Life cycle of Sahelian mesoscale convective cloud systems

Abstract

SUMMARYThis 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'. 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 lowresolution 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.

Late 20th century attribution of drying trends in the Sahel from the Regional Climate Model (RegCM3)

“…In this case the June, July and August correlations are (0.77), (0.86) and (0.90) between the location of the AEJ and Sahelian precipitation anomalies. This is plausible, given that most of the precipitation in West Africa is associated with MCSs [D' Amato and Lebel, 1998], which are organized along the AEJ [Mathon and Laurent, 2001]. Thus, an AEJ in lower latitudes would increase MCSs in lower latitudes and the Sahelian region would be drier than normal.…”

The propagation and diurnal cycles of deep convection in northern tropical Africa

“…Mathon et al (2002a) found that Sahelian organized systems identified by the 233 K threshold accounted for 90% of the seasonal rainfall observed by theÉtudes des Précipitations par Satellite (EPSAT)-Niger rain-gauge network over a nine-year period. Based on rain-gauge and IR satellite data from EPSAT, 213 K was optimal for identifying the peak in rainfall rate of Sahelian convective systems (Jobard and Desbois, 1992;Mathon and Laurent, 2001). Corresponding samples of radar-and satellite-based Hovmüller diagrams for the USA and satellite-diagrams for the African domain show that all thresholds exhibit similarity in the phase speeds but the middle threshold most closely matches the radar-derived streaks in the USA ( Figure 5).…”

“…The autocorrelation function for the cloud streaks was 30 grid points long (∼6°) and 15 grid points (∼7 h) in its cosine-weighted dimension. Sahelian MCSs that occurred between 12°and 16°N have mean duration of eight hours (Mathon and Laurent, 2001). The statistics of cloud streaks with durations greater than 3 h and span greater than 300 km are compiled.…”

“…For Sahelian systems larger than 5000 km 2 , Mathon and Laurent (2001) found that the mean system speed increased with longer lifetime. The mean MCS speed was similar to the mean zonal component of the AEJ (at about 650 hPa), except to the west of 5°E, where the mean MCS zonal speed was greater.…”

“…The mean MCS speed was similar to the mean zonal component of the AEJ (at about 650 hPa), except to the west of 5°E, where the mean MCS zonal speed was greater. Desbois et al (1988) calculated average speeds of 12-19 m s −1 , but used three-hourly data, while Mathon and Laurent (2001) used 30-minute data. Barnes and Sieckman (1984) found that fast-moving cloud lines over West Africa had speeds ranging from 7 to15 m s −1 .…”