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Understanding what causes rain

Weather models are not good at predicting rain. Particularly in hilly terrain, this can lead to great damage arising from late warnings of floods, or even none at all.

From June 1 to September 1, 2007 Delft University of Technology is participating in a major international experiment in Germany’s Black Forest, to learn more about what causes rain. Aircraft and an airship are to be used alongside ground-based observatories. Satellites will be used to gather the large-scale information.

The creation of rain is the result of a variety of physical processes. These processes influence each other and play out both at an extremely small scale (several micrometres) and on a very large one (100 kilometres). The spatial scale of weather models is a few kilometres, and physical processes which occur at a smaller scale have to be approximated. Cloud formation is an example of this. The complexity and differences in scale make weather modelling inaccurate in predicting the time and place of a downpour, and the quantity of rain which will ultimately fall.

Black Forest

The Black Forest has a lot of thunderstorms in summer, and the discrepancies between predictions and actual rainfall are enormous. This makes it an ideal natural ‘laboratory’. Spread over an area of 100 by 100 km, five temporary observatories are being set up with state-of-the-art remote sensing equipment to measure the atmosphere continuously. In July nine aircraft and an airship will also be deployed to carry out detailed measurements above, below and in the clouds. Satellites will be used to gather the large-scale data.

TU Delft

On June 4 TU Delft moved the TARA (Transportable Atmospheric RAdar) atmosphere radar to Germany. The instrument will be sited atop the Hornisgrinde (one of the highest peaks in the Black Forest). There it will measure the atmosphere together with other instruments (LIDARs [Light Detection And Ranging], radiometers, cloud radars). The TU Delft will also have access to two research aircraft (one French, the other German) which will fly through the clouds to measure their physical properties. These aircraft have been specially assigned to two Delft PhD candidates.

Within this international Convective and Orographically-induced Precipitation Study (COPS), TU Delft will be mainly concerned with the question as to how cloud and rain formation is influenced by dust particles in the atmosphere.

Climate models

The influence of the aerosol-cloud interaction on the earth’s radiation balance is also one of the greatest unknowns in climate models. The data collected during COPS will be suitable for improving models describing the relationship between atmospheric dynamics and cloud formation.


-TARA will be transported to Germany on June 4;
-TARA will be operational on June 11;
-Intensive measurement will be carried out from June 24 to August 1, with coordination between ground and aircraft measurement;

-From August 1 TARA will be used in standard mode to build up a sufficiently statistically representative dataset routinely.

Roy Meijer | alfa
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