Within the framework of the German IMPETUS (“An Integrated Approach to the Efficient Management of Scarce Water Resources in West Africa”) project, a surface and upper-air field campaign was conducted in the Soudanian climate zone of Benin (West Africa) between May and mid-October 2002. For the first time, twice-daily radiosonde measurements at a high vertical resolution were levied at the synoptic station of Parakou (9°N, 2°E) and assimilated in the (Re-)Analysis of the European Centre for Medium-Range Weather Forecasts (ECMWF). Moreover, hourly synoptic observations from stations in western Nigeria, Benin and Togo, as well as data from 50 recording rain gauges in an 20,000 km2 region in central Benin, were collected to establish an unprecedented instrumental data set in this usually data sparse region of the Tropics.

Using ECMWF Reanalysis (Nov. 1957- Aug. 2002) and operational analyses, as well as half-hourly METEOSAT Infra-red images, as complementary data sources, two squall lines, observed on 26 June and on 15 September 2002, respectively, are discussed. These rainfall events will be placed into the context of the synoptic-scale thermodynamic and kinematic environments, as well as into the seasonal cycle of 2002. In general, squall lines in 2002 occurred in a drier than normal West African monsoon year with an anomalous equatorward position of the mid-level African easterly jet (AEJ).Whereas the September case happened during a period when African Easterly Waves (AEW) activity was strong, any consistent westward propagation a southerly moist AEWs was hardly detectable during the June case. In the September case, the squall line occurred in the ridge of well developed southerly AEW whose preceding trough later developed into Hurricane Lilli. This system developed over the Nigerian Jos Plateau and crossed central Benin around midnight, close to the climatological diurnal peak of squall occurrence. In contrast, the June squall line developed from a regenerating old cloud cluster just east of the study region in the afternoon hours and, while crossing central and south Benin around 18 UTC, it developed into a large squall cluster affecting southern Togo, Ghana, and Ivory Coast. The respective roles of the synoptic flow and the squalls themselves on the thermodynamic structure and vertical stability at Parakou are discussed.