GABLS

The GEWEX Atmospheric Boundary Layer Study

GABLS is one of the activities within the GEWEX Modeling and Prediction Panel. GABLS was originally initiated in 2001.
The chair of GABLS is Bert Holtslag.

The overall objective of the GEWEX Atmospheric Boundary Layer Study (GABLS) is to improve the understanding and the representation of the atmospheric boundary layer in regional and largescale climate models. GABLS aims to provide a platform in which scientists working on boundary layers at different scales will interact. Such activity is important in itself and also very relevant for other activities in GEWEX, and more generally for the activities within WCRP and the International Geosphere-Biosphere Program (Holtslag and Randall, 2001).

The first focus of GABLS is on the representation of the stable atmospheric boundary layer (SBL). It appears that much of the warming predicted by climate models occurs during stable conditions over land (either in winter or at night). At the same time, it is realized that the understanding and parameterization of the SBL is still poor.

(Figure courtesy of Anton Beljaars, ECMWF)

As an example, the figure shows the difference in the temperature at a height of 2 meters for January 1996 as calculated from two model runs with the same forcings, but with (slightly) different stability functions in the mixing scheme of the ECMWF model in stable conditions.
The scheme with more mixing, leads to higher temperatures over continental areas in winter. To obtain the same synoptic evolution in the two simulations, gentle relaxation towards the analysis is applied above 500 m above the surface. Also, the same prescribed values for the sea surface temperature are used . Notice that the differences in the mean temperatures over the land areas can take values up to 10K! To review our understanding and to discuss future directions on stable boundary layers, a workshop was held at the European Center for Medium-Range Weather Forecasting (ECMWF) in Reading, UK, on March 25-27, 2002. The workshop agenda covered the following topics: Modeling and parameterization experiences at the large-scale modeling centers; progress in theory and understanding of SBLs; Large-Eddy Simulation (LES); and observational data sets (Cabauw, CASES, ARM, Lindenberg, and others).