Real-world laboratories for studying anthropogenic aerosol impacts on clouds and Earth’s climate

Mapping Intimacies ◽

10.5194/egusphere-egu2020-4558 ◽

2020 ◽

Author(s):

Velle Toll ◽

Heido Trofimov ◽

Jorma Rahu

Keyword(s):

Real World ◽

Power Plants ◽

Satellite Images ◽

Climate Models ◽

Cloud Droplet ◽

Cloud Water ◽

Global Climate Models ◽

Anthropogenic Aerosols ◽

Anthropogenic Aerosol ◽

Oil Refineries

The cooling of the Earth&#8217;s climate through the effects of anthropogenic aerosols on clouds offsets an unknown fraction of greenhouse gas warming. We discuss how causal relationship between aerosols and clouds can be derived from contrast between clouds polluted by anthropogenic aerosols and nearby unpolluted clouds. Ship tracks have been long considered to be real-world laboratories of aerosol-cloud interactions. More recently, polluted cloud tracks induced by aerosols emitted from volcanoes and wildfires and various industrial sources - such as oil refineries, smelters, coal-fired power plants, and cities have been analysed (Toll et al. 2019; Nature, https://doi.org/10.1038/s41586-019-1423-9). In this research, we extend satellite observations of polluted cloud tracks from Toll et al. (2019) with analysis of smaller and larger scale polluted cloud areas detected in satellite images.&#160;Polluted clouds are detected in MODIS and SEVIRI satellite images as areas with strongly increased cloud droplet number concentrations. Polluted cloud tracks can be utilized to study frequency and magnitude of anthropogenic cloud droplet number perturbations and subsequent cloud adjustments. Anthropogenic aerosol perturbations on liquid-water clouds are detected in various major global industrial areas. Both tens of kilometres wide ship-track-like polluted cloud tracks and hundreds by hundreds of kilometres wide polluted cloud areas show that cloud water can both increase and decrease in response to aerosols depending on meteorological conditions. On average, there is relatively weak decrease in cloud water. Polluted cloud tracks also show that cloud fraction can both increase and decrease compared to nearby less polluted clouds. Applicability of pollution tracks to study impact of absorbing aerosols situated above clouds on below-lying clouds is discussed. We expect that utilization of real-world laboratories of aerosol impacts on clouds will lead to improved physical parameterizations in global climate models and more reliable projections of the future climate.

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Polluted cloud lines in satellite snapshots and satellite climatologies

10.5194/egusphere-egu21-7164 ◽

2021 ◽

Author(s):

Velle Toll ◽

Heido Trofimov ◽

Jorma Rahu ◽

Piia Post

Keyword(s):

Power Plants ◽

Climate Models ◽

Spatial Scales ◽

Cloud Water ◽

Global Climate Models ◽

Anthropogenic Aerosols ◽

Oil Refineries ◽

Aerosol Cloud ◽

Aerosol Cloud Interactions ◽

Cloud Response

It is challenging to separate the cause from effect in aerosol-cloud interactions. Anomalous cloud lines polluted by anthropogenic aerosols help distinguish the cause from effect as properties of polluted clouds can be directly compared to nearby unpolluted clouds&#8217; properties. Pollution tracks in clouds induced by localised aerosol emissions (Toll et al. 2019, Nature, https://doi.org/10.1038/s41586-019-1423-9)&#160; are visually detectable ship-track-like quasi-linear polluted cloud features in satellite snapshots. We detected similar anomalous polluted cloud lines in the long-term average satellite data, where cloud response to aerosol over a long time is recorded. Polluted cloud tracks are induced by various aerosol sources like oil refineries, smelters, coal-fired power plants, smaller industry towns, ships, and volcanoes. We detected polluted cloud tracks at spatial scales varying from tens of kilometres to thousands of kilometres (Trofimov et al. 2020; JGR Atmospheres, https://doi.org/10.1029/2020JD032575).&#160;&#160;&#160;Polluted cloud tracks detected in satellite snapshots are excellent for the process-level understanding of aerosol-cloud interactions. Polluted cloud tracks recorded in satellite climatologies are great for estimating the average cloud response to aerosols. MODIS snapshots of polluted cloud tracks show relatively weak cloud water response to aerosols at various spatial scales. High-resolution analysis of South-East Atlantic shipping corridor shows partial off-set of the Twomey effect by decreased cloud water. Cloud fraction sometimes increases in the polluted cloud tracks and sometimes decreases compared to the nearby unpolluted clouds. The temporal evolution of cloud responses in pollution tracks estimated from geostationary SEVIRI data and meteorological conditions favourable for pollution track occurrence is presented. We expect that the utilisation of these real-world laboratories of aerosol impacts on clouds helps to improve global climate models&#8217; physical parameterisations.

