Turbulence in the Atmospheric Boundary Layer

Objectives

The course aims to give the students an introduction into turbulence and turbulent fluxes of matter and energy in the atmospheric boundary layer. It forms the basis for further advanced studies in this field by giving the students an appropriate background for assessing the relative importance and relevance of turbulence related to meteorology and climate.

Content

The course covers homogenous turbulence theory, including spectral descriptions, definition and measurement of turbulent fluxes and the influence of stratification in the atmospheric boundary layer. Starting from the basic equations of conservation of mass and energy, a set of equations valid for boundary layer flow is derived. This is achieved by using appropriate approximations based on scale analysis. Based on that, prognostic equations for the mean variables in turbulent flow and for covariances and variances are deduced. Based on the latter, the budget equation for the turbulent kinetic energy is derived, and the different terms are discussed. Salient features of various turbulence related processes in the atmospheric boundary layer are introduced. The profiles of scalars, such as temperature, humidity, and tracer concentration as well as velocity and their respective turbulent fluxes in the atmosphere are described and discussed in different forcing conditions. The turbulence closure problem is introduced and the most common local and non-local closure approaches are presented. Specific mathematical methods related to the description of turbulence, as FFT and Buckingham-Pi analysis in similarity theory are shortly covered. An overview over the common instrumentation and measurement methods is given, as well as a short introduction in Large Eddy Simulation (LES) as turbulence modeling tool.

On completion of the course the student should have the following learning outcomes defined in terms of knowledge, skills and general competence:

Knowledge

The student

• has clear understanding of the salient features of turbulent boundary layers in the atmosphere
• has advanced knowledge on the spectral evolution of turbulence, and the dominant terms of the budget equations for turbulence
• has knowledge on sources of (i.e. shear and bouyancy) and mechanisms affecting (e.g.stratification) turbulence and turbulent transport in the atmosphere
• has good understanding of the theoretical and mathematical models and methods commonly used in atmospheric boundary layer research
• has a general knowledge of instrumentation and measurement principles in use for atmospheric boundary layer research

Skills

The student

• can explain and apply the basic general concepts in boundary layer meteorology
• is able to explain the main steps in the derivation of the budget equation for turbulent kinetic energy (TKE)
• can explain the relevant terms in the budget equation for turbulent kinetic energy
• can sketch and explain the vertical profiles of turbulent fluxes under different stability conditions and their importance for and interaction with the corresponding profiles of the mean parameters
• is able to explain the turbulence closure problem and different local and non-local methods to overcome it
• can choose relevant instrumentation and method to measure and analyze data to describe the key turbulent parameters and fluxes in the atmosphere
• can calculate and interpret key turbulent parameters using direct and indirect methods from a corresponding data set
• can estimate and discuss mixing and spreading of properties vertically and laterally under different forcing conditions
• can explain the spectral behavior and characteristics of atmospheric turbulence

General competence

The student

• has a general overview over theoretical and experimental approaches in boundary layer meteorology
• can assess the role of turbulence for the structure and dynamics of the atmospheric boundary layer and its importance for the transport of matter and energy on different scales
• can assess the importance of turbulence and turbulent exchange for meteorology and climate
• can analyze and interpret state-of-the art boundary layer measurements
• can work in a team and produce and give a short presentation of this data analysis