Atmospheric energy budget response to idealized aerosol perturbation in tropical cloud systems

Dagan, Guy; Stier, Philip; Christensen, Matthew; Klocke, Daniel; Seifert, Axel

doi:10.5194/acp-20-4523-2020

Cited by 13 publications

(9 citation statements)

References 109 publications

(172 reference statements)

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“…As the simulation progresses the daily mean ERF becomes increasingly positive due to an increasing LW component, driven by enhanced ice, which results in a positive ERF ARI and ERF ACI . A positive ERF ACI due to the ice phase response was also reported by Dagan et al (2020). By day 5 the ERF is close to 10 Wm −2 , driven predominantly by a strong LW component of 13 Wm −2 .…”

Section: Toa Radiative Effectssupporting

confidence: 70%

“…We use the ICOsahedral Non-hydrostatic (ICON) modelling framework in a regional, limited-area, configuration; a full description and evaluation of the model is provided by Zängl et al (2015). This setup has been used in previous studies (Dagan et al, 2020;Klocke et al, 2017) to study clouds and ACI. The model domain, covering an area of ∼6 × 10 6 km 2 , is centered on the Amazon between longitudes 68°W-4°W and latitudes 15°S-3°N (Figure 1) on a triangular grid with a horizontal resolution of ∼1,500 m, which is able to represent convection without the need for sub-grid scale parameterizations of mid-level and deep convection (Klocke et al, 2017).…”

Section: Model Description and Setupmentioning

confidence: 99%

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Isolating Large‐Scale Smoke Impacts on Cloud and Precipitation Processes Over the Amazon With Convection Permitting Resolution

2021

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Absorbing aerosol from biomass burning impacts the hydrological cycle and radiation fluxes both directly and indirectly via modifications to convective processes and cloud development. Using the ICOsahedral Non-hydrostatic modelling framework in a regional configuration with 1,500 m convectionpermitting resolution, we isolate the response of the Amazonian atmosphere to biomass burning smoke via enhanced cloud droplet number concentrations N d (aerosol-cloud interactions; ACI) and changes to radiative fluxes (aerosol-radiation interactions; ARI) over a period of 8 days. We decompose ARI into contributions from reduced shortwave radiation and localized heating of the smoke. We show ARI influences the formation and development of convective cells: surface cooling below the smoke drives suppression of convection that increases with smoke optical depth, while the elevated heating promotes initial suppression and subsequent intensification of convection overnight; a corresponding diurnal response (repeating temporal response day-after-day) from high precipitation rates is shown. Enhanced N d (ACI) perturbs the bulk cloud properties and suppresses low-to-moderate precipitation rates. Both ACI and ARI result in enhanced high-altitude ice clouds that have a strong positive longwave radiative effect. Changes to low-cloud coverage (ARI) and albedo (ACI) drive an overall negative shortwave radiative effect, that slowly increases in magnitude due to a moistening of the boundary layer. The overall net radiative effect is dominated by the enhanced high-altitude clouds, and is sensitive to the plume longevity. The considerable diurnal responses that we simulate cannot be observed by polar orbiting satellites widely used in previous work, highlighting the potential of geostationary satellites to observe large-scale impacts of aerosols on clouds. Plain Language SummaryThere remain important uncertainties on how smoke from forest and grassland fires impacts the past, present, and future climates. In this study, we use a detailed model of the atmosphere over the Amazon rainforest to understand and quantify the processes by which smoke influences clouds and rain via two pathways: the first driven by changes to the absorption of solar radiation through the smoky atmosphere, and the second driven by an increase in the number of cloud droplets due to smoke particles. We find that the diurnal cycle of convection that drives much of the Amazon rainfall is greatly affected by changes to the radiation with less activity during the day and increasing activity overnight. The more numerous cloud droplets make the clouds brighter and help suppress rainfall rates. Changes to both radiation and cloud droplet number result in more extensive and thicker ice-phase clouds that exert a warming effect on the climate; the longer the smoke plume persists for, the stronger the warming effect. Our findings highlight important processes that are not sufficiently represented in global climate models, and also highlight a need to use time-resolved geostationa...

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Section: Toa Radiative Effectssupporting

confidence: 70%

Section: Model Description and Setupmentioning

confidence: 99%

Isolating Large‐Scale Smoke Impacts on Cloud and Precipitation Processes Over the Amazon With Convection Permitting Resolution

2021

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“…Koren et al (2008) report an increase in cloud fraction and taller convective clouds at small AOD perturbations, and Ten Hoeve et al ( 2011) reported similar behaviour with COTliquid. This is consistent with ACI-induced warm phase invigoration in shallow convection and in the warm base of deep convective cells (Marinescu et al, 2021;Koren et al, 2014;Seiki and Nakajima, 2014;Igel and van den Heever, 2021;Dagan et al, 2020), or through anomalous thermal buoyancy due to the fire itself (Zhang et al, 2019). The reduction in cloud top REice (Figure 7c and d) with AOD for high-IWP scenes suggests more cloud droplets are reaching the freezing level; this may due to ACI processes or enhanced aerosol activation through thermally-induced anomalous buoyancy, making attribution of the dominant mechanism difficult.…”

Section: Discussionsupporting

confidence: 79%

Satellite Observations of Smoke-Cloud-Radiation Interactions Over the Amazon Rainforest

