Observed Modulation of the Tropical Radiation Budget by Deep Convective Organization and Lower-Tropospheric Stability

This study analyzes the observed monthly deseasonalized and detrended variability of the tropical radiation budget and suggests that variations of the lower-tropospheric stability and of the spatial organization of deep convection both strongly contribute to this variability. Satellite observations show that on average over the tropical belt, when deep convection is more aggregated, the free troposphere is drier, the deep convective cloud coverage is less extensive, and the emission of heat to space is increased; an enhanced aggregation of deep convection is thus associated with a radiative cooling of the tropics. An increase of the tropical-mean lower-tropospheric stability is also coincident with a radiative cooling of the tropics, primarily because it is associated with more marine low clouds and an enhanced reflection of solar radiation, although the free-tropospheric drying also contributes to the cooling. The contributions of convective aggregation and lower-tropospheric stability to the modulation of the radiation budget are complementary, largely independent of each other, and equally strong. Together, they account for more than sixty percent of the variance of the tropical radiation budget. Satellite observations are thus consistent with the suggestion from modeling studies that the spatial organization of deep convection substantially influences the radiative balance of the Earth. This emphasizes the importance of understanding the factors that control convective organization and lower-tropospheric stability variations, and the need to monitor their changes as the climate warms. Plain Language Summary Anomalies of the tropically averaged radiative balance determine the time variations of the tropical climate. The stability of the lower atmosphere has been shown to influence this balance because increased stability favors the formation of low-level clouds and the reflection of solar radiation to space. Modeling studies have suggested that the spatial distribution of deep convection, especially the degree of clustering of deep clouds, could also impact humidity and cloud coverage, and thus the radiative balance of the Earth system. However, the relationships between cloud clustering, humidity, and the radiation budget have never been observed at the scale of the tropics. By analyzing long time series of satellite observations, we show that monthly variations of lower-atmospheric stability and convective clustering are both strongly correlated with variations of the radiative cooling of the tropics and that their contributions to the modulation of the radiation budget are complementary and equally important. These observational results thus confirm modeling inferences and emphasize that to predict the future of our climate, it will be necessary to determine how the stability and the clustering of deep convection will change with warming. Key Points The monthly variability of deep convective organization in the tropics is investigated using satellite observations An enhanced organization of deep convection is associated with a drier troposphere, fewer high clouds, and a radiative cooling of the tropics Observations suggest equal and complementary modulations of the tropical radiation budget by convective organization and low-level stability