Identification and Climatology of Rainfall Types over Southern West Africa

Rainfall in the West African monsoon seasons originates from a variety of rainfall systems ranging from isolated warm rain showers to large organized convective cloud clusters termed “Mesoscale Convective Systems (MCS)”. The climatology and contribution to rainfall of the latter has been studied extensively using infra-red and passive microwave imagery, with the bulk of the studies concentrating on the West African Sahel. Fewer studies have focused on the populous and wetter region of southern West Africa (SWA) where the importance of MCSs to rainfall totals diminishes at the expense of other types. Moreover, there is a general paucity of studies looking into the (thermo-) dynamical environments in which the different rainfall types tend to occur. The present study combines the advantages of describing characteristics of convective events by TRMM-based radar data with the sufficient spatio-temporal sampling by infra-red (IR) imagery from METEOSAT SEVIRI instrument to establish a climatology of regional rainfall types for SWA for the period 1998-2013. Furthermore, the contribution of the different rainfall types to total rainfall and typical synoptic environments in which they grow are explored.

An extended version of the classification method proposed by Houze et al. (2015, Rev. Geophys.) was used to define rainfall types which exhibit different degrees of convective organization. They cover the convective spectrum beginning from shallow and isolated warm-rain events to rainfall systems with an extensive convective part and, in later stages, a broad region of stratiform rainfall which typically represent MCSs. These highly organized rainfall types are altogether the main contributor to the total annual rainfall over SWA. However, their contribution exhibit a latitudinal dependence, being highest in the Sudanian zone (9°N - 12°N) with 71% and decreasing to 56% in a 100-km wide strip of the Guinea coast. At the same time, the contribution increases southward for less-intense convective elements which typically represent diurnal rainfall systems. Irrespective of type, rainfall occurs in conjunction with midlevel anomalies of relative vorticity which alter the vertical structure of horizontal wind and thus low-level wind shear. The preferred location for organized convection is a highly sheared environment, i.e. the region of midlevel northeasterlies, while intense rainfall types are suppressed in a regime of deep westerly anomalies with weaker low-level wind shear. Furthermore, our results suggest that both strongest enhancement and suppression of convective organization are typically associated with African Easterly Waves. While the strength of vertical windshear is key for further development of a convective system, the magnitude of the thermodynamical factors convective available potential energy (CAPE), convective inhibition (CIN) and downdraft CAPE (DCAPE) play an inferior role in their organization. Combining TRMM-PR rainfall elements with IR-based cloud tracking further reveals that organized convection is almost always long-lived (>9h) but varies significantly with respect to propagation speed (5–15 m/s). Less intense rainfall types tend to be short-lived, diurnal phenomena.

The novel approach of a merged radar-IR analysis for the understudied SWA stresses the importance of midlevel (wave) disturbances on type and lifetime of convective systems and thus the rainfall amount in this region. This study suggests that further investigations on the character of the disturbances and their implications for the development of both rainfall type and environmental controls are necessary to improve future operational forecasting of quantitative rainfall over SWA.