The Atmospheric Boundary Layer in Numerical Weather Prediction
(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 constituents 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 characteristics 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 and their organization on the boundary layer in atmospheric models?
- What are the dominant small-scale motions in the ABL over flat and heterogeneous terrain? By which processes are they caused and how can the impact of these processes be represented in atmospheric models?
- What is the impact of small-scale orography on the ABL and the lower atmosphere? How do topographically induced motions such as, for example, gravity waves, slope and valley winds, impact the exchange of momentum, heat, and mass in the mountain boundary layer?
These questions are addressed by combining theory, observations, and atmospheric models of various complexity. Our main research tools are large-eddy simulation (LES; link) and Doppler Lidars. LES with a resolution of O(1-100 m) 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. Doppler Lidars enable the measurement of coherent motions and turbulence statistics in the ABL. Based on these simulations and theoretical developments, parameterization ideas are developed and tested in single-column and numerical weather prediction models.