Observation of the Diurnal Cycle in the Low Troposphere of West Africa

Abstract

The authors give an overview of the diurnal cycle of the low troposphere during 2006 at two different sites, Niamey (Niger) and Nangatchori (Benin). This study is partly based on the first observations of UHF wind profilers ever made in West Africa in the context of the African Monsoon Multidisciplinary Analysis (AMMA) project. Also used are the radiosoundings made in Niamey and ground station observations at Nangatchori, which allow for the study of the impact of the dynamics on the water vapor cycle and the turbulence observed at the ground.Profiler measurements revealed a very consistent year-round nocturnal low-level jet maximal around 0500 UTC and centered at 400-m above the ground, with wind speed around 15 m s Ϫ1 . This jet comes either from the northeast during the dry season or from the southwest during the wet season, in relation with the position of the intertropical discontinuity. The radiosoundings made in Niamey highlight both the role of the nocturnal jet in bringing water vapor from the south during the night when the intertropical discontinuity has reached the vicinity of the considered area at the end of the dry season and the role of the daytime planetary boundary layer in mixing this water vapor within a larger depth of the troposphere. The planetary boundary layer processes play a large role in the diurnal cycle of the position of the intertropical discontinuity itself.The observations of turbulence made at the ground in Nangatchori showed that the best signature of the nocturnal jet close to surface can be seen in the turbulent kinetic energy and skewness of the air vertical velocity, rather than on the mean wind itself. They reveal the downward transport of momentum from the jet core aloft to the surface.

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.…”

Parameterization of convective transport in the boundary layer and its impact on the representation of the diurnal cycle of wind and dust emissions

“…hel (Parker et al, 2005;Lothon et al, 2008;Guichard et al, 2009;Schepanski et al, 2009). This maximum is associated with the low-level jet which forms at a few hundred meters above the surface after sunset, when the near-surface boundary layer turbulence collapses (see e.g., Bain et al, 2010;Gounou et al, 2012;Fiedler et al, 2013).…”

Evening evapotranspirative cooling in relation to vegetation and urban geometry in the city of Ouagadougou, Burkina Faso

“…The wind system creates winds from the southeast in the central parts of Ouagadougou, while a regional wind blows from the northeast. At times the wind at the surface and also the atmospheric stability appear to be influenced by a nocturnal low-level jet (NLLJ) such as the one centred at 400 m above ground level, with wind speeds of around 15 ms −1 and reaching maximum speed at approximately 05 : 00 hours UTC as described by Lothon et al (2008). In November and December the NLLJ generally comes from the northeast and is present in over 90% of the nights.…”

“…One possible cause of the increase in wind speed and cloud cover during moderately stable nights could be the NLLJ over West Africa, described by Lothon et al (2008) and Schrage and Fink (2010). During NLLJ events, both the near ground wind speed and the amount of low clouds increase.…”