Weather and climate in West Africa are determined by the pronounced contrast between tropical, moist air masses over the Gulf of Guinea in the south and the dry desert climate over the Sahara in the north. The sharp border between the two air masses exhibits a pronounced meridional annual cycle and follows the path of the sun northward. This circulation system is called "West African monsoon". In the past, the knowledge about the factors that control the monsoon and its strength was limited due to the small number of high-quality observations. Therefore, little is known about the reasons for the significant decline of annual rainfall over the Sahel area during the past 40 years which represents the most pronounced climatic signal worldwide. During the past few years, intensive atmospheric observations were performed in the framework of the international project "African Monsoon Multidisciplinary Analyses" (AMMA) in order to obtain high-quality data and to improve the process understanding. This work gathered and analyzed ground-based remote sensing observations which were performed in Benin and Niger during the AMMA field campaigns. These data give an insight into diurnal and annual cycles of atmospheric parameters, such as water vapor, temperature profiles, cloud cover, cloud liquid water content, or wind with a temporal resolution never reached before. Particular attention is paid to the atmospheric water which is recognized to be a critical parameter for many other atmospheric variables, e.g. the vertical temperature distribution, the long-wave radiation balance, and many more. The new type of observations revealed a diurnal cycle of the position of the Intertropical Discontinuity (ITD) prior to the start of the rainy season. The ITD represents the convergence zone at the surface between the dry and hot north-easterly trade winds and the moist and cooler south-westerly monsoon flow. Associated with this sharp front between the two air masses, strong water vapour changes occurred which could be observed in detail. This data set was then used for a comparison with the mesoscale atmospheric model Méso-NH which was run for a case study in April 2006. It is shown that the model reproduces the observed processes quite well, despite the low number of in-situ data which were assimilated in the model. Therefore, the model is suited to describe the processes in the lower atmospheric layers around the ITD
