Mesoscale Convective Complexes over Southern Africa

Abstract

The maximum spatial correlation technique (MASCOTTE) is an objective and automated method developed to simultaneously determine both the structural properties and evolution (tracking) of cloud shields of convective systems. Originally designed to monitor systems over the Amazon region, this method has now been adapted for subtropical southern Africa. In this paper, a detailed climatology of 70 mesoscale convective complexes (MCCs) that occurred during the austral summer months over southern Africa during the 1998-2006 period are presented. Most MCCs are clustered along the eastern regions of southern Africa, adjacent to the warm waters of the Mozambique Channel and Agulhas Current. A few infrequent systems are found to be developing in Namibia and Botswana. The systems are found to predominantly occur during the months of November-February, with maximum activity occurring in November and December. The transition from a more midlatitude-dominated circulation to a tropical circulation over the region during the late summer leads to an uncharacteristic equatorward migration of the MCC distribution then. The analysis also suggests that there is variability in MCC frequency on monthly and seasonal time scales. Although fewer in number (about nine per season) compared to MCC populations in other regions, the systems do tend to follow the nocturnal life cycle as documented elsewhere.

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“…The automated method objectively classifies the systems within images (Augustine andHoward 1988, 1991;Machado et al 1998;Carvalho and Jones 2001;Morel and Senesi 2002;Jirak et al 2003;Zheng et al 2004Zheng et al , 2008García-Herrera et al 2005b;Li et al 2012). The half-automated method, also named the hybrid approach, usually consists of two steps: the automated identification of convective clouds followed by a subjective tracking of these systems (e.g., Durkee and Mote 2010) or automated classification of the systems followed by a visual check to verify their evolution (e.g., Blamey and Reason 2012). The automated method is undoubtedly the most efficient and objective way to classify MCSs using a long series of satellite images.…”

“…Among the most common problems is the identification of splitting and merging of MCSs as they evolve (Machado et al 1998;Carvalho and Jones 2001;Durkee and Mote 2010). Thus, in this study we propose to use a half-automated approach similar to Blamey and Reason (2012), in which systems are identified and tracked through an automated method subsequently followed by a visual inspection.…”

“…Systems are then tracked based on the maximum spatial correlation between the target system and all other systems in a consecutive image. Once a system is identified, the method calculates several properties of the cloud features such as mean and minimum cloud shield temperatures, area of the MCS and of the coldest cluster, number of cold clusters (Blamey and Reason 2012). The hybrid method used in this study is performed as follows.…”

“…They considered that the 2528C threshold can adequately represent the evolution of the storm (Kane et al 1987;Augustine and Howard 1988). This modified MCC definition has been adopted by many other studies on MCCs (Augustine and Howard 1991;Jirak et al 2003;Durkee and Mote 2010;Blamey and Reason 2012). The present study also uses the modified MCC definition to examine the MCCs (Table 1).…”

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“…Another physically plausible cause of enhancement could be linked to the cloud system development. According to Blamey and Reason (2012), the meso-scale convective storms that develop in the HPA during summer have a general initiation time of 13 -19 LST. It corresponds well to this study's afternoon maxima in temperature , and similar maxima in the relative humidity.…”