On the role of transitioning weather states in the initiation of African easterly waves

A recent analysis using a weather state (cloud regimes) data from the International Satellite Cloud Climatology Project (ISCCP) suggests that stronger latent heating and negative radiative cooling over eastern Africa are associated with the development of large and well organized convective systems (referred to as weather state 1,WS1). Smaller, less-organized convective systems (referred to as weather state type 3, WS3) that originate over the Arabian Sea/northern Indian Ocean region set off a chain of events that leads to the appearance of better organized mesoscale scale convective systems (MCSs), just to the west of the Ethiopian highlands. The sequence of events shows that enhanced activity by the scattered and less-organized convective systems interacts with the Ethiopian highlands producing a transition to the MCS systems and, almost at the same times, easterly wave perturbations emerge. Past studies have not elaborated the precise mechanism for this transition from a less-organized and relatively weak convection to more organized MCS-type convection and the role of topography in it. A preliminary analysis based on a “Convective Tracking” data set (also based on ISCCP) from the Remote Sensing of Climate Group of The City College, confirms that larger convective systems (mean radius of >150km) west of Ethiopian mountains are indeed associated with WS1, while smaller convective systems (mean radius < 60km) to the east of Ethiopia are associated with WS3. This presentation describes further work to explore the mechanisms that determine such transitions, including the role of topography on convective growth and easterly wave initiation over East Africa.