The second GABLS experiment—what did we learn?

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. In total, 35 singel-column model results are analyzed and when possible compared with observations. The model closures range from first order to various turbulent kinetic energy closures and includes research, operational weather forecast and global climate models. There is a large variation in how the schemes represent the diurnal cycle and the vertical structure of the PBL. The results of the experiment will be discussed and also some discussions on the lesson we learned of how to set up intercomparison projects.