The interaction between convection and African Easterly Waves: a model case study

The aim of this study is to investigate the interaction between African easterly waves (AEWs), and the convection that develops within them. Over land this convection takes the form of mesoscale convective systems (MCS) which move across West Africa. A number of MCS may develop and decay within a given AEW. Over the Atlantic, the organised convection within an AEW may develop into a tropical cyclone. On the one hand we are interested in how the AEW provides a favourable environment for the development of convection. On the other hand, we wish to examine the role of the convection in modifying the AEW. In addition, we are interested in the differences in this relationship over land and over water.

We conduct a model case study using the non-hydrostatic limited area COSMO model of the German Weather Service (DWD). Our study focuses on the period between 09-16 September 2006. On 09 September, an MCS developed over Burkina Faso within the trough of an AEW and grew rapidly into a mature MCS during the following 12 hours. It moved westwards, reached its maximum strength early on 10 September and then began to decay. New convective systems developed within the AEW as it approached the Atlantic. On 11 September this complex moved over the Atlantic, underwent further development. On 12 September, the system was organised enough to become a tropical depression and it was upgraded to Tropical Storm Helene on 14 September 2006. After steady intensification Helene became a Hurricane on 16 September 2006.

We present results from 72 h model runs initialised from ECMWF analyses on different days within the period of interest. The horizontal resolution is 2.8 km so that the convective parametrisation is switched off. We use the model output to diagnose the interaction between the synoptic and the convective scale over land. Various budgets are calculated (e.g. heat moisture, vorticity) and a potential vorticity diagnosis is carried out. The model results are also compared to observations obtained during the AMMA campaign.