Analysis of the large-scale environment preceding the development of an African disturbance

It has been previously hypothesized that African disturbances can develop as far eastward as the Ethiopian Highlands (EH). African disturbances propagate westward through the Sahel along the African Easterly Jet (AEJ), which results from thermal balance between the hot, dry air mass over the Saharan Desert and the cooler, humid air mass over the tropical rainforests.

Satellite data have shown that convection in association with a disturbance often begins over central or eastern Africa, or over the Asir Mountains of the Arabian Peninsula, yet convection typically does not develop further to the east (or upstream). This suggests that pivotal atmospheric ingredients come together to foster development of an African disturbance in the vicinity of the EH.

In the case of the African disturbance that precedes the genesis of Hurricane Alberto (2000), a number of large-scale features appear to come together to instigate the development of the disturbance. According to ECMWF analyses, the AEJ increases in speed and is directed in a NE-SW orientation over Sudan and the Red Sea approximately 24 h prior to development of the pre-Alberto disturbance. Intensification of both a deep-layer anticyclone over the northern Arabian Peninsula and a monsoon trough along the south coast of the Arabian Peninsula appear to be the cause of the AEJ increase and orientation, which in turn provides vertical shear and mid-level dry air advection conducive to the development of a mesoscale convective system (MCS) near the EH, as well as cyclonic horizontal shear to support a cyclonic wave over Sudan.

ECMWF Reanalysis (ERA) data indicate that a 700-hPa stationary wave may be a common phenomenon over southern Sudan during July and August. This suggests that the interaction of a MCS with the stationary wave may the necessary trigger for the growth of an African Easterly Wave (AEW) to support development of the (coincidentally?) pre-cyclogenic disturbance.

Analyses over a 3-day period preceding disturbance development are to be presented to show how large-scale dynamics may have converged over the EH region to give rise to the disturbance.