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Large-scale industrial cloud perturbations confirm bidirectional cloud water responses to anthropogenic aerosols

10.5194/egusphere-egu2020-8909 ◽

2020 ◽

Author(s):

Heido Trofimov ◽

Velle Toll

Keyword(s):

Large Scale ◽

Satellite Images ◽

Climate Models ◽

Hot Spot ◽

Cloud Water ◽

Global Climate Models ◽

Small Scale ◽

Liquid Water Path ◽

Anthropogenic Aerosols ◽

Anthropogenic Aerosol

Aerosols offset poorly quantified fraction of anthropogenic greenhouse gas warming, whereas the aerosol impact on clouds is the most uncertain mechanism of anthropogenic climate forcing. In this research, we extend satellite observations of polluted cloud tracks from Toll et al. (2019, Nature, https://doi.org/10.1038/s41586-019-1423-9) with analysis of larger scale polluted cloud areas detected in MODerate-resolution Imaging Spectroradiometer satellite images. We demonstrate that large-scale anthropogenic aerosol-induced cloud perturbations exist at various major industrial aerosol source regions. The areal extent of the polluted cloud areas detected in MODIS satellite images extended to hundreds by hundreds of kilometres. Polluted clouds detected in satellite images in the global anthropogenic air pollution hot spot of Norilsk, Russia, and in other regions show close compensation between aerosol-induced cloud water increases and decreases. On average, there is relatively weak decrease in cloud water in the large areas with strong decreases in cloud droplet radii. This is in very good agreement with previous results based on small-scale polluted cloud tracks (Toll et al., 2019) and strongly disagrees with unidirectionally increased liquid water path in global climate models.

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Aerosol indirect effects in a multi-scale aerosol-climate model PNNL-MMF

Atmospheric Chemistry and Physics Discussions ◽

10.5194/acpd-11-3399-2011 ◽

2011 ◽

Vol 11 (1) ◽

pp. 3399-3459 ◽

Cited By ~ 5

Author(s):

M. Wang ◽

S. Ghan ◽

M. Ovchinnikov ◽

X. Liu ◽

R. Easter ◽

...

Keyword(s):

Indirect Effects ◽

Large Scale ◽

Climate Models ◽

Climate Model ◽

Global Climate ◽

Global Climate Models ◽

Anthropogenic Aerosols ◽

Anthropogenic Aerosol ◽

Aerosol Indirect Effects ◽

Multi Scale

Abstract. Much of the large uncertainty in estimates of anthropogenic aerosol effects on climate arises from the multi-scale nature of the interactions between aerosols, clouds and large-scale dynamics, which are difficult to represent in conventional global climate models (GCMs). In this study, we use a multi-scale aerosol-climate model that treats aerosols and clouds across multiple scales to study aerosol indirect effects. This multi-scale aerosol-climate model is an extension of a multi-scale modeling framework (MMF) model that embeds a cloud-resolving model (CRM) within each grid cell of a GCM. The extension allows the explicit simulation of aerosol/cloud interactions in both stratiform and convective clouds on the global scale in a computationally feasible way. Simulated model fields, including liquid water path (LWP), ice water path, cloud fraction, shortwave and longwave cloud forcing, precipitation, water vapor, and cloud droplet number concentration are in agreement with observations. The new model performs quantitatively similar to the previous version of the MMF model in terms of simulated cloud fraction and precipitation. The simulated change in shortwave cloud forcing from anthropogenic aerosols is −0.77 W m−2, which is less than half of that in the host GCM (NCAR CAM5) (−1.79 W m−2) and is also at the low end of the estimates of most other conventional global aerosol-climate models. The smaller forcing in the MMF model is attributed to its smaller increase in LWP from preindustrial conditions (PI) to present day (PD): 3.9% in the MMF, compared with 15.6% increase in LWP in large-scale clouds in CAM5. The much smaller increase in LWP in the MMF is caused by a much smaller response in LWP to a given perturbation in cloud condensation nuclei (CCN) concentrations from PI to PD in the MMF (about one-third of that in CAM5), and, to a lesser extent, by a smaller relative increase in CCN concentrations from PI to PD in the MMF (about 26% smaller than that in CAM5). The smaller relative increase in CCN concentrations in the MMF is caused in part by a smaller increase in aerosol lifetime from PI to PD in the MMF, a positive feedback in aerosol indirect effects induced by cloud lifetime effects. The smaller response in LWP to anthropogenic aerosols in the MMF model is consistent with observations and with high resolution model studies, which may indicate that aerosol indirect effects simulated in conventional global climate models are overestimated and point to the need to use global high resolution models, such as MMF models or global CRMs, to study aerosol indirect effects. The simulated total anthropogenic aerosol effect in the MMF is −1.05 W m−2, which is close to the Murphy et al. (2009) inverse estimate of −1.1 ± 0.4 W m−2 (1σ) based on the examination of the Earth's energy balance. Further improvements in the representation of ice nucleation and low clouds are needed.

