Single column modeling of the diurnal cycle based on CASES99 data – GABLS second intercomparison project

The second GABLS (GEWEX Atmospheric Boundary Layer Study) intercomparison case was selected from the CASES99 data set. The purpose of the study is to intercompare boundary layer closures performance for a diurnal cycle in a highly controlled single column setting. To keep the simulation simple, all models were forced with the observed surface temperature and a constant geostrophic wind base on the observations. The simulations covered two full diurnal cycles and the first night plus a full diurnal cycle it is possible to compare with the observations, the rest of the simulation may be used for model intercomparison.

In this study, 23 different models so far are participating and there is a good spread in their turbulence closures, with first order models as well as models with prognostic TKE. The participants are from research groups but many operational centers have also contributed to this study. In additions, some of the models are run with both high and operational (low) vertical resolution.

The analysis shows that even for this very controlled simulation there is a large variability in the model results especially when it comes to the turbulent fluxes and the boundary layer height and the deviations from the observations are large. There is higher agreement between the models when it comes to the 2m-temperature (naturally because of the surface temperature forcing) and the 10m-windspeed and direction. However, the models in general do not show the observed increase in wind speed during the daytime. Thus, the modeled wind speed is to high during the night (due to less stability than observed) and to low during the daytime.

To further add to the information of the case, a three-dimensional simulation of the selected period was performed with one of the contributing models, COAMPS. The analysis of this simulation gives some indication of how 3-dimensional the flow situation was and what can or can't be expected by the single column models.

In addition, an effort is made by another group to make a LES simulation of this diurnal cycle. If simulations are completed in time, the results from this simulation will also be compared with the single-column model results.