Satellite data used to provide life-saving weather forecasts in tropical Africa

By: Peter Hill

Much of the population of tropical Africa are vulnerable to severe weather, often caused by intense storms that can generate heavy rainfall, strong winds and flooding. For instance, thousands of fishermen drown each year in Lake Victoria as a result of accidents caused by storms. As a result, improved weather forecasting systems in tropical Africa could save lives and protect livelihoods.

The Global Challenges Research Fund (GCRF) African Science for Weather Information and Forecasting Techniques (SWIFT) project aims to enable African weather forecasting services to develop such improved weather forecasting systems. A partnership between meteorologists from Senegal, Ghana, Nigeria, Kenya and the UK, including several scientists at the University of Reading, SWIFT is striving to improve forecasts from timescales of a few hours to a few weeks ahead.

Much of my work in the SWIFT project involves very short-range predictions – from 0 to 12 hours ahead – based directly on observations, something meteorologists term “nowcasting”. The simplest nowcasts take weather observations and extrapolate them forwards in time, using the assumption that the weather will continue to develop along the same trajectory as the recent past. Nowcasts can be crucial for severe weather events, providing timely information to enable authorities and the public to respond appropriately to safeguard lives and livelihoods.

One of the major obstacles to nowcasting in tropical Africa is the lack of rainfall radar observations, which are used for nowcasting in other parts of the world, including the UK. Passive satellite observations, which measure the naturally occurring energy at the top of the atmosphere, provide a less direct measure of weather systems. Yet in the absence of other observations, this satellite data can provide vital information for nowcasting purposes.

To this end, the SWIFT project has made satellite-based nowcasts for tropical Africa freely available from a new website. Figure 1 provides examples of two such products. These nowcasts are based on software provided by the European Nowcasting Satellite Applications Facility (NWCSAF). However, these products have been calibrated and validated for mid-latitude European weather systems and it is therefore necessary to evaluate how well they perform for tropical Africa.

Figure 1: Examples of two NWCSAF products over tropical Africa. (a) shows the convective rainfall rate in different regions (b) shows the rapidly developing thunderstorms convection-warning product over the Guinea Coast region.

To understand the suitability of this NWCSAF software for tropical Africa, I compared the two products shown in Figure 1 to higher quality satellite rainfall estimates that incorporate data from multiple sources including direct rainfall estimates from rain gauges at the surface. This higher quality data cannot be used for nowcasting because it is not available sufficiently quickly.

The comparison demonstrates that both NWCSAF products provide useful information, despite some limitations. For instance, the convective rain rate product has valuable skill for predictions at least 90 minutes ahead (Figure 2). The rapidly developing thunderstorms product can also identify the occurrence of heavy precipitation, correctly identifying around 60% of strong (5 mm of rain per hour) events at least one hour before they occur. These products could be used to inform flood warnings, disaster response, or provide warnings to fishermen.

Figure 2: Skill metrics for the convective rainfall rate product, compared to predictions based on the historical occurrence of rainfall events. Hit rate means the proportion of true rainfall events that are successfully identified, and false alarm ratio means the proportion of the predicted rainfall events that do not occur in reality. “Retrieval” here is the skill of the satellite products, versus higher quality data. This higher quality data is regarded as “truth” but is not available sufficiently quickly to be useful for nowcasts. The “extrapolation” is the forecast made by projecting the observed storms forward in time. The “climatology” skill is skill from assuming today’s storms can be predicted using previous years storms on the same time of day and time of year.

This analysis is crucial in providing forecasters with confidence in the products which GCRF African SWIFT has made available to them to issue warnings. It has also highlighted some aspects of both the convective rain rate and rapidly developing thunderstorm – convection warning products that could be improved upon. Future work will aim to further develop these products to provide better nowcasts for tropical Africa.

Ongoing work within the African SWIFT project is also enabling African groups to generate these products locally, as well as supporting forecasters to understand and use these products effectively to minimise adverse impacts of severe weather on lives and livelihoods in Africa.

References:

Roberts, A.J., Fletcher, J.K., Groves, J., Marsham, J.H., Parker, D.J., Blyth, A.M., Adefisan, E.A., Ajayi, V.O., Barrette, R., de Coning, E., Dione, C., Diop, A.-L., Foamouhoue, A.K., Gijben, M., Hill, P.G., Lawal, K.A., Mutemi, J., Padi, M., Popoola, T.I., Rípodas, P., Stein, T.H.M., Woodhams, B.J. 2021. Nowcasting for Africa: advances, potential and value. Weather (In press).

Hill, P. G., Stein, T. H. M., Roberts, A. J., Fletcher, J. K., Marsham, J. H. & Groves, J. 2020. How skilful are Nowcasting Satellite Applications Facility products for tropical Africa? Meteorological Applications, 27(6). DOI: https://doi.org/10.1002/met.1966

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