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New cloud parameterization with relative dispersion in CAM5.1: model evaluation and impacts on aerosol indirect effects

10.5194/acp-2016-1172 ◽

2017 ◽

Author(s):

Xiaoning Xie ◽

He Zhang ◽

Xiaodong Liu ◽

Yiran Peng ◽

Yangang Liu

Keyword(s):

Northern Hemisphere ◽

Radiative Forcing ◽

Climate Models ◽

Cloud Droplet ◽

Effective Radius ◽

Global Climate Models ◽

Relative Dispersion ◽

Dispersion Effect ◽

Anthropogenic Aerosols ◽

Cloud Radiative Forcing

Abstract. Aerosol-induced increase of relative dispersion of cloud droplet size distribution ε exerts a warming effect and partly offsets the cooling of aerosol indirect radiative forcing (AIF) associated with increased droplet concentration by increasing the cloud droplet effective radius (Re) and enhancing the cloud-to-rain autoconversion rate (Au) (labeled as dispersion effect), which can help reconcile global climate models (GCMs) with the satellite observations. However, the total dispersion effects on both Re and Au are not fully considered in most GCMs, especially in different versions of the Community Atmospheric Model (CAM). In order to accurately evaluate the dispersion effect on AIF, the new complete cloud parameterizations of Re and Au explicitly accounting for ε are implemented into the CAM version 5.1 (CAM5.1), and a suite of sensitivity experiments is conducted with different representations of ε reported in literature. It is shown that the shortwave cloud radiative forcing is much better simulated with the new cloud parameterizations as compared to the standard scheme in CAM5.1, whereas the influences on longwave cloud radiative forcing and surface precipitation are minimal. Additionally, consideration of dispersion effect can significantly reduce the changes induced by anthropogenic aerosols in the cloud top effective radius and the liquid water path, especially in Northern Hemisphere. The corresponding AIF with dispersion effect considered can also be reduced substantially, by a range of 0.10 to 0.21 W m−2 at global scale, and by a much bigger margin of 0.25 to 0.39 W m−2 for the Northern Hemisphere in comparison with that fixed relative dispersion, mainly dependent on the change of relative dispersion and droplet concentrations (Δε / ΔNc).

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The diurnal evolution of anthropogenic aerosol impacts on clouds

10.5194/egusphere-egu21-11700 ◽

2021 ◽

Author(s):

Jorma Rahu ◽

Piia Post ◽

Velle Toll

Keyword(s):

Cloud Droplet ◽

Cloud Water ◽

Climate Projections ◽

Anthropogenic Aerosols ◽

Anthropogenic Aerosol ◽

Average Decrease ◽

Complex Processes ◽

Cloud Properties ◽

European Part Of Russia ◽

Aerosol Cloud Interactions

Reducing uncertainty in aerosol-cloud interactions is necessary for more reliable climate projections. Understanding the effects of anthropogenic aerosols on clouds remains a challenge due to complex processes governing the cloud adjustments to increased cloud droplet numbers. Using SEVIRI data, we study the daily evolution of polluted cloud tracks induced by strong pollution sources in the European part of Russia. We use semi-automated cloud droplet effective radius based statistical classification algorithm to differentiate between polluted and nearby unpolluted pixels in the satellite images. We use the 15-minute resolution Cloud Physical Properties product by KNMI to study changes in polluted cloud properties during the daytime. In some cases, cloud water increases during the day and in some cases decreases in polluted clouds compared to the nearby unpolluted clouds. On average, the diurnal evolution of cloud water is very similar between polluted and unpolluted clouds. Interestingly, there is less cloud water in polluted clouds already in the morning, suggesting that cloud water decreases more in polluted clouds during the night. The relatively weak average decrease in cloud water agrees with MODIS-based estimate (Toll et al 2019, Nature, https://doi.org/10.1038/s41586-019-1423-9).

