The diurnal cycle of lower boundary‐layer wind in the West African monsoon

Abstract

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. Copyright

Interactions of atmospheric gases and aerosols with the monsoon dynamics over the Sudano-Guinean region during AMMA

“…The meridional wind is also presented as color shading for the NLLJ intensity. The first period (8 June at 23:00 UTC and 9 June at 11:00 UTC) corresponds to a classical monsoon case, often observed and described in the literature (Abdou et al, 2010;Lothon et al, 2008). At night, surface pollutants are concentrated in a shallow layer (less than 200 m), corresponding to nocturnal surface layer and to the lowest part of the NLLJ (represented by the dark blue shaded area in Fig.…”

The West African Sahel: A Review of Recent Studies on the Rainfall Regime and Its Interannual Variability

“…It lies generally within the layer 200 to 400 m above the surface [146,147]. Studies at Niamey, Niger (c. 14 ∘ N) and at Nangatchori, Benin (c. 10 ∘ N) showed that this feature tends to occur throughout the year.…”

Formation and Maintenance of Nocturnal Low-Level Stratus over the Southern West African Monsoon Region during AMMA 2006

“…The NLLJ is a typical feature of the stable nocturnal boundary layer, which was first theoretically explained by Blackadar (1957) using ideas of inertial oscillations [see also recent refinement by van de Wiel et al (2010)]. NLLJs in the WAM region have been documented and discussed by Parker et al (2005), Lothon et al (2008), Abdou et al (2010), Bain et al (2010), Schrage and Fink (2012), and others.…”