Turbulence in the atmospheric boundary layer strongly influences the weather and climate we experience in our daily life. As such, it not only affects near surface temperature, humidity and winds, but also the occurrence of fog, air pollution and aerosols, and exchange of greenhouse gasses between the land surface and the atmosphere. Our research focuses on cloudy boundary layers and on stable boundary layers. Extensive observations are made at our observational site CESAR in Cabauw. Using these, model parameterizations, as part of the current operational ECMWF, HIRLAM and HARMONIE and our own regional climate model RACMO, have been developed. An extensive inter-comparison between different state-of-the-art climate and weather prediction models with observations is presently done in the KNMI Parameterization Testbed (KPT).

A major part of the uncertainty in predictions of climate change is related to radiative feedbacks from clouds, in particular in the atmospheric boundary layer. Here, experience in our group with modelling the atmospheric boundary layer, observations of clouds, radiation and turbulence at Cabauw, and satellite based observations meet with global climate modelling activities. On the regional scale, aerosols, together with interactions of aerosols with clouds, are of interest, in particular in explaining why Europe has warmed so much during the last 30 years.

Events of extreme precipitation have a huge influence on society, as they are associated with flooding, erosion and water damage, and may have impacts on transport and safety. Precipitation extremes are expected to increase in a warmer climate because a warmer atmosphere can ‘hold’ more moisture. We study the impact of moisture on precipitation extremes in observations and in long simulations with our regional climate model RACMO. Emphasis is also on the North Sea, which has a considerable influence on the Dutch climate, and the role of the surface and land-use changes. One important finding is that short duration extremes could increase much faster than expected

The potential collapse of large ice sheet, and resulting sea level rise, is one of the largest concerns for a low lying country as The Netherlands. RACMO is used extensively over the ice sheets of Greenland and (West)Antarctica to establish, combined with observations, the ice mass balance. Besides these polar regions, RACMO is applied for Africa and (in the near future) Indonesia.