Abstract
AbstractWind 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. Copyright © 2009 Royal Meteorological Society
Highlights
A complex dynamical structure governs the West African atmosphere during the boreal summer
Wind profiler observations over Niamey and Djougou completed by European Centre for Medium-Range Weather Forecasts (ECMWF) analyses allow us to study the intraseasonal variability of the low and mid-troposphere at several scales, down to the diurnal scale
450 km apart, give a continuous view of the African Easterly Jet (AEJ) from two different perspectives, since the AEJ crosses the latitude of Djougou during the monsoon onset and retreat, whereas it is set over the latitude of Niamey during the active monsoon phase
Summary
A complex dynamical structure governs the West African atmosphere during the boreal summer. The West African Monsoon (WAM) is a recurrent large-scale pattern arising from the thermal contrast existing between the warm African continent and the cooler southern Atlantic Ocean during the boreal summer. This meridional circulation results in the transport of moist air from the Atlantic and dry air from the Sahara. We use the continuous measurements of Ultra High Frequency (UHF) and Very High Frequency (VHF) wind profilers deployed respectively at two sites, Niamey in Niger and Djougou in Benin, during AMMA, to study the intraseasonal variability of the low and mid troposphere and their interactions down to the diurnal time-scale.
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