Study of the stably stratified atmospheric boundary layer through Large-Eddy simulations and high resolution mesoscale modelling

Principal Investigator

Dr Joan Cuxart Rodamilans
Departament de Fisica
Universitat de les Illes Balears
Carretera de Valldemossa, km 7.5
07071 Palma de Mallorca
SPAIN

Joan.Cuxart@uib.es

Other researchers: Maria Antonia Jimenez Cortes , Antoni Mira Navarro, Daniel Martínez Villagrasa

Project description

The study of the Stably stratified atmospheric Boundary Layer (SBL) is of great interest for a large number of applications, especially those related to the numerical modelling of the atmosphere, but also for specific subjects such as frost or fog formation, with important impact on sectors like the agriculture, the aviation or the road security.

The representation of the SBL in the numerical models of the atmosphere is far from optimal. When the stability of stratification is weak (presence of moderate wind and/or clouds) the turbulence is more or less continuous and most of the current schemes are able to simulate the main characteristics of this regime. But for moderate to strong stability (usually in weak pressure gradient conditions and clear skies), the turbulence is weak, sporadic or intermittent, many times of elevated origin, and highly dependent on the physiographical characteristics of every particular location. Most of the parameterisation schemes fail to properly represent these features. Since the SBL is very common in large areas of the planet, in areas of formation of air masses, its proper representation is crucial for a good modelling of the atmosphere, both for NWP purposes or for climate studies.

This project is double fold. Its first aspect is to perform Large-Eddy simulations (LES) of the SBL, of increasing complexity. The shear-driven SBL by the surface rugosity has already been explored during the first year of project using a subgrid-scale scheme based on the isotropy of the subgrid eddies. It has been shown that such an approach is only valid for weakly stable stratification, through comparison to available experimental data.

The current LES work consists on studying the main features of the nocturnal low-level jets and to focus on thermal inversions. To further increase the complexity of the LES of the SBL, the radiation scheme and the humidity have been considered in the runs. Both contributions are important in the surface cooling, especially under strong stratified conditions. Further tests on the surface characteristics are still needed to better capture the observed surface layer. On the other hand, a simple case of an LES of a Convective Boundary Layer (CBL) is performed to estimate the amount of mixing through an inversion by means of the comparison to data from tank experiments (with the collaboration of the Technical Delft University, where there is a saline water tank). From the preliminar runs, it is found that the advection scheme of the model has to be revised to better reproduce the entrainment zone.

The second aspect of this project is to explore the real SBL, over complex terrain. The inhomogeneities of the terrain are among the main factors producing coherent structures in stably stratified conditions. Gravity currents are associated to terrain slopes, excitation of gravity waves is very often related to obstacles in the terrain encountered by the air flow. In this project, simulations of real cases at the basin scale are performed. The default domains selected are the Balearic Islands and the Duero/Douro basin. These two areas are well defined hydrological basins, where mesoscale organisation takes place and modulates the SBL behaviour. Up to now, two more domains will be studied: the Hudson valley (UEA) and the Ebre river basin (north-east of Spain). Besides, the outputs are validated against observations: satellite images and automatic surface weather stations. The PDFs methodology is also used to validate the fields. Moreover, the model outputs are used to study the katabatic winds developed in both domains and the findings complement the information obtained from the LES results and available observations.

All activities are made with the French community model Meso-NH, installed at the ECMWF. A version of it also exists at the University of the Balearic Islands, but with much less computational capacity. It is used for preliminary tests at low resolution.

For more details, please refer to the latest progress report.

Additional information

This is a new project, started in 2002.

Would accept support for 1 year only, if necessary.