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Phenomenology of Sahelian convection observed in Niamey during the early monsoon
Author(s) -
Dione C.,
Lothon M.,
Badiane D.,
Campistron B.,
Couvreux F.,
Guichard F.,
Sall S. M.
Publication year - 2013
Publication title -
quarterly journal of the royal meteorological society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.744
H-Index - 143
eISSN - 1477-870X
pISSN - 0035-9009
DOI - 10.1002/qj.2149
Subject(s) - convection , convective available potential energy , convective inhibition , atmospheric convection , free convective layer , climatology , atmospheric sciences , geology , deep convection , monsoon , convection cell , troposphere , meteorology , environmental science , combined forced and natural convection , natural convection , physics
Abstract This study aims to achieve a better understanding of the initiation of deep convection in the Sahel by using the African Monsoon Multidisciplinary Analyses (AMMA) dataset. Based on the Massachusetts Institute of Technology (MIT) radar, wind proler, satellite data, surface ux and meteorological stations, we have characterised the atmospheric convection which occurred over Niamey during the onset period of the monsoon. From 6 to 31 July, radar reectivity elds combined with brightness temperatures were used to classify the type of convection observed each day within a 50 km radius of the MIT radar location. Four types of convection have been identied: fair weather (FW) with a clear sky throughout the entire day, shallow convection (SH), afternoon locally initiated deep convection (LC), and propagating deep convection (PC). Subsequently, the mechanisms responsible for the initiation of local deep convection were investigated. Neither early morning convective available potential energy nor the convective triggering potential allowed the onset of local deep convection to be predicted correctly. In eect, they were both favourable to deep convection most of the time, while convective inhibition was typically quite large. Our results show that the daytime growth of the atmospheric boundary layer needed to be sucient for local deep convection to occur during that period. Convergence lines, which grew within the morning clear‐air roll organisation, were found to be precursors of local deep convection. Classes FW, SH and LC ultimately behaved quite similarly, with notable convergence in the lower troposphere, but FW showed smaller boundary‐layer growth, and FW and SH classes revealed a signicant divergence above the boundary layer. Most cases of LC generated a circular gust front. These density currents almost always generated new convective cells.