The organization and kinematics of tropical rainfall systems ground tracked at mesoscale with gages: First results from the campaigns 1999–2006 on the Upper Ouémé Valley (Benin)

Abstract : A dense network of rain gages, set up in the Upper Oueme Valley in Benin is used to study the spatial organization and the kinematics of the convective systems that cross the region. The study area is situated under Soudanian climate and set up as part of the AMMA-CATCH (African Monsoon Multidisciplinary Analysis – Couplage de l'Atmosphere Tropicale et du Cycle Hydrologique) observing system. Previous works focusing on the rainy events that occur in the Sahelian region of Niamey have shown that most of the rainfall in that region is provided by Organized Convective System that cover several thousand km2 and usually propagate with a strong westward component. It was shown also that the time evolution of these Sahelian rainy events usually exhibits a convective peak followed by longer lasting and weaker stratiform rainfall. The aim of the present study is to analyze the spatial organization and kinematics of the rainy events occurring further south under the distinct, much more humid, Soudanian climate. These events have been poorly documented so far and the extent to which the Soudanian rainfall events behave like their Sahelian counterparts remains unclear. Seven years of rainfall data gathered over the AMMA-CATCH Benin site are studied. A new method called the ‘Average Synchronized Hyetograph' (ASH) is proposed to analyze the kinematics of the rain patterns. The method also allows the assessment of the spatial organization of the system. A classification of the rainy events is proposed. It is based on assessing if (i) the rain patterns show a global propagation velocity and direction and (ii) if the time evolution of the rain rate within the network is typical of organized tropical Mesoscale Convective Systems (MCS) with a well-defined convective peak. The present study shows that about 55% of the events have a signature typical of those of MCS. Conversely, about 27% of rainfall events do not show evidence of being associated with MCS or even propagating. The kinematic properties of the events classified as MCS appear to be globally consistent with what was observed for the Sahelian zone from satellite tracking; the dominant direction is south-southwest but with a large departure from this average trend; their velocity ranges mostly between 20 and 50 km/h but a significant number of events are faster than that. MCS remains the dominant type of events during the whole rainy season but the period between mid July and mid August. The classification of rainfall events resulting from ground tracking has been compared with METEOSAT satellite tracking for the years 2003 and 2004. There is a significant correspondence of classes of rainfall events between the two tracking methods despite differences of scales and criterions in their definition.