Diurnal cycle of the intertropical discontinuity over West Africa analysed by remote sensing and mesoscale modelling

The diurnal cycle of the Intertropical discontinuity (ITD), i.e. the interface at the ground between moist monsoon air and dry Harmattan air, is an important factor in the West African monsoon system. During the whole of 2006, high-resolution ground-based remote-sensing measurements were performed in the area of Djougou, Benin, which made it possible to observe the ITD and the associated sharp gradient of temperature and humidity in detail. In order to extend the point measurements to a three-dimensional view of the ITD and to enhance the knowledge of the related processes, the mesoscale atmospheric model Meso-NH was run for a case study (84 h period) in April 2006. In addition Meteosat infrared observations were used to determine the ITD position and its movement. From these observations a northward propagation of the moist air front (ITD) of 8-12 m s^-1 was calculated. The propagation speed of the front evaluated from the satellite observations was replicated by the model and the time of the front arrival over Djougou was simulated with a maximum error of 1 hour. With respect to the ground-based observations, the model was able to reproduce the dynamics and thermodynamics of the front as well as the diurnal cycle of the planetary boundary layer over the study area. This agreement suggests the use of the model to further describe processes in the lower atmosphere at high resolution.