Wind profiler analysis of the African Easterly Jet in relation with the boundary layer and the Saharan heat-low
Résumé
Wind profiler measurements in Niger and Benin during the African Monsoon Multidisciplinary Analysis project are used to study the intraseasonal variability of the low and mid troposphere at several time-scales. We focus on the African Easterly Jet (AEJ) and its interaction with the Saharan Heat-Low (HL) and the planetary boundary layer (PBL). We find a pronounced diurnal cycle of the AEJ, characterised by a decrease of wind speed during the afternoon, reaching a minimum at 1800 UTC of about 15-20% of the daily average during the pre-onset period. This decrease is out of phase with the HL intensity, but in phase with the daytime turbulent mixing associated with the PBL. The interaction between the PBL and mid-troposphere is likely responsible for this daily decrease of the AEJ. During the transition periods (dry to wet or wet to dry), the HL seems to govern the AEJ; however, slightly before the monsoon onset, it has no direct influence on the jet. During that time, we find smaller AEJ wind speed for deeper PBL, as found at the diurnal time-scale. This is consistent with the still large surface heating at that time, which favours deep PBL growth, with a top inversion often higher than the shear layer between the monsoon and the easterlies. After the monsoon onset, deep convection, African Easterly Waves (AEWs) and dry intrusions make the synoptic environment complex and blur the interaction between AEJ and PBL. We still find weaker AEJ for deeper PBL, but likely without a direct connection between them.
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