Most of us take the accuracy of weather forecasts for granted, not giving too much thought to the science, technology and global cooperation behind the information we receive.
The use of satellite data in weather forecasting has demonstrably increased accuracy. What challenges exist to achieve still greater accuracy?
Impact of satellite data
European Centre for Medium-Range Weather Forecasts (ECMWF) Principal Scientist, Dr Tony McNally, from the EUMETSAT Numerical Weather Prediction SAF, says regular investigations into how well weather forecasts perform, both in “normal” conditions and “extreme” weather events, are carried out to evaluate the impact of satellite data on accuracy.
“Time and time again, these studies show satellite data are the most important source of observations in our system,” Tony said.
“In routine conditions, northern hemisphere forecasts would be around half a day to a day less accurate without the input of satellite observations.
“Southern hemisphere forecasts would be up to two or even three days less accurate.”
The impact on forecasts of severe weather events, such as storms, hurricanes and tropical cyclones, can be even more significant.
Models showing the expected paths of hurricanes which hit the Caribbean and the United States are generally highly accurate.
“But when we rerun some of those cases without satellite data, the results can be dramatic,” Tony said
An investigation of the forecast path of Hurricane Sandy, in 2012, showed that, without the input of satellite data, ECMWF model forecasts would not have provided critical guidance five or six days in advance that the storm would strike the New Jersey coast, which it ultimately did with devastating effect.
Similar studies during the 2017 Atlantic hurricane season have confirmed the key role played by satellite data in producing accurate forecasts.
“We obviously can’t stop the hurricanes but we can provide the best possible guidance that allows people time to make preparations and, if necessary, evacuate,” Tony said.
“We take a systematic and forensic approach to evaluating the quality of our forecasts, finding what went right and what went wrong in a particular forecast, to learn how to make the systems better.”
How much more accuracy is needed?
“The reliability of our medium-range weather forecasts, e.g. around day five, is currently very high and arguably sufficient for many people’s requirements,” Tony said.
“But the challenge, and what users need, is for us to extend that accuracy out to longer range forecasts – producing equally reliable predictions 10 days or more ahead of time,” he said.
“If you want a forecast for half-an-hour’s time, you could just look out of the window at the clouds,” Tony said.
“But if you need to make longer range forecasts, you need a global picture of what the atmosphere is doing right now – and that’s where satellites are particularly crucial. Only satellites can give you this global picture.
“For example, if you want to know what the weather will be like in Europe in eight days’ time, you need to look at what is happening over the Pacific Ocean. What’s happening over the Pacific Ocean now will affect Europe eight days from now.”
To achieve longer range forecast accuracy, our models will expand to take into account all aspects of the entire Earth system (atmosphere, land, ice, ocean and composition) on ever finer temporal and spatial scales.
These models will require new types of satellite observations providing highly detailed and accurate information, not just on meteorological parameters such as temperature and wind, but on variables such as aerosol, carbon dioxide, ocean state, ocean salinity and more.
In a few years’ time, EUMETSAT will begin launching its next generations of geostationary and low-Earth orbiting satellites, with state-of-the-art instruments.
These satellites, together with those of international partners and the European Copernicus Sentinel satellites, will dramatically increase the amount of weather and climate observation data.
But this is just one side of the equation when it comes to increasing weather forecasting accuracy.
Along with more and better satellites, scientists will have to be better trained to exploit these new measurements, Tony said.
The EUMETSAT NWP SAF provides training to scientists from National Meteorological Centres, satellite agencies (including EUMETSAT) and academic institutions who are either new to satellite data assimilation or wish to learn more about a specific area.
It’s a one-week course conducted as part of a larger training provided by the ECMWF.
“It’s a face-to-face course limited to around 30 people and is typically oversubscribed,” Tony, who is responsible for the course, said.
“We aim to explain the most important science issues related to satellite data assimilation, show how things are done in practice at the ECMWF and incorporate a ‘clinic’ element to the course, where participants can interact with our experts and ask questions about different problems or situations in their own area.”