Study of the dynamics of the atmosphere in the Sahel region of Niger

This paper presents the results of a study of the dynamics of the lower level of the atmosphere in the Sahel region. The areas of wind, horizontal divergence, vorticity and vertical velocity were determined from pilot data supplied by the national meteorology observation stations and ASECNA (Africa and Madagascar Agency for Air Navigation Safety). The study was carried out as part of the summer 1980 program of ECLATS (Study of the dry tropical atmosphere boundary layer). The methods of computing the mesoscale parameters (hodographs of the wind, wind field, horizontal divergence, vorticity and vertical velocity) are set out. Movements are from east to northeast at the lower levels, but ill-defined at altitude, where a drop in wind strength and a wide variability in wind direction were observed. Examination of the vertical and diurnal variations of the wind pinpoints a morning surge at low level, a gradual rotation from northeast to east, mainly at 6 UTC, and a wind modul us maximum of around 300 metres at low altitude. This is a basic characteristic of the lower-level movements in the region. Examination of the fields of wind, divergence, vorticity and vertical velocity shows that the area of study is liable to have average vertical velocities of around 0 . 1 cm/sec. Finally, applying the study of the field of wind and vertical velocity to the potential temperature evolution equation made it possible to estimate the relative importance of the different factors involved in the heat balance of the Sahel boundary layer: local advection, turbulence and subsidence exchange. These are of the same order (104°C, s-1). These data are needed for the validation of numerical models of aerosol movements in these regions and also for the experiments which could be regarded as validating these models.