Energy, water and matter are transported from the earth s surface into the atmosphere by turbulent, convective and mean motion. In weather situations with divergent flow - high pressure systems - a boundary layer forms which restricts the transport to the lowest one or two kilometers of the troposphere by a temperature inversion. The realistic simulation of the atmospheric boundary layer (ABL) in models is therefore essential, in particular in climate models.

In a first step simulations of the three-dimensional hydrostatic regional model REMO in the forecast mode are compared to the observed ABL structure. The measurements have been performed with a ground-based Differential Absorption Lidar System (DIAL) for water vapour during three measuring campaigns at an agricultural site in Eastern Germany. Time-height-sections of water vapour from 06 to 18 UT and up to 3000 m above ground are available for 25 days. For special episodes - undisturbed ABL evolution and disturbed ABL by synoptic-scale features - mean water vapour profiles, time-height sections and turbulent water vapour fluxes within the ABL are compared.

- The onset and growth of the ABL in the morning is realistically simulated.

- The height of the ABL and the humidity gradient at the top of ABL are naturally not precise, depending on the vertical resolution of the model at the respective height level.

- But ABL-height determination from DIAL which is located at a fixed place is uncertain, too, because intervals of elevated and subsided top heights of unfavourably some 10 minutes duration may give unrepresentative averages.

- Methods of comparing ABL-height and gradients in models and observations are discussed.

- The entrainment of dry air from above into the ABL is investigated.

- The influence of synoptic-scale disturbances on the simulated ABL appears to be realistically reproduced.

Since ground-based lidar measurements give quasi-continuous time-height-sections of atmospheric parameters, they are an appropriate tools for the validation of ABL simulations in climate models.