Seasonal variations in rain cells propagation over Central Africa and association with diurnal rainfall regimes

Three-hourly data from two satellite rainfall estimates products, PERSIANN and TMPA, are analysed to document the seasonal patterns of diurnal rainfall distribution over the Congo Basin and neighbouring areas. PERSIANN data for 2001-2017, at a one-hour time-scale, are further used to identify rain cells (>= 4 mmh-1) in an attempt to explain the diurnal rainfall variations. Over land areas, an afternoon rainfall maximum is clearly shown, but over much of the region only a minor part of the rains (20%-30%) falls in the wettest 3-h period. Substantial rains (often 50%-60%) occur in the evening and at night, as a progressively delayed peak from east to west, but a seasonal change is found in the meridional propagation of the peak diurnal rainfall, in a south-westerly direction in January, and a north-westerly direction in July. Rain cells have prominent genesis areas west of high terrain, but can develop over most regions, with a peak genesis time slightly ahead the diurnal phase of the rains. The size, mean lifetime and mean rainfall intensity of the rain cells are strongly related to each other and display a semi-annual cycle not fully in phase with the seasonal cycle of the rains. The mean rain cell propagation speed (6.7 ms-1) is much lower than in previous studies, which focused on mesoscale convective systems. Rain cells which have a longer lifetime move much faster, the mean speed of those lasting less than 6 h being half that of those lasting at least 24 h. Most (86%) of the mobile rain cells propagate westward, but the meridional component of their propagation shows an annual cycle (southward in austral summer, northward in boreal summer) which matches the mid-tropospheric winds and explains the seasonal changes in the diurnal rainfall peak. The diurnal cycle of rainfall over Central Africa shows a dominant afternoon maximum, with frequent delays in the diurnal peak from east to west and a seasonal change in the meridional propagation. A tracking of rain cells demonstrates slower motion than in previous studies, though rain cells with a longer lifetime move much faster. Besides the dominant westward propagation, a seasonally-varying meridional component is found which matches the mid-tropospheric winds and explains the seasonal changes in the diurnal rainfall peak. image