On the sensitivity of aerosol-cloud interactions to changes in sea surface temperature in radiative-convective equilibrium

Abstract

Abstract. Clouds play a crucial role in regulating Earth's energy balance and are influenced by anthropogenic aerosol concentration (Na) and sea surface temperature (SST) changes. However, these two factors – aerosols and SST – are typically studied independently. In this study, we employ idealized cloud-resolving, radiative-convective-equilibrium simulations to explore aerosol-cloud interactions (ACI) under different SSTs. Our findings reveal that ACIs are dependent on the prescribed SST, even at equilibrium conditions. Specifically, we show that increasing Na leads to a decline in top-of-atmosphere (TOA) energy gain across SSTs due to changes in the cloud radiative effect, both in the short-wave and the long-wave parts of the spectrum. TOA short-wave flux changes with an increase in Na are found to be more sensitive to the underlined SST conditions compared to long-wave radiation. The variations in how the clouds' short-wave radiative effect responds to Na at various SSTs are explained by variations in the sensitivity of the water content in the cloud. Specifically, the sensitivity of the water content to Na decreases with SST due to deepening of the warm, liquid portion of the cloud. This deepening results in clouds that are less responsive to aerosol-induced warm rain suppression. Furthermore, with an increase in Na, we observe an increase in latent heat release at the upper troposphere associated with heightened production of snow and graupel. We show that this trend, which is consistent across all SSTs, affects the anvil cloud cover by affecting the static–stability at the upper troposphere via a similar mechanism to the iris–stability effect, resulting in a decline in TOA long-wave energy gain. In conclusion, under the ongoing climate change, studying the sensitivity of clouds to aerosols and SST should be conducted concomitantly as mutual effects are expected.

Large‐Scale Tropical Circulation Intensification by Aerosol Effects on Clouds

“…Specifically, Figure 2c illustrates an increase in OLR with SST but a decrease with N a (for a given SST). Interestingly, the trends in IWP and CF ice with N a presented here are opposite to the trends presented by Lorian and Dagan (2023), based on simulations conducted with the same model and micro‐physical scheme but in small‐domain simulations that do not include convective self‐organization and large‐scale overturning circulation (Muller & Held, 2012; Wing et al., 2020). Thus, we can conclude that the trends in IWP and CF ice with N a presented here are likely driven by large‐scale circulation changes.…”

“…To better understand the causes of the changes in ω d and hence in $$\mathcal{I}$$ and in w ↓ with changes in N a , I calculate ω d (Equation ) assuming that either Q r or S is held fixed at the value it attains at a reference N a of 200 cm −3 for each SST (a similar calculation to that presented in Bony et al. (2016) and in Lorian and Dagan (2023)). Figure 3c illustrates that both the N a ‐driven and the SST‐driven changes in w ↓ are captured by the ω d calculated for a given S ( ω d fixed S ).…”

“…The weakest increase in water vapor concentration and in the magnitude of Q r at the mid‐troposphere is observed in the highest SST case considered. This trend may be explained by the fact that, under high SST, the freezing level is relatively high, leading to a deepening of the warm section of deep convective clouds (Lorian & Dagan, 2023; Figure S3a in Supporting Information ). Consequently, under high SST conditions, warm rain has a deeper layer in which it can form, making it less sensitive to changes in N a compared to lower SST conditions (Freud & Rosenfeld, 2012; Lorian & Dagan, 2023; Figure S3b in Supporting Information ).…”

“…This trend may be explained by the fact that, under high SST, the freezing level is relatively high, leading to a deepening of the warm section of deep convective clouds (Lorian & Dagan, 2023; Figure S3a in Supporting Information ). Consequently, under high SST conditions, warm rain has a deeper layer in which it can form, making it less sensitive to changes in N a compared to lower SST conditions (Freud & Rosenfeld, 2012; Lorian & Dagan, 2023; Figure S3b in Supporting Information ). This reduced sensitivity of warm rain to N a under high SSTs diminishes the mixing and humidification of the environment.…”

The Potential of Absorbing Aerosols to Enhance Extreme Precipitation

“…Domain mean clouds, precipitation and radiation properties of these baseline simulations (i.e., excluding aerosol absorption) are presented in Figure S1 in Supporting Information . In addition, the reader is referred to Lorian and Dagan (2023) for further details about clouds, precipitation and radiation properties in similar simulations.…”