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Sensitivity study of cloud parameterizations with relative dispersion in CAM5.1: impacts on aerosol indirect effects

Atmospheric Chemistry and Physics ◽

10.5194/acp-17-5877-2017 ◽

2017 ◽

Vol 17 (9) ◽

pp. 5877-5892 ◽

Cited By ~ 10

Author(s):

Xiaoning Xie ◽

He Zhang ◽

Xiaodong Liu ◽

Yiran Peng ◽

Yangang Liu

Keyword(s):

Northern Hemisphere ◽

Radiative Forcing ◽

Climate Models ◽

Cloud Droplet ◽

Effective Radius ◽

Global Climate Models ◽

Relative Dispersion ◽

Dispersion Effect ◽

Anthropogenic Aerosols ◽

Cloud Radiative Forcing

Abstract. Aerosol-induced increase of relative dispersion of cloud droplet size distribution ε exerts a warming effect and partly offsets the cooling of aerosol indirect radiative forcing (AIF) associated with increased droplet concentration by increasing the cloud droplet effective radius (Re) and enhancing the cloud-to-rain autoconversion rate (Au) (labeled as the dispersion effect), which can help reconcile global climate models (GCMs) with the satellite observations. However, the total dispersion effects on both Re and Au are not fully considered in most GCMs, especially in different versions of the Community Atmospheric Model (CAM). In order to accurately evaluate the dispersion effect on AIF, the new complete cloud parameterizations of Re and Au explicitly accounting for ε are implemented into the CAM version 5.1 (CAM5.1), and a suite of sensitivity experiments is conducted with different representations of ε reported in the literature. It is shown that the shortwave cloud radiative forcing is much better simulated with the new cloud parameterizations as compared to the standard scheme in CAM5.1, whereas the influences on longwave cloud radiative forcing and surface precipitation are minimal. Additionally, consideration of the dispersion effect can significantly reduce the changes induced by anthropogenic aerosols in the cloud-top effective radius and the liquid water path, especially in the Northern Hemisphere. The corresponding AIF with the dispersion effect considered can also be reduced substantially by a range of 0.10 to 0.21 W m−2 at the global scale and by a much bigger margin of 0.25 to 0.39 W m−2 for the Northern Hemisphere in comparison with that of fixed relative dispersion, mainly dependent on the change of relative dispersion and droplet concentrations (Δε∕ΔNc).

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Comparing multiple model-derived aerosol optical properties to spatially collocated ground-based and satellite measurements

Atmospheric Chemistry and Physics ◽

10.5194/acp-17-4451-2017 ◽

2017 ◽

Vol 17 (7) ◽

pp. 4451-4475 ◽

Cited By ~ 10

Author(s):

Ilissa B. Ocko ◽

Paul A. Ginoux

Keyword(s):

Climate Models ◽

Climate Modeling ◽

Global Climate ◽

Measurement Techniques ◽

Global Climate Models ◽

Orthogonal Polarization ◽

Geophysical Fluid Dynamics Laboratory ◽

Multiple Model ◽

Anthropogenic Aerosols ◽

Aerosol Properties

Abstract. Anthropogenic aerosols are a key factor governing Earth's climate and play a central role in human-caused climate change. However, because of aerosols' complex physical, optical, and dynamical properties, aerosols are one of the most uncertain aspects of climate modeling. Fortunately, aerosol measurement networks over the past few decades have led to the establishment of long-term observations for numerous locations worldwide. Further, the availability of datasets from several different measurement techniques (such as ground-based and satellite instruments) can help scientists increasingly improve modeling efforts. This study explores the value of evaluating several model-simulated aerosol properties with data from spatially collocated instruments. We compare aerosol optical depth (AOD; total, scattering, and absorption), single-scattering albedo (SSA), Ångström exponent (α), and extinction vertical profiles in two prominent global climate models (Geophysical Fluid Dynamics Laboratory, GFDL, CM2.1 and CM3) to seasonal observations from collocated instruments (AErosol RObotic NETwork, AERONET, and Cloud–Aerosol Lidar with Orthogonal Polarization, CALIOP) at seven polluted and biomass burning regions worldwide. We find that a multi-parameter evaluation provides key insights on model biases, data from collocated instruments can reveal underlying aerosol-governing physics, column properties wash out important vertical distinctions, and improved models does not mean all aspects are improved. We conclude that it is important to make use of all available data (parameters and instruments) when evaluating aerosol properties derived by models.

