On the relationship between convective activity and easterly waves over Tropical North Africa: a weather state perspective

In the literature, a general consensus is lacking as to where and how the African easterly waves (AEWs) are generated. Earlier literature showed that the AEWs develop and grow in association with the middle level jet over tropical West Africa, while most recent work showed that these waves, indeed, are forced in association with convective activity over central and East Africa. Past work used proxy data for diagnosing convection. In this work, we follow a different approach and use a weather state data from the International Cloud Climatology Project (ISCCP). A weather state analysis is based on cloud-top pressure and cloud optical thickness joint histograms produced by ISCCP. In contrast to proxy data, the Weather State data provides a unique opportunity to study individual tropical convectively-active cloud regimes and their association with wave disturbances. Various statistical analysis tools are used to investigate the relationship between wave phases and convectively-active cloud regimes. Preliminary results show that deep convectively-active cloud regimes associated with mesoscale systems (referred to weather state 1; WS1) precede wave signatures over East Africa, suggesting an important influence of WS1 on wave initiation. Results also show that the WS1 transitions to a less active cloud type including cumulus congestus (referred to as WS3) over East Africa, while farther west over West Africa, WS3 transitions to WS1 and together they enhance the wave activity over central Africa. Details of these findings and the relationship between various cloud regimes and AEWs will be presented. Important questions such as where does a particular convectively active cloud type fall with respect to a convective maximum expressed by minimum outgoing infrared radiation or brightness temperature will be illustrated.