Large-eddy simulations of the diurnal cycle in the atmospheric boundary layer and the impact of land surface conditions and geostrophic forcings

In this study we use Large Eddy Simulation (LES) of a multi-day evolution of the ABL in comparison with observations of the CASES99 field campaign. The results obtained from these simulations are used as a benchmark for the single column model investigations for GABLS2 (see also submitted abstract by Svensson and Holtslag) in addition to the observations. Subsequently we analyze the impact of geostrophic forcing and surface boundary conditions. Simulations are performed with several combinations of surface boundary conditions (temperature and heat flux) and geostrophic forcings (constant, time-varying, time and height varying). The main aim towards using different setups was to assess the role of surface flux boundary condition and geostrophic forcing in determining the degree of agreement with observations. To the best of our knowledge, this was the first time LES was applied towards simulation of ABL flow over multiple days to allow detailed comparison with the well-documented observational dataset of CASES99. Besides of detailed results of LES in comparison with the CASES99 observations and the ensemble mean of single column models for GABLS2, also lessons for future set up of comparison of LES with field data will be given.