Anatomy of an observed African easterly wave in July 2006

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 Hovmoller diagram of meridional winds did not detect the wave, and a Hovmoller 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. Copyright (C) 2011 Royal Meteorological Society and British Crown Copyright, the Met Office