Thresholds for Atmospheric Convection in Amazonian Rainforests

Abstract

The Amazon rainforest is known as the “Green Ocean” for its maritime-like convection and cloud microphysics during the wet season. Although previous studies suggest the dominant thermodynamic processes involved in deep convection may differ between land and ocean, a comprehensive understanding of the thermodynamics of Amazonian convection is lacking. Using 404,971 daytime precipitating cloud profiles from the CloudSat satellite, we observe a regime transition from congestus dominance to cumulonimbus dominance when convective available potential energy exceeds a threshold in Amazonia and also in shrublands, but not in oceanic regions. In addition, the cloud regime transition is linked to boundary layer moisture in the two continental regions, while it is linked to lower-free-tropospheric moisture in the oceanic region. As the dry season progresses in Amazonia and modifies the free-tropospheric stability, a moderate plant water stress and increased incoming solar energy facilitate the initiation of deep convection and the onset of the wet season.