Multi‐scale description of a Sahelian synoptic weather system representative of the West African monsoon

A 13-year (1998-2010) climatology of mesoscale convective characteristics associated with the West African monsoon are investigated using precipitation radar and passive microwave data from the NASA Tropical Rainfall Measuring Mission satellite. Seven regions defined as continental northeast and northwest, southeast and southwest, coastal, and maritime north and south are compared to analyse zonal and meridional differences. Data are categorized according to identified African easterly wave (AEW) phase and when no wave is present. While some enhancements are observed in association with AEW regimes, regional differences were generally more apparent than wave vs. no-wave differences. Convective intensity metrics confirm that land-based systems exhibit stronger characteristics, such as higher storm top and maximum 30 dBZ heights and significant 85 GHz brightness temperature depressions. Continental systems also contain a lower fraction of points identified as stratiform. Results suggest that precipitation processes also varied depending upon region and AEW regime, with warm-rain processes more apparent over the ocean and the southwest continental region and ice-based microphysics more dominant over land, including mixed-phase processes. AEW regimes did show variability in stratiform fraction and ice and liquid water content, suggesting modulation of mesoscale characteristics possibly through feedback with the synoptic environment.

Section: Introductionmentioning

confidence: 99%

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

Guy

Rutledge

2011

“…Rainfall in West Africa is primarily associated with MCSs (Le Barbé and Lebel, 1997;Diongue et al, 2002;Redelsperger et al, 2002;Fink et al, 2006). Due to the scarcity of radar and hourly rainfall data, most studies of mesoscale precipitation in Africa used satellite IR or passive microwave data (e.g.…”

Section: Introductionmentioning

confidence: 99%

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

Laing

Carbone

Levizzani

et al. 2008

149

132

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.

“…Our understanding of AEWs remains founded on climatological studies which are limited by the upper-air network, with only a handful of detailed casestudies available (e.g. Redelsperger et al, 2002;Berry and Thorncroft, 2005). In reality, there are wide differences in the structure of individual AEWs within a given season which do not adhere to the climatological composites.…”

Section: Introductionmentioning

confidence: 99%

Anatomy of an observed African easterly wave in July 2006

Bain

Parker

Dixon

et al. 2011

The detailed structure of an African easterly wave (AEW) observed during the AMMA field campaign is analysed. The wave was present from 25 to 29 July 2006. A complex circulation pattern was observed: the overall structure of convection and the positive vorticity of the trough region had an elongated inverted-V appearance, wrapped around an area of low winds and clear skies. Satellite imagery showed that the AEW was a significant influence on the modulation of convection on the large scale.The wave was identified initially through its strong signature on soil moisture and convection. The AEW structure observed was not anticipated and has not been discussed in previous literature. In addition, wave tracking using a Hovmöller diagram of meridional winds did not detect the wave, and a Hovmöller of vorticity showed the wave moved at a slower speed than other AEWs in July.New schematics explaining the structure are presented, describing the case as observed by satellites and analysed by a limited-area version of the Met Office Unified Model. It is proposed that the positive vorticity branches of the inverted-V can be regarded as analogous to atmospheric fronts, with characteristic gradients in winds and thermodynamic properties, acting as locations for enhanced convection. The implications of the new case are discussed in relation to previous theory and it is suggested that the accepted model of an idealised AEW is incomplete and should be extended to include more complex structures.