The propagation and diurnal cycles of deep convection in northern tropical Africa

Abstract

ABSTRACT:The propagation and diurnal cycle of organized convection in northern tropical Africa are examined using five years (1999)(2000)(2001)(2002)(2003) of digital infrared imagery for May-August. Reduced-dimension techniques are used to document the properties of cold clouds -proxies for deep convection and precipitation. Large-scale environments are diagnosed from global analyses.Organized convection in Africa consists of coherent sequences or episodes which span an average distance of about 1000 km and last about 25 h. A substantial fraction of events exhibits systematic propagation at regional to continental scales while undergoing decay and regeneration. Episodes with 36 h duration and 1472 km span recur at a one-per-day interval. Most episodes have phase speed of 10-20 m s −1 , which is faster than most African easterly waves. Convective episodes tend to initiate in the lee of high terrain, consistent with thermal forcing from elevated heat sources. Average diurnal frequency maxima result from the superposition of local diurnal maximum with the delayed-phase arrival of systems propagating from the east. Propagation occurs with moderate low-to mid-tropospheric shear, which varies with the African easterly jet migration and West African monsoon phases. Frequent deep convection occurs with local shear maxima near high terrain. For the peak monsoon period and for 10°W-10°E, where easterly waves and convective systems are frequent, 35% of cold cloud episodes occur east of the wave trough compared with about 24% to the west. Based on the coherent behaviour of organized, propagating convection, inferences may be made regarding the prediction of precipitation beyond one or two days.

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

“…As mentioned previously, MCSs often travel faster than AEWs, and therefore life-cycle characteristics could be aliased to wave regimes other than the formative and decay stages. Despite this limitation, occurrence frequency results were consistent with previous studies (most of which concentrated on genesis/lysis of MCS with respect to AEW phase) that have found that deep convection occurs east of AEW troughs near the continental sites and west of and in the trough near the coast and Atlantic (Carlson, 1969;Payne and McGarry, 1977;Reed et al, 1977;Diedhiou et al, 1999;Fink et al, 2006;Kiladis et al, 2006;Laing et al, 2008). Fink and Reiner (2003) and Laing et al (2008), primarily using microwave and infrared observations for squall line and deep convective tracking, respectively, found a large fraction of systems were not associated with any AEW phase.…”

A nudging scheme to assimilate satellite brightness temperature in a meteorological model: Impact on representation of African mesoscale convective systems

“…Both systems are characterized by a marked diurnal cycle with development of convection after 1200 UTC, a maximum around 1800 UTC and a rapid decrease from 0600 to 1200 UTC. This is in agreement with the convection cycle evaluated from the MSG climatology over Africa presented in Laing et al (2008).…”

“…A realistic diurnal cycle is reproduced with generation of convection at 1200 UTC on each simulation day and a maximum around 1800 UTC. From CTBT maps at 1400 UTC (not shown), it is possible to observe that convection initiates on the lee side of the Darfur mountains, in agreement with the analyses of Laing et al (2008) and Mekonnen et al (2006).…”

“…Mesoscale convective systems (MCSs) are responsible for most of the rain production in the Sahelian region, playing a key role in the rainfall variability over a large domain of spatial and temporal scales (Laing et al, 2008). The MCS definition includes fast-moving squall lines (10 to 15 m s −1 ) which can be large systems (up to 1000 km according to Redelsperger et al, 2002), and are responsible for 80% of the annual rainfall in the sub-Saharan region (Mohr et al, 1999).…”

“…The AEW forms at the north (15 • N) and south (5 • N) sides of the AEJ over WA, and converges over the Atlantic at a latitude of 17.5 • N (Burpee, 1972;Diedhiou et al, 1999). It is well recognized that AEWs can play an important role in the organization and modulation of convection (Duvel, 1990;Redelsperger et al, 2002;Mekonnen et al, 2006), but it is still under debate whether the enhancement of convection and precipitation rate is ahead of or behind the trough axis of the AEW (Laing et al, 2008). Conversely, the role of convection on wave activity was studied by Berry and Thorncroft (2005), who stressed the potential role of diabatically generated potential vorticity anomalies in sustaining AEW activity.…”