The third GABLS Single Column Model intercomparison: Unraveling interactions between physical processes in the stable boundary layer

For the third GEWEX Atmospheric Boundary Layer Study (GABLS) a case with realistic geostrophic forcings and dynamical tendencies has been set up to study the diurnal cycle over land such that a quantitative comparison with observations is possible. Emphasis is given to the stable boundary layer. The case has been selected from the Cabauw data archive and shows a clear decoupling at sunset, a well-defined inertial oscillation resulting in a low-level jet during the subsequent night, and a well-defined morning time transition. In total 18 state of the art single column models from various research groups participated in the intercomparison study. Comparison of the model results shows a large variation in night time minimum temperature. In fact, most of the models attain lower temperatures than observed. Also a large variation in wind speed maximum at 200 m is observed. All models underestimate this wind maximum.

At previous BLT and other meetings we already introduced the set-up of the case and the variations in model results. In order to explain the relatively large differences among model results, here our aim is to unravel the interactions between the dominant processes in this stable boundary layer, i.e. turbulent mixing, radiation and coupling to the land surface. To start with, a characterization of the models has been made in terms of these three dominant processes. We show that relations can be found between parameters that define the strength of each process relatively independent from the other processes. Subsequently, we study the parameters that are the net result of the interacting between the processes, like 2 m temperature, 200 m wind and boundary-layer height. By using the model characterizations and by using sensitivity runs of one model, we are able to quantify the relative importance of each process in determining the behavior of these parameters. For example, it is shown that variation in the coupling of the boundary layer to the land surface among the models plays a larger role than the spread in turbulent mixing efficiency in explaining the observed spread in 2m minimum temperature.

References:

Bosveld F.C., P. Baas, E. van Meijgaard, E.I.F. De Bruijn, G.-J. Steeneveld and A.A.M. Holtslag (2012). The third GABLS intercomparison case for model evaluation, part A: Case Selection and Set-up. In preparation for Boundary Layer Meteorology.

Bosveld F.C., P. Baas, G.-J. Steeneveld and A.A.M. Holtslag et al. (2012). The third GABLS intercomparison case for model evaluation, part B: Single Column Model results and Process Understanding. In preparation for Boundary Layer Meteorology.