Aerosol midlatitude cyclone indirect effects in observations and high-resolution simulations

McCoy, Daniel; Field, Paul R.; Schmidt, Anja; Grosvenor, Daniel P.; Bender, Frida A.‐M.; Shipway, Ben J.; Hill, Adrian A.; Wilkinson, Jonathan; Elsaesser, Gregory S.

doi:10.5194/acp-18-5821-2018

Cited by 40 publications

(41 citation statements)

References 110 publications

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“…In other words, the fact that a model has an ECS within the range provided by independent estimates (e.g., Stevens et al, 2016) does not imply that the model is representing all the feedbacks correctly. From the model-development perspective, our efforts as a community should focus on improving understanding of the underlying processes and finding observational process-based metrics that are capable of constraining the processes that control the models' radiative feedbacks, expanding on the recent work on feedback proxies, both in the midlatitudes (e.g., Ceppi et al, 2016;Gordon & Klein, 2014), and in the tropics (e.g., McCoy et al, 2018;Myers & Norris, 2015;Qu et al, 2015). Of course, the assumption here is that the relationship between these processes and the long-term feedbacks holds in the real world.…”

Section: 1029/2019ms001688mentioning

confidence: 99%

Strong Dependence of Atmospheric Feedbacks on Mixed‐Phase Microphysics and Aerosol‐Cloud Interactions in HadGEM3

Bodas‐Salcedo

Mulcahy

Andrews

et al. 2019

J Adv Model Earth Syst

119

132

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We analyze the atmospheric processes that explain the large changes in radiative feedbacks between the two latest climate configurations of the Hadley Centre Global Environmental model. We use a large set of atmosphere‐only climate change simulations (amip and amip‐p4K) to separate the contributions to the differences in feedback parameter from all the atmospheric model developments between the two latest model configurations. We show that the differences are mostly driven by changes in the shortwave cloud radiative feedback in the midlatitudes, mainly over the Southern Ocean. Two new schemes explain most of the differences: the introduction of a new aerosol scheme and the development of a new mixed‐phase cloud scheme. Both schemes reduce the strength of the preexisting shortwave negative cloud feedback in the midlatitudes. The new aerosol scheme dampens a strong aerosol‐cloud interaction, and it also suppresses a negative clear‐sky shortwave feedback. The mixed‐phase scheme increases the amount of cloud liquid water path (LWP) in the present day and reduces the increase in LWP with warming. Both changes contribute to reducing the negative radiative feedback of the increase of LWP in the warmer climate. The mixed‐phase scheme also enhances a strong, preexisting, positive cloud fraction feedback. We assess the realism of the changes by comparing present‐day simulations against observations and discuss avenues that could help constrain the relevant processes.

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Section: 1029/2019ms001688mentioning

confidence: 99%

Strong Dependence of Atmospheric Feedbacks on Mixed‐Phase Microphysics and Aerosol‐Cloud Interactions in HadGEM3

Bodas‐Salcedo

Mulcahy

Andrews

et al. 2019

J Adv Model Earth Syst

119

132

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“…estimate components of the ERFaer due to changes in specific cloud properties, such as the RFaci (e.g. Quaas et al, 2008;Gryspeerdt et al, 2017;McCoy et al, 2018), the change in liquid f c ( f l ) (Gryspeerdt et al, 2016;Christensen et al, 2017), L (Gryspeerdt et al, 2018a) or cloud albedo (Lebsock et al, 2008;Christensen et al, 2017) due to the difficulty in isolating specific processes in the atmosphere. In contrast, model studies are able to isolate the radiative forcing due to aerosol impacts on individual processes (e.g.…”

mentioning

confidence: 99%

Surprising similarities in model and observational aerosol radiative forcing estimates

Gryspeerdt¹,

Mülmenstädt²,

Gettelman³

et al. 2019

Preprint

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Abstract. The radiative forcing from aerosols (particularly through their interaction with clouds) remains one of the most uncertain components of the human forcing of the climate. Observation-based studies have typically found a smaller aerosol effective radiative forcing than in model simulations and were given preferential weighting in the IPCC AR5 report. With their own sources of uncertainty, it is not clear that observation-based estimates are more reliable. Understanding the source of the model-observational difference is thus vital to reduce uncertainty in the impact of aerosols on the climate. These reported discrepancies arise from the different decompositions of the aerosol forcing used in model and observational studies. Applying the observational decomposition to global climate model output, the two different lines of evidence are surprisingly similar, with a much better agreement on the magnitude of aerosol impacts on cloud properties. Cloud adjustments remain a significant source of uncertainty, particularly for ice clouds. However, they are consistent with the uncertainty from observation-based methods, with the liquid water path adjustment usually enhancing the Twomey effect by less than 50&#8201;%. Depending on different sets of assumptions, this work suggests that model and observation-based estimates could be more equally weighted in future synthesis studies.

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“…Within this framework we focus on the midlatitudes. Previous work has performed 15 similar analysis on cyclonic midlatitude systems, finding a distinct increase in LWP at a fixed precipitation rate (McCoy et al, 2018c). These large synoptic systems account for roughly half the midlatitudes (Bodas-Salcedo et al, 2014).…”

Section: Introductionmentioning

confidence: 84%

“…Here we focus on the northern midlatitudes, where the majority of anthropogenic aerosol is emitted (Myhre et al, 2013). Previous work has examined aerosol-cloud adjustments in midlatitude cyclones, showing that cyclone liquid water path (LWP) 25 increases with cloud droplet number concentration (Nd) (McCoy et al, 2018c). This work focuses on the remainder of cases in the midlatitudes when there is no cyclone center within 2000 km (roughly 42% of the time between 30°N-60°N, see Fig.…”

Section: Discussionmentioning

confidence: 99%

“…Here, the liquid water path in clouds is calculated following Elsaesser et al (2017). Previous analysis has shown that the midlatitude partitioning or rain and cloud in MAC-LWP compares favorably with convection-permitting simulations (McCoy et al, 2018c). The MAC-LWP data set estimates that 7% of total liquid path is rain in the NH midlatitudes outside of cyclones.…”

Section: Observationsmentioning

confidence: 99%

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Untangling causality in midlatitude aerosol-cloud adjustments

McCoy¹,

Field²,

Gordon³

et al. 2019

Preprint

Self Cite

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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 10 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 cloud macrophysical state. Once variability associated with meteorology has been removed, covariance between the liquid water path averaged across cloudy and clear regions (LWP, here, characterizing the 15 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 20 outside of low-pressure centers associated with cyclonic activity. Observations from MAC-LWP, and MODIS are compared to simulations in the MetOffice 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 model and 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 25 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% 30 overestimate in associated radiative forcing.

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Aerosol midlatitude cyclone indirect effects in observations and high-resolution simulations

Cited by 40 publications

References 110 publications

Strong Dependence of Atmospheric Feedbacks on Mixed‐Phase Microphysics and Aerosol‐Cloud Interactions in HadGEM3

Strong Dependence of Atmospheric Feedbacks on Mixed‐Phase Microphysics and Aerosol‐Cloud Interactions in HadGEM3

Surprising similarities in model and observational aerosol radiative forcing estimates

Untangling causality in midlatitude aerosol-cloud adjustments

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