Abstract
Abstract. Changes in global-mean precipitation are strongly constrained by global radiative cooling, while regional rainfall changes are less constrained because energy can be transported. Absorbing and non-absorbing aerosols have different effects on both global-mean and regional precipitation, due to the distinct effects on energetics. This study analyses the precipitation responses to large perturbations in black carbon (BC) and sulfate (SUL) by examining the changes in atmospheric energy budget terms on global and regional scales, in terms of fast (independent of changes in sea surface temperature, SST) and slow responses (mediated by changes in SST). Changes in atmospheric radiative cooling/heating are further decomposed into contributions from clouds, aerosols, and clear–clean sky (without clouds or aerosols). Both cases show a decrease in global-mean precipitation, which is dominated by fast responses in the BC case and slow responses in the SUL case. The geographical patterns are distinct too. The intertropical convergence zone (ITCZ), accompanied by tropical rainfall, shifts northward in the BC case, while it shifts southward in the SUL case. For both cases, energy transport terms from the slow response dominate the changes in tropical rainfall, which are associated with the northward (southward) shift of the Hadley cell in response to the enhanced southward (northward) cross-equatorial energy flux caused by increased BC (SUL) emission. The extra-tropical precipitation decreases in both cases. For the BC case, fast responses to increased atmospheric radiative heating contribute most to the reduced rainfall, in which absorbing aerosols directly heat the mid-troposphere, stabilise the column, and suppress precipitation. Unlike BC, non-absorbing aerosols decrease surface temperatures through slow processes, cool the whole atmospheric column, and reduce specific humidity, which leads to decreased radiative cooling from the clear–clean sky, which is consistent with the reduced rainfall. Examining the changes in large-scale circulation and local thermodynamics qualitatively explains the responses of precipitation to aerosol perturbations, whereas the energetic perspective provides a method to quantify their contributions.
Highlights
Aerosols have been proposed to affect clouds and precipitation to a large extent by interacting with clouds and radiation (Ramanathan et al, 2001)
Since fast and slow responses are examined from an energetic perspective, we focus on how the atmospheric diabatic cooling (Q) and energy transport terms (H ) respond to aerosol perturbations in fixed sea surface temperatures (fSST) and mixed-layer ocean (MLO) simulations
Averaged precipitation is decreased in both the black carbon (BC) and SUL experiments, and the associated reduced latent heating is primarily balanced by decreased atmospheric radiative cooling (ARC) (Table 1)
Summary
Aerosols have been proposed to affect clouds and precipitation to a large extent by interacting with clouds and radiation (Ramanathan et al, 2001). Knowledge about the chain of processes, from aerosol emission to acting as cloud condensation nuclei (CCN) or ice nuclei (IN) and to cloud microphysics, and dynamics, is critical for reducing the uncertainties and understanding the climate system (Ghan et al, 2016), which is referred to as a “bottom-up” approach. This is challenging, considering uncertainties can arise from aerosol emissions, activation, cloud microphysics
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