Thresholds for Atmospheric Convection in Amazonian Rainforests

• Published in: Geophysical research letters 2019-08, Vol.46 (16), p.10024-10033
• Main Authors: Wu, M., Lee, J.‐E.
• Format: Article
• Language: English
• Subjects: Atmospheric convection; Boundary layer transition; Boundary layers; Cloud microphysics; Cloud types; Clouds; CloudSat; Convection; Convective available potential energy; Daytime; Dominance; Dry season; Energy; Microphysics; Moisture; Oceans; Potential energy; Profiles; Rainforests; Rainy season; Regions; Satellite observation; Satellites; Seasons; Solar energy; Stability; tropical rainforest; Troposphere; Vertical profiles; Water stress; Wet season
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1029/2019GL082909

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. Plain Language Summary This paper presents new analyses of satellite observations that link the atmospheric energetics and vertical structure of daytime precipitating clouds, focusing on the unique features over the Amazon rainforest. The dominant cloud type over the Amazon rainforest is similar to the dominant cloud type observed in oceanic regions most time of the year, so it is often regarded as a “Green Ocean.” However, here we show a continental cloud regime transition from midlevel cloud dominance to deep cloud dominance when atmospheric buoyant energy or boundary layer moisture exceeds a threshold in Amazonian rainforests. Key Points The Amazon rainforest exhibits a cloud regime transition when daytime convective available potential energy exceeds a threshold A moderate plant water stress and increased incoming solar energy facilitate deep convection and the onset of the wet season Boundary layer moisture is crucial for deep convection over land, while lower‐free‐tropospheric moisture is more important over ocean