Part of the Hans-Ertel-Centre for Weather Research

The atmospheric boundary layer (ABL) plays an important role in numerical weather prediction and climate simulations. Its structure and evolution have a strong impact on near-surface weather and climate. ABL processes, such as turbulence and coherent motions, for example, contribute to the formation and development of clouds and thunderstorms. They also largely control the exchange of momentum, heat, water and other constitutents between the land surface and the free atmosphere.

Representing the stable ABL in weather and climate models, in particular, poses a great challenge. In the stable ABL, turbulence is weak and intermittent. Other processes such as radiation and small-scale coherent motions become more important in determining the charateristics of the ABL. The influence of these other processes on the ABL is poorly known and currently not, or only crudly, represented in weather and climate models.

Our group studies the atmospheric boundary layer over complex terrain, with the goal of improving our understanding of its structure and evolution and our ability to represent and predict it in weather and climate models. Research questions that our group are addressing include:

• How can we best represent the impact of small-scale clouds on the daytime convective boundary layer in atmospheric models?
• What is the influence of coherent motions on the nighttime stable boundary layer and how can the impact of these motions be represented in atmospheric models?
• How can we best use surface-sensitive satellite observations (e.g. skin temperature) to improve our understanding and the modeling of land-atmosphere coupling and the diurnal evolution of the ABL?

These questions are addressed by combining theory, observations, and atmospheric models of various complexity. Our main research tool is large-eddy simulation (LES) with a resolution of O(1-100 m) that allows for the explicit representation of the largest turbulent eddies and small-scale coherent motions in the atmospheric boundary layer under stable and convective conditions, respectively. Based on these simulations parameterizaton ideas are developed and tested in single-column and numerical weather prediction models.