Life cycle of Sahelian mesoscale convective cloud systems

Abstract

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 km2. 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.

A meteorological and chemical overview of the DACCIWA field campaign in West Africa in June–July 2016

“…Mesoscale convective system (MCS) identification: the evolution of deep convective clouds is monitored by applying an overlap-based tracking algorithm (Mathon and Laurent, 2001;Schröder et al, 2009;Williams and Houze, 1987) to the 15 min infrared data of SEVIRI. In two successive images, cold cloud regions are identified first and then connected in time by determining the highest accordance with respect to area, area overlap and spatial translation.…”

“…∼ 900 km 2 ). The former criterion is widely used as a proxy for deep precipitating convection in tropical regions, whereas the latter excludes convective systems with low contribution to total cold cloud cover (Mathon and Laurent, 2001;Schröder et al, 2009 (Stein et al, 2015) driven by GDAS (Global Data Assimilation System) winds. Both models were run for 24 h with continuous emissions of the tracer.…”

On the spatial coherence of rainfall over the Saloum delta (Senegal) from seasonal to decadal time scales

“…Intermittent regional modes have been identified, which represent an envelope modulating the occurrences of mesoscale convective systems Fritsch, 1993, 1997;Hodges and Thorncroft, 1997;Mathon and Laurent, 2001). They are controlled on the one hand by internal atmospheric dynamics; in particular the development of synoptic scale African Easterly Waves (AEW; Reed et al, 1977;Duvel, 1990;Thorncroft and Hoskins, 1994a,b;Diedhiou et al, 1999;Kiladis et al, 2006) or synoptic scale Kelvin waves (Mounier et al, 2007).…”

Changes in Vegetation and Rainfall over West Africa during the Last Three Decades (1981-2010)

“…Fensholt and Rasmussen [4] have shown that vegetation is mainly influenced by rainfall in semiarid areas, i.e., where annual rainfall amount is around 150 mm·yr −1 . The regional rainfall variability has a strong seasonal link with convection processes [5]- [7] and the thermal gradient between the ocean and the continent [8]. Many research programmes are held over West Africa these last years in order to enhance the knowledge about the West African Monsoon (WAM).…”