Surface conditions affect megastorm predictability, research suggests

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Research from the UK Centre for Ecology & Hydrology (UKCEH) has found that land surface conditions frequently affect the direction and intensity of megastorms after they have formed. As a result, it is claimed that the findings from the study will make it easier to predict the path of some of the world’s most powerful storms, enabling communities to better protect themselves against severe flooding.

Mesoscale convective systems (MCSs) are ‘megastorms’ that affect large parts of the world, including Africa, Australia, Asia and the Americas, causing human and livestock deaths plus major damage to infrastructure. They can potentially last from several hours up to two days; release energy equivalent to the UK consumption for an entire year; be bigger than the size of England and travel 1,000km in distance; and unleash over 100mm of rainfall in just an hour. In Sahelian Africa, these extreme storms have tripled in frequency since the 1980s due to global warming.

Until now, it was thought that the path of these complex weather systems was largely unpredictable. But the research is helping scientists to develop online tools to better forecast the path and strength of an approaching storm, which will inform alert systems for communities across Africa, providing them with up to six hours’ warning. This includes Senegal, where UKCEH is working with the national meteorological service, ANACIM, to see how useful very short-term forecasts are for local emergency responses.

The new study, published in the journal PNAS, was funded by the Department for International Development (DFID) and Natural Environment Research Council (NERC) as part of the ‘Future Climate for Africa’ UK aid research programme. The researchers looked at satellite data on the activity of thousands of storms, plus land temperatures, in the Sahel for the period 2006 to 2010.

Lead author Dr Cornelia Klein of UKCEH explained, “It is well known that heat provides thunderstorms with great energy, but it was commonly thought that once they are moving, they were not affected by the state of the ground over which they traveled. However, we found that drier soils increased the intensity of an MCS mid-storm, affecting the amount of rainfall they release and also where they travel. Conversely, we found storms were often weakened over moister soils.

“Our finding means that, for the first time, we can predict, from satellite-observed surface conditions, how these extremely large West African storms may behave when, for example, they approach a city. A more effective alert system will enable local people to take action to protect themselves as well as their homes, livestock and possessions, plus plan emergency responses.”

Flash flooding frequently occurs during the storm season in the Sahel, peaking between June and September, and can have a serious impact, with water entering homes and people losing property and a safe, dry space to live. Flooding can also cause sewage overflow from inadequate drainage systems, posing a health risk to humans and animals.

The study’s authors say the results have important implications for ‘nowcasting’ (forecasting for several hours ahead) of severe weather not just in the Sahel, but potentially other MCS hotspot regions of the world.

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