Stephan's homepage

I am very much interested in the dynamics of boundary layer clouds like stratocumulus and shallow cumulus clouds. To this end our group (Harm Jonker and Pier Siebesma) at TU Delft is using a Large-Eddy model to simulate turbulence in these clouds. Most of our modeling exercitions are performed at the Dutch Supercomputer "Cartesius" installed in Amsterdam. We typically use observations as a basis for model initialization, while, in turn, observations of turbulence can be used to validate the modeling results. If the model is capable of reproducing the observations, one is in a position to perform sensitivity studies to learn about the generic behavior of cloud systems. This approach, and the use of simple conceptual models like the mixed-layer model are very helpful in getting a better understanding of the cloudy boundary layer.

Because weather forecast and climate models use a rather coarse grid resolution, the turbulent eddies in the boundary layer are not resolved by these models. Therefore the effect of turbulence on the vertical transport of heat, moisture and momentum must be calculated by means of parameterizations. To improve the parameterizations, we make intensive use of both observations and Large-Eddy Simulation results. With our colleagues from the United Meteorological Office we have set up a new model intercomparison case that deals with a cold-air outbreak. The aim is investigate the vertical turbulent transport in the so-called grey zone or Terra Incognita, that is at length scales between 100 m and 10 km, and in particular how this transport can be captured in weather forecast models using a horizontal resolution of the order of 1 km.

Our group has a strong collaboration with our colleagues from the Dutch Royal Meteorological Office (KNMI). Various students have done an internship or a Master's research project at the KNMI. In addition, observations collected at the 200 m tall Cabauw meteorological tower operated by the KNMI are frequently used for practical exercises.

We are currently involved in a European Union funded research programme, EUCLIPSE. The main question is to study the cloud feedback on a future climate change and to reduce the uncertainty in the representation of cloud processes and feedbacks in the new generation of cloud models. A nice movie about the project has been made for EURONEWS.