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Boundary layer measurements in Nangatchori, Benin were performed over the period of one full year (2006) using a novel ground-based microwave profiler and additional remote-sensing instruments. In this paper, the diurnal cycle of the ITD (Inter-Tropical Discontinuity) in the transition period between dry and wet season during the month of April is described in detail. Dry air masses from the north (Sahel) and moist air from the south (tropical Atlantic Ocean) cause very sharp temperature and humidity gradients in the low troposphere over West Africa. Continuous observations of these phenomena in terms of temperature and humidity profiles have been achieved for the first time with a high temporal resolution of less than 15 minutes. Especially the ability of the microwave radiometer to provide temperature profiles with high vertical resolution through multi-angle measurements gives detailed boundary layer information in conjunction with meteorological tower observations. Together with additional lidar ceilometer observations of aerosol backscatter the change of air masses can be seen very well. The high data availability of > 85 % allows a statistical analysis of the full month of April in which Nangatchori comes increasingly under the influence of tropical air. Thus the data set is well suited for an improved process understanding, model evaluation in a data sparse area, and possibly together with additional observations the development of improved boundary layer parameterizations for atmospheric models.