The diurnal cycle of precipitation is well documented over the Amazon Basin, but the mechanisms that drive daily variations in convection are not well understood. We don’t have a good picture of what determines the timing of early afternoon convective outbreaks, and of how long-lived (longer than a day), propagating squall lines modulate the diurnal rainfall signal.

Previous studies (TRMM, geostationary IR, ground-based radar) of diurnal rainfall variation for the wet season Amazon consistently report a secondary nocturnal maximum, in addition to the well-known afternoon maximum associated with explosive locally-formed convection. Using ground-based radar data, radiosonde, and geostationary infrared satellite data from a recent TRMM field campaign in the Amazon Basin, this paper implicates propagating, nocturnal mesoscale convective systems to explain this feature. At other times, widespread shallow stratiform rain forming after midnight, which appears to be locally generated, also contributes to the nocturnal precipitation maximum.

These nocturnal systems are further shown to suppress the next day's explosive growth of convective cells near local noon. Post-sunrise destabilization of the boundary layer is thought to be inhibited by evaporative cooling from residual nocturnal stratiform rain, and by blocking of solar radiation from mid-level clouds in the morning.