Generation and propagation of African squall lines

Abstract

A study of squall lines affecting Minna, Nigeria during 1974–1976 emphasizes two particular aspects of their dynamics: that a strong jet around 650 mb is essential for their development, and that the forward edge of the squall line moves slightly faster than the maximum jet speed. A linear model of the squall line as a superposition of normal modes of an unstably stratified sheared flow sheds light on these observations. If the flow profile has a mid‐level maximum, constructive interference of normal modes gives rise to an expanding circle of convection with an arc front moving slightly faster than the maximum mean flow speed, followed by individual cells. If, however, shear is unidirectional, there is no arc front, and a line of cells is formed moving at the middle level mean flow speed. These two cases are suggested as models for tropical and mid‐latitude squall lines respectively, while a further case provides a possible model for storm‐splitting.

Tropical Squall Lines of the Arizona Monsoon

“…One of the earliest simulations of a tropical squall line was performed by Moncrieff and Miller (1976) experiments (Bolton 1984, Pointin 1985, Rotunno et al 1988, Redelsperger and Latore 1988, Nicholl. and Wei •• bluth 1988.…”

“…In the tormer case, cumulonimbus developed when the gust front moved under these clouds while in the latter, new cells were seen to torm approximately 10 km in advance of the main squall line updratt and low-level cold pool. Tripoli and Cotton (1989a,b) Wave theory has already been utilized by Raymond (1984) 24 in his wave-CISK model and by Bolton (1984) in a linear gravity wave model to simulate the formation and growth of tropical squall lines. The preceding results suggest gravity waves are produced by thunderstorms and boundary layer convection and that these waves may play an important role in both the evolution of the preexisting convection and in the development of new convection.…”

Radar characteristics of continental, coastal, and maritime convection observed during AMMA/NAMMA

“…Mesoscale convective systems (MCSs) are primary contributors of rainfall totals in West Africa (Le Barbé and Lebel, 1997;D'Amato and Lebel, 1998;Laurent et al, 1998;Mathon et al, 2002;Redelsperger et al, 2002;Fink et al, 2006), despite their small contribution to convective system population numbers (Mohr et al, 1999). Highly organized continental MCSs often take the form of squall lines (SLs) (Hamilton et al, 1945;Eldridge, 1957;Bolton, 1984;Houze et al, 1989), while systems in the adjacent east Atlantic region are often more stratiform in nature Houze, 2003, 2006;Fuentes et al, 2008), with a transitional region for systems exiting the West African coast (Sall and Sauvageot, 2005;DeLonge et al, 2010).…”

Regional comparison of West African convective characteristics: a TRMM‐based climatology

“…This instability leads to the formation and maintenance of (often intense) MCSs (Aspliden et al, 1976;Payne and McGarry, 1977;Houze and Betts, 1981;Barnes and Sieckman, 1984;Rowell and Milford, 1993;Hodges and Thorncroft, 1997;Mohr and Thorncroft, 2006;Nicholls and Mohr, 2010). These westward-moving systems generally exhibit a linear (squall line) morphology over the continent (Hamilton et al, 1945;Eldridge, 1957;Bolton, 1984), a non-squall (amorphous) morphology over the eastern Atlantic Houze, 2003, 2006;Fuentes et al, 2008), and a transition stage upon exiting the coast (Sall and Sauvageot, 2005;DeLonge et al, 2010).…”