Observation of the Diurnal Cycle in the Low Troposphere of West Africa
Résumé
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.
Domaines
Planète et Univers [physics]Origine | Fichiers éditeurs autorisés sur une archive ouverte |
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