Herbert

Stier

2022

Preprint

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Abstract. The Amazon rainforest routinely experiences intense and long-lived biomass burning events that result in smoke plumes that cover vast regions. The spatial and temporal extent of the plumes, and the complex pathways through which they interact with the atmosphere, has proved challenging to measure and gain a representative understanding of smoke impacts on the Amazonian atmosphere. In this study we use multiple collocated satellite sensors onboard AQUA and TERRA platforms to study the underlying smoke-cloud-radiation interactions during the diurnal cycle. An 18-year timeseries for both morning and afternoon overpasses is constructed providing collocated measurements of aerosol optical depth (column integrated aerosol extinction, AOD), cloud properties, top-of-atmosphere radiative fluxes, precipitation, and column water-vapour content from independent sources. The long-term timeseries reduces the impact of interannual variability and provides robust evidence that smoke significantly modifies the Amazonian atmosphere. Low loadings of smoke (AOD ≤ 0.4) enhance convective activity, cloudiness and precipitation, but higher loadings (AOD > 0.4) strongly suppress afternoon convection and promote low-level cloud occurrence. Accumulated precipitation increases with convective activity but remains elevated under high smoke loadings suggesting fewer but more intense convective cells. Contrasting morning and afternoon cloud responses to smoke are observed, in-line with recent simulations. Observations of top-of-atmosphere radiative fluxes support the findings, and show that the response of low-level cloud properties and cirrus coverage to smoke results in a pronounced and consistent increase in top-of-atmosphere outgoing radiation (cooling) of up to 50 Wm-2 for an AOD perturbation of +1.0. The results demonstrate that smoke strongly modifies the atmosphere over the Amazon via widespread changes to the cloud-field properties. Rapid adjustments work alongside instantaneous radiative effects to drive a stronger cooling effect from smoke than previously thought, whilst contrasting morning/afternoon responses of liquid and ice water paths highlight a potential method for constraining aerosol impacts on climate. Increased drought susceptibility, land-use change, and deforestation will have important and widespread impacts to the region over the coming decades. Based on this analysis, we anticipate further increases in anthropogenic fire activity to be associated with an overall reduction in regional precipitation, and a negative forcing (cooling) on the Earth’s energy budget.

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“…The increase of CF and TWP with N a in the tropics could be driven by local processes such as humidification of the troposphere (Abbott & Cronin, 2021; Dagan et al., 2020) and invigoration of convection (Altaratz et al., 2014). However, the increased tropical cloudiness under polluted conditions could also be driven by changes in the large‐scale conditions associated with the increase in N a in the sub‐tropics—effect which is not accounted for in the un‐coupled simulations, which demonstrate a weaker response (Figure 5).…”

Section: Resultsmentioning

confidence: 99%

Sub‐Tropical Aerosols Enhance Tropical Cloudiness—A Remote Aerosol‐Cloud Lifetime Effect

Dagan

2022

J Adv Model Earth Syst

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Earth's climate is changing (Masson-Delmotte et al., 2021). Reliable climate predictions are essential for developing climate change mitigation and adaptation strategies. However, climate predictions are still highly uncertain due largely to our limited understanding of the role of clouds in climate change. A major source of uncertainty is related to the cloud response to anthropogenic aerosols (Bellouin et al., 2019). It has been shown that aerosol-cloud-interaction (ACI) mechanisms are cloud-regime-dependent (Altaratz et al., 2014;Christensen et al., 2016;Dagan & Stier, 2020b;Gryspeerdt & Stier, 2012), and most previous studies have evaluated them on a per-regime basis. However, since the different cloud regimes in the atmosphere are connected by the global circulation, aerosol effects on one regime may also affect the properties of different regimes, as well as the transition between the different regimes (Christensen et al., 2020;Dagan & Chemke, 2016;Goren et al., 2019). These inter-cloud regime connections are under-explored and not fully understood.High-resolution, limited-area, cloud-resolving model (CRM) simulations and large-eddy simulations (LES) are the main tools to explore ACI mechanisms. However, traditionally, these tools are unable to directly account for changes in the dynamics and thermodynamics of the large-scale climate system, and hence lack an important component of clouds' response (Abbott & Cronin, 2021;Anber et al., 2019). For example, it was recently shown that the representation of large-scale effects on the CRM domain, that is, the lateral boundary conditions or "large-scale forcing" (LSF), could artificially determine clouds' response to aerosol perturbation (Dagan, Stier, Spill, et al., 2022;Spill et al., 2021). Specifically, it was shown that the recent efforts to couple local cloud response with changes in the large-scale vertical velocity (Abbott & Cronin, 2021) could result in an unrealistic Abstract Clouds' susceptibility to aerosols is considered to be a leading source of uncertainty in climate research. An inability to account simultaneously for the large range of scales involved in cloud-aerosol-climate interactions has hindered the progress of research. In this study, using a novel system of idealized large-eddysimulations that explicitly resolves clouds but also accounts, in an idealized manner, for large-scale changes in the thermodynamic and dynamic conditions and for inter-cloud regime coupling, it is shown that aerosol perturbation in the sub-tropics increases cloudiness in the tropics. Specifically, aerosol-driven sub-tropical rain suppression leads to increased advection of cold and moist air from the sub-tropics to the tropics, thus enhancing tropical cloudiness. The increased tropical cloudiness has a strong cooling effect by reflecting more of the incoming solar radiation. The classical "aerosol-cloud lifetime effect" is shown here to have a small local effect (in the sub-tropics) but a strong remote effect (sub-tropical aerosols increase cloudiness in the tropics)...

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Atmospheric energy budget response to idealized aerosol perturbation in tropical cloud systems

Cited by 13 publications

References 109 publications

Isolating Large‐Scale Smoke Impacts on Cloud and Precipitation Processes Over the Amazon With Convection Permitting Resolution

Isolating Large‐Scale Smoke Impacts on Cloud and Precipitation Processes Over the Amazon With Convection Permitting Resolution

Satellite Observations of Smoke-Cloud-Radiation Interactions Over the Amazon Rainforest

Sub‐Tropical Aerosols Enhance Tropical Cloudiness—A Remote Aerosol‐Cloud Lifetime Effect

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