Research on Stable Boundary Layers
The boundary layer is the interface between the earth surface and the free atmosphere. As such, it is a key component of the climate system. Energy fluxes of momentum, heat and moisture couples the atmosphere to the surface in this layer. Depending on daytime, latitude, and other conditions, its depth ranges between a few metres (polar night in Antarctica) to a few kilometres (Sahara during mid-day). As many processes in this layer are governed by turbulence, it is very challenging to represent them appropriately in coarse resolution NWP and climate models.
Energy fluxes in the Stable Boundary Layer
Turbulence is generally to small to resolve it explicity in coarse-resolution models. Most current NWP and climate models currently represent the fluxes in the boundary layer with a relatively straightforward and simplified bulk parametrisation (e.g. Louis et al., 1982). It is however known that there are a range of problems with this simple parameterisation. Effectively, it can lead to a de-coupling between the earth surface and the free atmosphere under very stable conditions. Stable boundary layers often persist at high latitudes and during polar night.
In this work we examined how predicted energy fluxes according to the parameterisation of Zilitinkevich et al., 2002 compared to measurement data from the FINTUREX field campaign. During this campaign, which took place in summer 1994 on the Ekstrom ice shelf near the German Neumayer station, eddy correlation measurements and tower measurements were carried out for a period of about 1 month.
Figure 1: Time series of the stratification at the Neumayer Station, Antarctica, during several days of the FINTUREX experiment. Gray: stable stratification, blue: unstable stratification.
Figure 1 shows a temporal sequence of the stability profile in the lowest 10m above the surface during the FINTUREX measurement campaign. As can be seen from the gray colours, the stable stratification in the middle and upper surface layer becomes neutral to unstable (blue colours) with the diurnal cycle. In the lowest half metre above ground, however, stable stratification persists. This persistent near-surface inversion layer was confirmed by other studies in Greenland, and shows some similarities with stability profiles above the ocean. The inversion layer effectively decouples the heat fluxes from the surface temperature. It could therefore be important to include this phenomenon in energy flux parameterisations for high latitudes.
Relevant publications
- Sodemann, H. and Foken, T., 2005: Special characteristics of the temperature structure near the surface, Theor. Appl. Climatol.,80(2-4),81-89,doi:10.1007/s00704-004-0092-1. [Abstract, Paper, PDF, 500KB]
- Sodemann, H. and Foken, T., 2004: Parameter estimation for an extended theory for stable atmospheric boundary layers, Quart. J. Royal Meteorol. Soc.,130(602A),2665-2672. [Abstract, Paper, PDF, 190KB]
- Sodemann, H.,2002. Evaluation of a parameterisation for turbulent fluxes of momentum and heat in stably stratified surface layers. Diploma Thesis, Department of Micrometeorology, University of Bayreuth, Germany, 98 pp. [ PDF, 3MB]