Fascinated by rain


Fascinated by algorithms and downpours

Gepubliceerd op
5 juli 2013

‘Spectacular!’ That’s how Remko Uijlenhoet described the video footage showing precisely how raindrops burst apart as they fall. He was not the only person who got excited when the images were shown at the International Precipitation Conference at de Reehorst in Ede, jointly organised by Wageningen University and the Royal Netherlands Meteorological Institute.

More than one hundred and fifty meteorologists, hydrologists, mathematicians, statisticians, physicists and engineers visited Wageningen and Ede on 1, 2 and 3 July to discuss all sorts of themes relating to precipitation – including the relevance of the way in which a raindrop bursts.

Preventing false weather alarms

Uijlenhoet tried to explain it in layman’s terms: ‘Let’s say you have measured high levels of reflection. The first thing you need to do is find out why.’ ‘Consider drizzle,’ adds Hidde Leijnse, joint organiser of the conference along with Uijlenhoet. ‘You can get pretty wet because the intensity is high, but the reflection is low.’ ‘Before you issue a weather warning, you need to know what you’re talking about,’ adds Uijlenhoet. In other words, if the weatherman forecasts extreme rainfall and things turns out to be not that bad, thousands of people will have left work early for no reason. That’s why you not only have to look at the satellite pictures of where it’s raining but find out the ‘raindrop size distribution’ as well.

Phone masts double up as rain gauges

Satellite observations are validated by measurements on the ground to improve the algorithms used in weather forecasting. Ground readings can be taken with old-fashioned rain gauges, but the conference-goers were exploring smarter - and more enjoyable - methods. Sensors are more affordable nowadays and can transmit data fast. TU Delft has devised a plan to fit sensors on umbrellas and use crowdsourcing in each street to get an idea of how much rain falls. Wageningen researchers are already using phone masts to measure rainfall. When it rains, the signal between the masts weakens a little. When this information is linked to other data, such as satellite images, it improves the accuracy of weather forecasts. This is important, according to Remko Uijlenhoet, ‘especially, when you consider that there are hardly any rain gauges in South America and Africa but plenty of mobile telephone masts.’

'Grass roots initiative'

Many of the presentations at the eleventh International Precipitation Conference were of a very technical nature, relating to processes high in the atmosphere, measuring techniques, algorithms and statistics. ‘But compared with previous conferences, the emphasis was more on applications, such as urban water management,’ says Remko Uijlenhoet. ‘The conference is organised by a different group every two years. The organisers choose the theme. This is a real grass roots initiative, not connected to an organisation or a project. It started in Caracas, Venezuela, in 1986, and we have just agreed to hold the next one in Sao Paolo, Brazil.’