The diurnal cycle of lower boundary-layer wind in the West African monsoon
Résumé
Measurements of lower boundary-layer winds collected from four Sonic Detection And Ranging (SODAR) systems during the African Monsoon Multidisciplinary Analysis (AMMA) are presented. The SODARs were located in a mesoscale network in the vicinity of Niamey, Niger, in the period 31 May to 17 August 2006; from this network 41 days of good measurements have been obtained from at least one of the SODARs, while there are several days in which two or more SODARs produced good simultaneous data. The average variation of the winds in the lower boundary layer is here presented, as a function of height and of time of day. The results confirm previous studies of the diurnal cycle of winds, and quantify the variation of this diurnal cycle with height at low altitudes. On many nights, a distinct nocturnal low-level jet was observed, at 200–400 m altitude. In profiles averaged over all of the sample days, the peak wind shear was in the layer below 300 m, with the strongest wind shear from 0000 to 0600 UTC. In particular, the magnitude of low-level wind shear, which is a significant hazard for aviation, is shown to be strongly dependent on time of day. Strong low-level wind shear (more than 4 m s−1 per 100 m), was observed on more than 35% of days in the 0000–0300 UTC period.