Life cycle of Sahelian mesoscale convective cloud systems

This paper provides an eight-year high-resolution climatology of Sahelian mesoscale convective systems (MCSs) during the summer. MCSs are defined as convective cloud clusters larger than 5000 km(2). They are extensively tracked from METEOSAT full-resolution infrared images (time resolution 0.5 h and spatial resolution about 5 km). The method enables every MCS to be tracked throughout its entire lifetime. For each time step, the MCS location and its morphological and radiative characteristics are computed for three different brightness temperature thresholds. The methodology is presented, evaluated and compared with previous studies using low-resolution data. Statistical MCS distributions, diurnal cycle and spatial variability of MCS characteristics are analysed on the basis of this high-resolution tracking. It is shown that a few large and long-lived cloud clusters contribute most of the total cloud cover. Sahelian cloud clusters propagate westward at a greater speed when very deep convection is well developed. The diurnal organization of the convection has been analysed, and has proved that the merging of MCSs is partly explained by the actual merging of independent convective entities, whereas the splitting of MCSs is mostly associated with weakening of convection. The importance of mesoscale convective complexes for the total MCS coverage has also been studied.