News - January 11, 2010

Rain radar reveals heavy rainfall

Besides showing if rain will fall, a rain radar also provides information to determine the chances of extreme rainfall.

To foretell heavy rainfall, you need to look back. 'You go into past sequences of rainfall and use statistics to calculate the chance of extreme rainfall. This applies also to extremes which exceed the measurements', explains hydrologist Aart Overeem. He graduated last month in researching into the use of radar to determine the chance of extreme rainfall.

6000 rain gauges
Rain gauges are the basics. The Royal Netherlands Meteorological Institute (KNMI) has thirty five of them throughout the country and these give continuous measurements. This is good enough for determining the rainfall for each region. However, the network is not dense enough to characterize, for example, an entire water board area in a province, while the rain radar can. 'The KNMI has been making detailed radar images of the entire country for more than ten years. A new image of six thousand pixels is generated every five minutes, and each image covers six square kilometers in the Netherlands. That would be equivalent to six thousand rain measurement instruments.
The use of radar images has provided Overeem with an abundance of new data, enabling him to make useful and detailed statements on the chance of extreme rainfall. In Wageningen, for example, we can expect once in an average of twenty years 60 millimeters of rainfall in one downpour, whereas this would be more than 75 millimeters in the province of South Holland. These regional differences could not be shown in such detail in the past. Besides, the radar can now provide much more information about heavy rainfall within short periods, such as a quarter of a hour. Such information is much sort after by water managers. Overeem says: 'It is especially important for the design of drains and pumping stations. It's also important in public information. The public wants to know how extreme a heavy downpour can be.'