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Reply to “Comment on ‘Rethinking the Lower Bound on Aerosol Radiative Forcing’”

Journal of Climate ◽

10.1175/jcli-d-17-0034.1 ◽

2017 ◽

Vol 30 (16) ◽

pp. 6585-6589 ◽

Cited By ~ 4

Author(s):

Bjorn Stevens ◽

Stephanie Fiedler

Keyword(s):

Twentieth Century ◽

Lower Bound ◽

Radiative Forcing ◽

Climate Models ◽

Anthropogenic Aerosols ◽

Anthropogenic Aerosol ◽

Aerosol Radiative Forcing ◽

Clear Sky ◽

The North ◽

Aerosol Forcing

Kretzschmar et al., in a comment in 2017, use the spread in the output of aerosol–climate models to argue that the models refute the hypothesis (presented in a paper by Stevens in 2015) that for the mid-twentieth-century warming to be consistent with observations, then the present-day aerosol forcing, [Formula: see text] must be less negative than −1 W m−2. The main point of contention is the nature of the relationship between global SO2 emissions and [Formula: see text] In contrast to the concave (log-linear) relationship used by Stevens and in earlier studies, whereby [Formula: see text] becomes progressively less sensitive to SO2 emissions, some models suggest a convex relationship, which would imply a less negative lower bound. The model that best exemplifies this difference, and that is most clearly in conflict with the hypothesis of Stevens, does so because of an implausible aerosol response to the initial rise in anthropogenic aerosol precursor emissions in East and South Asia—already in 1975 this model’s clear-sky reflectance from anthropogenic aerosol over the North Pacific exceeds present-day estimates of the clear-sky reflectance by the total aerosol. The authors perform experiments using a new (observationally constrained) climatology of anthropogenic aerosols to further show that the effects of changing patterns of aerosol and aerosol precursor emissions during the late twentieth century have, for the same global emissions, relatively little effect on [Formula: see text] These findings suggest that the behavior Kretzschmar et al. identify as being in conflict with the lower bound in Stevens arises from an implausible relationship between SO2 emissions and [Formula: see text] and thus provides little basis for revising this lower bound.

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Exploring aerosol–cloud interaction using VOCALS-REx aircraft measurements

Atmospheric Chemistry and Physics ◽

10.5194/acp-19-7955-2019 ◽

2019 ◽

Vol 19 (12) ◽

pp. 7955-7971 ◽

Cited By ~ 2

Author(s):

Hailing Jia ◽

Xiaoyan Ma ◽

Yangang Liu

Keyword(s):

Climate Models ◽

Global Climate ◽

Cloud Droplet ◽

Global Climate Models ◽

Relative Dispersion ◽

Aircraft Measurements ◽

Aerosol Cloud ◽

Dispersion Effects ◽

Entrainment Zone ◽

Entrainment Mixing

Abstract. In situ aircraft measurements obtained during the VAMOS (Variability of the American Monsoons) Ocean-Cloud-Atmosphere-Land Study Regional Experiment (VOCALS-REx) field campaign are analyzed to study the aerosol–cloud interactions in the stratocumulus clouds over the southeastern Pacific Ocean (SEP), with a focus on three understudied topics (separation of aerosol effects from dynamic effects, dispersion effects, and turbulent entrainment-mixing processes). Our analysis suggests that an increase in aerosol concentration tends to simultaneously increase both cloud droplet number concentration (Nd) and relative dispersion (ε), while an increase in vertical velocity (w) often increases Nd but decreases ε. After constraining the differences of cloud dynamics, the positive correlation between ε and Nd becomes stronger, implying that perturbations of w could weaken the aerosol influence on ε and hence result in an underestimation of dispersion effect. A comparative analysis of the difference of cloud microphysical properties between the entrainment and non-entrainment zones suggests that the entrainment-mixing mechanism is predominantly extremely inhomogeneous in the stratocumulus that capped by a sharp inversion, whereby the variation in liquid water content (25 %) is similar to that of Nd (29 %) and the droplet size remains approximately constant. In entrainment zone, drier air entrained from the top induces fewer cloud droplets with respect to total in-cloud particles (0.56±0.22) than the case in the non-entrainment zone (0.73±0.13) by promoting cloud droplet evaporation. This study is helpful in reducing uncertainties in dispersion effects and entrainment mixing for stratocumulus, and the results of this study may benefit cloud parameterizations in global climate models to more accurately assess aerosol indirect effects.

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