Boundary layers

Course goals

After taking this course, the student:
- Understands the various physical mechanisms that define the ABL, i.e., the layer in which the exchange of heat, moisture, and momentum with the surface of the Earth occurs at short (sub-daily) time scales.
- Is acquainted with the overall characteristics of atmospheric boundary layers (ABL), i.e., the unstable (convective, mixed) and stable (nocturnal) ABL.
- Is acquainted with the physical mechanisms that generate turbulence near the surface of the Earth (i.e., wind shear and buoyancy), and can apply that knowledge to the everyday meteorological situation.
- Is acquainted with the various statistical terms used in the description of turbulence (variance, covariance, correlation, standard deviation) and the definition of turbulent fluxes (kinematic and dynamic).
- Can apply Reynold's decomposition to the governing conservation equations to yield additional terms that describe the divergence of the turbulent fluxes of momentum, heat, and moisture.
- Understands closure through the flux-profile relationships, in which the turbulent fluxes are coupled to the vertical gradients of the average quantities.
- Understands the surface energy balance and its connection to the above terms.
- Can derive and mathematically manipulate simple theoretical models of the katabatic and convective ABL.
- Can interpret global ABL-relevant fields from atmospheric reanalyses (model with assimilated observations), using the Python programming language (or similar), and report on the results.
 

Content

This 3.75 ECTS course is concerned with the atmospheric boundary layer (ABL), roughly the lowest 10% (~1 km) of the atmosphere. The ABL is defined as the air layer which exchanges momentum, heat and moisture with the surface of the Earth at short sub-daily) timescales. We will discuss the average nature of this turbulent exchange and how it can be expressed in terms of the mean quantities (closure). We will discuss surface exchange processes (sensible and latent heat, friction) and how this influences the ABL structure. We discuss various ABL types (convective, stable, neutral) and also the katabatic boundary layer that is often observed over ice sheets and glaciers.