The impact of a mesoscale convective system cold pool on the northward propagation of the intertropical discontinuity over West Africa

The interaction between a mesoscale convection system cold pool and the intertropical discontinuity (ITD) is investigated on two consecutive days (5 and 6 June 2006), in the framework of the African Monsoon Multidisciplinary Analysis Special Observing Period (SOP 1a). Satellite imagery and surface meteorological observations are used to track the movement of the ITD prior to and after its interaction with the cold pool. The impact of the northward propagating cold pool on the Sahelo-Saharan planetary boundary layer (PBL) thermodynamics and aerosol vertical distribution is analyzed by means of airborne and ground-based measurements over the two-day period. Complementary European Centre for Medium-range Weather Forecasts (ECMWF) analyses are also used. The influence of the cold pool persists as a coherent dynamic and thermodynamic structure over these two study days, influencing a front at least 1000 km long and approximately 200 km wide, spreading across southern Mali. Dropsonde measurements revealed that the cold pool was bringing moisture quite far north over the northern Sahel and to the southern fringes of the Sahara, as well as cooling the desert surface. As a result, the vertical development of the Sahelo-Saharan PBL was found to be delayed on 6 June compared with 5 June. Further north, over the part of the Sahara not impacted by the propagating cold pool, the airborne measurements suggest that the large amounts of dust in the upper part of the Saharan aerosol layer, as observed by lidar, may also have had an impact on the Saharan PBL, by reducing the insolation in this region. The airborne observations were also used to assess the quality of the ECMWF analyses at 1200 UTC on 5 and 6 June, as well as +36 h, +24 h and +12 h forecasts valid at 1200 UTC on 6 June. A quantitative improvement was seen in the 6 June forecast valid at 1200 UTC once information relevant to the propagation of the ITD was assimilated, i.e. for the +24 h and +12 h forecasts. The analyses were able to reproduce the northward surge of moisture associated with the cold pool, suggesting that ECMWF analyses may be used to analyze the role of propagating density currents on the so-called monsoon onset mechanism.