Forecasts call for stormy weather

International students of the future will have even more bad Dutch weather to complain about, as the country’s wet and windy weather will become more extreme owing to the greenhouse effect.

Three TU Delft researchers are currently conducting joint-research aimed at helping the Netherlands prepare for the much heavier rainfall to come.

“Statistics indicate that due to a rise in temperatures there will be more rain during the winter in the Netherlands and more severe storms in the summer,” says prof.dr. Nick van de Giesen, the chair of Water Resources Management at the faculty of Civil Engineering and Geosciences. He, together with fellow TU Delft researchers, prof.dr. Herman Russchenberg and prof.dr. Harm Jonker, is conducting nationally important, multidisciplinary research that focuses on three different aspects of heavy rainfall.

Russchenberg, of the EEMCS faculty’s Remote Sensing of the Environment section, is developing new techniques to observe clouds and impending rain. Jonker, the head of the Clouds, Climate & Air Quality group at the faculty of Applied Sciences, uses this observational knowledge to gain a better understanding of cloud formation. Van de Giesen meanwhile is studying ways to protect the environment against large amounts of water falling from the sky.
Russchenberg stresses the importance of this combined research: “To prepare well for heavier rainfall, we need to be able to predict exactly where and when it will start to storm. If we can do that, then, for example, aircraft and farmers can anticipate it.”

Van de Giesen adds that current climate models all focus on temperature rise, yet knowledge about rainfall is also very important: “We’re certain that we know very little about rain and how much exactly it will increase and influence water levels.”

For the observation of the weather, Russchenberg works with several techniques. The most important device in weather forecasting is radar. “The Royal Netherlands Meteorological Institute now uses one radar to observe clouds that are twohundred kilometers away. However, the beam from the radar spreads as it gets further away and therefore measures less accurately. Furthermore, the Earth’s surface isn’t flat, so after a certain distance the radar looks above the clouds.”

Russchenberg’s solution to this problem is to place multiple radar systems closer together: “A storm cloud is very small, and therefore to accurately predict where and when it will start to rain we need one radar system per city or regional waterboard. However, for a good weather forecast, not only is good observation necessary, but also a good understanding of the processes occurring inside the clouds.”

To better understand the process of cloud formation, Jonker uses numerical techniques, while trying to imitate cloud formation processes in laboratory experiments. “We simulate the cumulo nimbus, a storm cloud,” he explains. “These clouds can become twelve kilometers high and grow out to become real monsters. Computers have only recently become powerful enough to simulate these clouds.”

For clouds, size matters. “Small clouds are much less complex, because they interact with the Earth’s surface in a simple way,” Jonker explains. “But large clouds, like cumulo nimbi, have their own dynamics. Moreover, cumulo nimbi require calculations on a very small scale. For example, to simulate the ice crystals inside these clouds, a scale of micrometers is necessary, while the smallest possible measurement scale currently available is ten meters. Another complex part of the cumulo nimbus is that it’s difficult to determine whether it will result in heavy rainfall or not, and thus the Royal Netherlands Meteorological Institute will sometimes issue a weather warning, yet nothing ultimately happens.”

Conversely, Van de Giesen mentions times when there were severe storms, yet they weren’t predicted. “On some of these occasions the sewers of Rotterdam flooded,” he says. And Russenberg recalls a heavy storm that hit in September 2001: “The farmers around Delft suffered forty million euro in damage.”
For these reasons and others, it’s important to adapt the environment to the heavy rainfall. Van de Giesen is currently studying several methods for protecting a city against large volumes of water.
“Since the 1970s, the rain and waste water in most newly built neighborhoods around Rotterdam is kept separate,” he explains. “The rain water is directly dropped into the surface water, and consequently the sewerage systems do not flood when it rains heavily. However, to prevent pollution of the surface water,

it’s very important that the two water flows do not get mixed up.”
Van de Giesen adds that ‘green roofs’ are also an option: “These are flat roofs with soil and plants on them that absorb some of the rain water. Rotterdam already has plans in place for implementing green roofs around the city.” Another option for storing water is water squares, which are like normal residential squares, or playgrounds, but are built a bit lower than their surroundings and can temporarily be submerged to store rain water from heavy rainfalls. These water squares have also recently been introduced in Rotterdam.

Not surprisingly, sewers – repositories of all manner of natural and unnatural waste – are notoriously difficult to analyze. Van de Giesen: “To further improve sewer systems, we need more knowledge about the water flows in them. When detectors are placed in a sewer, not only does the water pass by the detectors, but also leaves and other matter, and these disturbances make it very difficult to study sewer systems.”
Van de Giesen cites an article published this month about optical fibers, developed at the TU Delft, which can be used for measuring the temperature in sewerage. “With this new technique, we can learn more about the water flows inside a sewer system,” he says. “Two PhD students have just started a project on this subject.”

According to the researchers, we need more radar, better simulations and more knowledge about sewerage, but which of these is most urgent? Russchenberg says the Randstad (the Amsterdam-Rotterdam-Hague-Utrecht conurbation) demands the most attention: “The Randstad will continue to become more and more crowded and also have more road traffic. One minor storm can bring city life to a standstill. Furthermore, there are fewer possibilities for storing water, because buildings, roads and sidewalks are everywhere.” Van de Giesen agrees that cities are important, but adds that “some aspects must also be regulated nationally, like, for example, the country’s rivers and streams.”
Gazing out the window, there are fortunately no big ‘monster’ clouds rapidly forming in the sky today, but beware, because the weather forecaster on the radio just said “there will be some rain tomorrow”. 

Soldaten en Arabieren op de vakkendisco van Virgiel afgelopen zaterdag. Honderden studenten waren toe aan flink feesten na drie weken tentamens. Virgiel heeft dan ook vele studenten moeten teleurstellen. Het thema was dit keer ‘Desert Storm’, de naam van de Amerikaanse aanval op Irak in 1991.

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