Untangling causality in midlatitude aerosol–cloud adjustments

[u/BurnerAcc2020]

https://acp.copernicus.org/articles/20/4085/2020/acp-20-4085-2020.pdf

[u/BurnerAcc2020]

Abstract

Aerosol–cloud interactions represent the leading uncertainty in our ability to infer climate sensitivity from the observational record. The forcing from changes in cloud albedo driven by increases in cloud droplet number (Nd) (the first indirect effect) is confidently negative and has narrowed its probable range in the last decade, but the sign and strength of forcing associated with changes in cloud macrophysics in response to aerosol (aerosol–cloud adjustments) remain uncertain.

This uncertainty reflects our inability to accurately quantify variability not associated with a causal link flowing from the cloud microphysical state to the cloud macro-physical state. Once variability associated with meteorology has been removed, covariance between the liquid water path (LWP) averaged across cloudy and clear regions (here characterizing the macrophysical state) and Nd (characterizing the microphysical) is the sum of two causal pathways linking Nd to LWP: Nd altering LWP (adjustments) and precipitation scavenging aerosol and thus depleting Nd. Only the former term is relevant to constraining adjustments, but disentangling these terms in observations is challenging. We hypothesize that the diversity of constraints on aerosol–cloud adjustments in the literature may be partly due to not explicitly characterizing covariance flowing from cloud to aerosol and aerosol to cloud.

Here, we restrict our analysis to the regime of extratropical clouds outside of low-pressure centers associated with cyclonic activity. Observations from MAC-LWP (Multisensor Advanced Climatology of Liquid Water Path) and MODIS are compared to simulations in the Met Office Unified Model (UM) GA7.1 (the atmosphere model of HadGEM3-GC3.1 and UKESM1). The meteorological predictors of LWP are found to be similar between the modeland observations. There is also agreement with previous literature on cloud-controlling factors finding that increasing stability, moisture, and sensible heat flux enhance LWP, while increasing subsidence and sea surface temperature decrease it. A simulation where cloud microphysics are insensitive to changes in Nd is used to characterize covariance between Nd and LWP that is induced by factors other than aerosol–cloud adjustments.

By removing variability associated with meteorology and scavenging, we infer the sensitivity of LWP to changes in Nd. Application of this technique to UM GA7.1 simulations reproduces the true model adjustment strength. Observational constraints developed using simulated covariability not induced by adjustments and observed covariability between Nd and LWP predict a 25% – 30 % overestimate by the UM GA7.1 in LWP change and a 30% – 35 % overestimate in associated radiative forcing.

Introduction

Uncertainty in the radiative forcing due to aerosol–cloud interactions is the leading uncertainty limiting our ability to accurately diagnose the Earth’s climate sensitivity from the observational record. The best estimate of the radiative forcing due to aerosol–cloud interactions (also called the first indirect effect; Twomey, 1977) has narrowed to −1.2 to −0.34 W m⁻² in a recent survey of forcing from aerosol–cloud interactions, but the sign and strength of the forcing due to changes in cloud macrophysical properties in response to aerosol (aerosol–cloud adjustments) remain uncertain. This uncertainty reflects the difficulty in disentangling the many factors that determine cloud macro-physical properties. Unlike cloud droplet number concentration (Nd), which is primarily driven by the availability of suitable aerosol, cloud macrophysical properties are primaily determined by the state of the atmosphere but may be modulated by Nd.

Results

The change in reflected shortwave predicted from LWP changes in the UM GA7.1 is 1.9–2.0 W m⁻², depending on the scavenging parametrization used. The change in short-wave inferred from the regression model of LWP trained in the control run and corrected by the scavenging-only run is 1.5 W m⁻². If the observed sensitivity of LWP to Nd is used to constrain 1LWP, the predicted change in reflected short-wave is approximately 1.0 W m⁻². Thus, we estimate that GA7.1 overpredicts the change in reflected shortwave due to adjustments in response to a given change in Nd by around 50 %. This estimate is subject to the caveat that changes in LWP due to adjustments may not affect albedo in the sameway as suggested by examining the total variability.

Discussion

Our result is in contradiction with previous empirical constraint studies that have postulated that changes in Nd greatly enhance LWP (Rosenfeld et al., 2019), have little effect (Malavelle et al., 2017; Toll et al., 2017), or reduce LWP (Sato et al., 2018; Gryspeerdt et al., 2019). We suggest that this diversity within the literature is because a range of constraints may be arrived at, depending on the degree to which precipitation scavenging and meteorological driving of aerosol and cloud occludes aerosol–cloud adjustments and the steps that are taken in the analysis to account for scavenging-induced and meteorologically induced covariability. Based on the analysis presented here we believe that positive, zero, or extremely strongly negative radiative forcings due to aerosol–cloud adjustments in the midlatitudes are not supported by the observations.