Intense convective systems in West Africa and their relationship to the African easterly jet

Abstract

SUMMARYFor May-September 1998, convective systems in West Africa were identified from observations by the Tropical Rainfall Measuring Mission satellite Microwave Imager at 85 GHz. Using re-analysis data, the 10-day average position of the African easterly jet (AEJ) was diagnosed from the 700 hPa zonal winds. The distance from each convective system's centroid to the axis of the AEJ was calculated. Each convective system's minimum brightness temperatures were ranked so that intense convective systems were defined as those in the 10th percentile or lower. The weak (>10th percentile) and intense convective systems were represented statistically as two separate populations, the weak by the skewed Gumbel distribution and the intense by the normal distribution. From May to August, the peak in activity of weak convective systems remained south of 10 • N but shifted east of 10 • E. The peak in activity of the intense convective systems followed the seasonal migration of the AEJ northwards and became increasingly separate from the peak in activity of weak convective systems. The majority of both weak and intense convective systems occurred within 0.50 • of high terrain. The high convective available potential energy, high-shear AEJ environment in the vicinity of high terrain appeared to have the greatest probability of generating intense convective systems in the study area.

A note on long‐term variations of the African easterly jet

“…They found that the forcing from the high terrain and shear associated with the AEJ would favour producing intense convective systems. Laing et al (2008Laing et al ( ) employed 5 years (1999Laing et al ( -2003 of digital infrared imagery for May to August and confirmed the results of Mohr and Thorncroft (2006).…”

“…They have also found a negative meridional potential vorticity (PV) gradient, which is consistent with theoretical expectations. Mohr and Thorncroft (2006) examined the relationship between intense convective systems and the AEJ during May to September 1998. They found that the forcing from the high terrain and shear associated with the AEJ would favour producing intense convective systems.…”

“…These have generally used brief time periods, sometimes only 1 year (e.g. Mohr and Thorncroft, 2006), have examined only the long-term mean (e.g. Nicholson and Grist, 2001), or have analysed the jet's meridional cross section across a single longitude (e.g.…”

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

“…In agreement with Guy et al (2011), AEW regimes produced an increased occurrence of larger systems in the ConNE, ConNW and coastal regions. Environmental factors such as strong lowlevel vertical wind shear aid the organization of intense convective systems (Frank, 1978;Rowell and Milford, 1993;Johnson et al, 2005;Mohr and Thorncroft, 2006;Nicholls and Mohr, 2010) over land. Stratiform precipitation is formed through the decay of convective regions and broad mesoscale ascent in the associated stratiform region (Zipser, 1969;Houze, 1977;Houze et al, 1989).…”

“…Surface westerly winds are overlaid by the AEJ, with potentially unstable conditions at low levels. 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).…”