Abstract
A radar-based analysis of the structure, motion, and rainfall variability of westward-propagating squall-line mesoscale convective systems (SLMCSs) in Niamey, Niger, during the African Monsoon Multidisciplinary Activities (AMMA) 2006 special observing period is combined with an analysis of 700-mb (hPa) winds and relative vorticity to study the relationship between SLMCSs and African easterly waves (AEWs). Radar results show that SLMCSs were the most important rainmakers in Niamey and accounted for about 90% of the rainfall despite being present less than 17% of the time. Analysis of the 700-mb synoptic-scale flow revealed that during the 2006 West African monsoon season the African easterly jet vacillated between about 10° and 15°N on time scales of 1–2 weeks. AEWs followed the jet as it vacillated north and south, thereby producing two preferred paths for AEWs propagating past Niamey’s longitude, a northern track along 8°–16°N and a southern track along 2°–6°N. It was found that Niamey SLMCSs occurred westward of the trough of AEWs propagating along either track. The properties of SLMCSs must then be placed in the context of their location relative to these two AEW tracks, rather than in the trough and ridge pattern of a single AEW track. Radar analysis further indicated that although the total amounts of rainfall produced by SLMCSs occurring in both African easterly jet latitude regimes were similar, significant structural differences occurred between the two groups of systems. SLMCSs that formed to the west of AEW troughs propagating along the northern track had a significantly larger mean stratiform rain fraction in an environment of lower convective available potential energy when compared with the SLMCSs that occurred to the west of the troughs of AEWs in the southern track. The authors conclude that AEWs that propagated farther north provided a more favorable environment for stratiform rain production in Niamey SLMCSs than those AEWs located farther south. These results may be helpful to studies of the two-way interaction between AEWs and convection in West Africa.
Anomalous Atmospheric Circulation Associated with Recent West African Monsoon Rainfall Variability