Rainfall variability associated with the summer African monsoon: A satellite study

Instantaneous rainfall intensities retrieved by a multi-sensor precipitation estimation algorithm based on the Spinning Enhanced Visible and InfraRed Imager (SEVIRI) onboard Meteosat and on the Special Sensor Microwave Imager (SSM/I) data are used to investigate the dynamics and phenomenology associatedwith the African monsoon regime. A 5-year (2004–2008) climatology during the warm season (June–August) of coherent precipitation patterns is presented with emphasis on the intraseasonal and interannual variability of the tropical northern African Monsoon for the investigation of the longitudinal distribution of rainfall and the zonal component of motion. The coherence and phase speed of rain streaks are also quantified by means of a twodimensional autocorrelation analysis to derive the zonal-span and the duration properties of the identified rain systems. The periodicity of the precipitating episodes is finally investigated through harmonic analysis performed in different longitudinal bands of the studied domain. Rainfall episodes tend to initiate in the lee of steep topography (maxima in correspondence of the Ethiopian highlands), consistently with the thermal heating forcing from elevated terrain. Such an organized convection consists of coherent sequences of precipitation episodes, which span an average distance of about 460 km and last about 10 h. The diurnal cycle of summer precipitation is characterised by afternoon and early evening maxima located mainly downwind of the major mountain chains, as also the spectral analysis has clearly highlighted.