Airborne observations of the impact of a convective system on the planetary boundary layer thermodynamics and aerosol distribution in the inter‐tropical discontinuity region of the West African Monsoon

Abstract

ABSTRACT:The impact of a convective system downdraught and associated gravity current on the monsoon and harmattan flow structure as well as on aerosol vertical distribution over the Sahel and the Sahara is investigated using dropsondes and an airborne lidar. Complementary ground-based and satellite observations, as well as European Centre for Medium-range Weather Forecasts analyses are also used. The mission was conducted on 5 June 2006, in the framework of the African Monsoon Multidisciplinary Analysis Special Observing Period. The targeted area was the Saharan heat low region, and Mali in particular, over which the inter-tropical discontinuity was strongly perturbed by the convective system. To the north, away from the influence of the gravity current, the atmosphere exhibited a two-layer structure, with a well mixed 3 km deep internal boundary layer capped by a stable layer 2-2.5 km deep layer referred to as the Saharan aerosol layer (SAL). The aerosol loading in the internal boundary layer (as determined by lidar) was observed to be much less than in the SAL above. Plumes of dust generated by strong low-level winds west of the Hoggar as observed in Spinning Enhanced Visible and Infra-Red Imager (SEVIRI) images, and possibly injected in the upper SAL through orographic lifting, contributed to the dust loading of the elevated SAL over much of the domain observed by the lidar. In the northernmost part of the domain investigated, the depth of the SAL top was decreased as the result of subsidence enhanced by the presence of an elevated jet associated with the monsoon trough and/or related to the return circulation of plain-mountain wind system involving the Atlas range. Associated with the gravity current passing over dust sources, a large aerosol plume was observed by means of lidar measurements and SEVIRI imagery. The plume was seen to reach heights of approximately 3 km above ground level a few tens of kilometres behind the leading edge. Lidar measurements suggest that a fraction of the dust lifted by the gravity current is mixed into the SAL, and in some instances injected above the SAL.

Disagreements in Low-Level Moisture between (Re)Analyses over Summertime West Africa

“…At this stage SEVIRI imagery shows that there is a large northward surge of moist air behind the MCS (dark blue showing elevated humidity and magenta regions indicating dust lifted by a cold pool). This modification of the low-level moisture distribution by an MCS cold pool is consistent with the event that occurs between 8 and 10 June 2010 studied in Roberts and Knippertz (2014), as well as cases discussed in Cuesta et al (2010) and Flamant et al (2007). The region of anomalously moist air over the Sahara remains over parts of Mali, Mauritania, and Algeria and by DAY 2 0.75 a group of convective cells is produced as far north as 258N (Fig.…”

“…8). Large MCSs produced close to the ITD are able to significantly advance the position of the monsoon flow by the generation of large evaporatively generated cold pool outflows (Flamant et al 2007;Marsham et al 2008). However, it is also the case that meteorological features that favor the development of MCSs can be responsible for similar northward surges of the monsoon flow.…”

“…Processes that are known to influence the position of the ITD include interactions with the extratropics Vizy and Cook 2009;Knippertz and Todd 2010;Roehrig et al 2011; Denotes Open Access content. Roberts and Knippertz 2014), changes to the strength of the SHL (Parker et al 2005), African easterly waves (AEWs) on the African easterly jet (AEJ) (Berry and Thorncroft 2005;Couvreux et al 2010;Bain et al 2011), convectively generated cold pools (Flamant et al 2007;Cuesta et al 2010;Marsham et al 2008Marsham et al , 2013bGarcia-Carreras et al 2013;Roberts and Knippertz 2014), and eastward-propagating convectively coupled Kelvin waves (Mounier et al 2007;Mekonnen et al 2008;Mera et al 2014). Increased boundary layer moisture south of the ITD, in regions where the monsoon flow is deep, allows for the production of mesoscale convective systems (MCSs).…”

“…MCSs in the northern Sahel and southern Sahara are capable of lifting large amounts of dust through the production of convective cold pools known as haboobs (Sutton 1925;Hamilton et al 1945;Flamant et al 2007;Marsham et al 2008;Roberts and Knippertz 2012;Marsham et al 2013b). The monsoon front has also been implicated in dust-lifting processes (Bou Karam et al 2008;Marsham et al 2008;Burton et al 2013).…”

“…Flamant et al (2007) showed how a cold pool outflow from an MCS formed the leading edge of the monsoon (ITD) for one case during the AMMA field campaign. Comparing simulations with parameterized and explicit convection and observations, Marsham et al (2013a) show that the representation of convection has impacts on the entire WAM and its diurnal cycle, and that cold pools contribute a substantial part of the monsoon flow in explicit simulations, but are missing when convection is parameterized.…”