the Royal Meteorological Society

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Bain C (2011) Anatomy of an observed African easterly wave in July 2006 in Quarterly Journal of the Royal Meteorological Society

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Berkes F (2012) Convective Squalls over the Eastern Equatorial Atlantic in Weather and Forecasting

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Birch C (2013) Wave-cloud lines over northwest Australia in Quarterly Journal of the Royal Meteorological Society

 
Description The project has used field measurements to test weather and climate forecast models over West Africa. We have made some important steps in understanding the links between land surface state and rainfall, in West Africa. These are results which appear to hold in all the tropical continental regions. We find that afternoon rain is favoured over dry surfaces, and suppressed over cooler, moist surfaces. This means that after rainfall, rain on the subsequent days is more likely to be initiated over areas which remained dry. The results also apply to the control of rainfall by vegetation: when forest is cleared for farming, rainfall is increased over the cleared areas, but decreases over the remaining forests. We speculate that this effect may put stress on the survival of the remaining forest. We have developed case studies of the atmospheric response to surface patterns of vegetation and soil moisture, and the ways in which this response conditions the rainfall on the next day. We have also made statistical analysis of satellite observations and high-resolution models, to show that the land-atmosphere coupling is robust over many rainfall events. We have shown that current climate models have the wrong sign of response of the rainfall to surface wetness: this is a major concern in the interpretation of model projections of the water cycle over the tropical continents in a future climate. We have also shown that high-resolution models capture the feedbacks faithfully, and we have quantified the implications of the errors for the prediction of future drought periods in Africa. We have used the latest generation of high-resolution models to explain how climate models handle the water cycle of the region, including the feedbacks between rain-bringing storms and the surface state.
Exploitation Route Our results offer methodologies for the planning of future land use in the tropics, in relation to its influence on rainfall. Our results are directly relevant to the improvement of global weather and climate models, and are being used to develop these models through our partnership with the Met Office. The methodologies developed in this project are also being used in a number of spin-off research activities, such as a project with the Met Office, analysing the changes in rainfall which result from changing land-cover in Africa. We are developing the material into guidance for weather forecasters as part of our Impact Plan. This includes leading a new textbook on weather forecasting in West Africa.
Sectors Environment

 
Description The findings of this project have been used in the weather and climate prediction services, in the UK, Europe and within Africa. Our results have also pointed out fundamental physical processes which need to be represented in models of the climate system, in order to correctly link the land surface, rainfall, and large-scale circulations. These results are influencing the development of the next generation of models for the Met Office. As part of the Impact Plan for this project, we have led the preparation of a "Forecasters' Handbook for West Africa";. This project is a major knowledge exchange between researchers and forecasters, in Africa and around the world, and will be used to train the next generation of weather forecasters in Africa. This is a unique resource for weather forecasting, globally. The book was published in April 2017. https://www.wiley.com/en-gb/Meteorology+of+Tropical+West+Africa%3A+The+Forecasters%27+Handbook-p-9781118391303 The book was translated into French and published in 2018: https://laboutique.edpsciences.fr/produit/1038/9782759821808/Meteorologie%20de%20lAfrique%20de%20lOuest%20tropicale More than 300 copies have been supplied to the African weather services and training centres, funded by the Met Office and Meteo-France.
First Year Of Impact 2009
Sector Aerospace, Defence and Marine,Environment,Transport
Impact Types Societal,Economic

 
Description Forecasters' Handbook for West Africa
Geographic Reach Africa 
Policy Influence Type Influenced training of practitioners or researchers
Impact We have published a textbook for the training of forecasters in the field of African meteorology. This is the first such book of its kind anywhere in the world, and targets one of the most environmentally vulnerable regions. The book was generated as a Knowledge Exchange activity associated with the AMMA project (several NERC and other funded projects), bringing together forecasters and researchers from Africa, the EU and USA. It has been supplied to weather services and training centres in Africa and we are working to embed the material into the syllabus of training programmes in the weather services and universities in Africa. The outcomes of this are intended to be improved skills of weather forecasters, and improved predictions, benefitting vulnerable people in Africa, as well as some major economic sectors in the region, including power generation, agriculture, fisheries and transport.
 
Description International funding for Forecasting Workshop in West Africa
Amount £54,460 (GBP)
Organisation Meteorological Office UK 
Sector Public
Country United Kingdom
Start 03/2013 
End 03/2013
 
Description International funding for Forecasting Workshop in West Africa
Amount £54,460 (GBP)
Organisation Meteorological Office UK 
Sector Public
Country United Kingdom
Start 03/2013 
End 03/2013
 
Description CEH-Leeds 
Organisation Natural Environment Research Council
Department Centre for Ecology & Hydrology (CEH)
Country United Kingdom 
Sector Academic/University 
PI Contribution My team in Leeds conduct atmospheric studies using observations, models and theoretical ideas. I have also led a number of projects and field experiements in which we have collaborated with CEH.
Collaborator Contribution Expertise in land-surface processes. Expertise in land-atmosphere interactions. Expertise in land-atmosphere climate dynamics. Data analysis, especially remote sensing of rainfall and land surface state. Leadership of projects. Co-supervision of PhD students.
Impact This is a multidisciplinary partnership in the area of land-atmosphere interaction. It has resulted in a large number of high-impact papers, successful jointly-supervised PhD studentships, and successful impacts, especially in Africa.
 
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.