Technical Note: Estimating aerosol effects on cloud radiative forcing

Abstract. Estimating anthropogenic aerosol effects on the planetary energy balance through the aerosol influence on clouds using the difference in cloud radiative forcing from simulations with and without anthropogenic emissions produces estimates that are positively biased. A more representative method is suggested using the difference in cloud radiative forcing calculated as a diagnostic with aerosol scattering and absorption neglected. The method also yields an aerosol radiative forcing decomposition that includes a term quantifying the impact of changes in surface albedo. The method requires only two additional diagnostic calculations: the whole-sky and clear-sky top-of-atmosphere radiative flux with aerosol scattering and absorption neglected.

Download Full-text

Technical Note: Estimating aerosol effects on cloud radiative forcing

Atmospheric Chemistry and Physics Discussions ◽

10.5194/acpd-13-18771-2013 ◽

2013 ◽

Vol 13 (7) ◽

pp. 18771-18777

Author(s):

S. J. Ghan

Keyword(s):

Radiative Forcing ◽

Technical Note ◽

Anthropogenic Aerosol ◽

Cloud Radiative Forcing ◽

Aerosol Radiative Forcing ◽

Clear Sky ◽

Aerosol Scattering ◽

The Difference ◽

Aerosol Effects ◽

The Impact

Download Full-text

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.

Download Full-text

MACv2-SP: a parameterization of anthropogenic aerosol optical properties and an associated Twomey effect for use in CMIP6

Geoscientific Model Development ◽

10.5194/gmd-10-433-2017 ◽

2017 ◽

Vol 10 (1) ◽

pp. 433-452 ◽

Cited By ~ 57

Author(s):

Bjorn Stevens ◽

Stephanie Fiedler ◽

Stefan Kinne ◽

Karsten Peters ◽

Sebastian Rast ◽

...

Keyword(s):

Optical Properties ◽

Radiative Forcing ◽

Industrial Emissions ◽

Max Planck ◽

Aerosol Optical Properties ◽

Anthropogenic Aerosol ◽

Aerosol Radiative Forcing ◽

Clear Sky ◽

Aerosol Forcing ◽

Aerosol Radiative

Abstract. A simple plume implementation of the second version (v2) of the Max Planck Institute Aerosol Climatology, MACv2-SP, is described. MACv2-SP provides a prescription of anthropogenic aerosol optical properties and an associated Twomey effect. It was created to provide a harmonized description of post-1850 anthropogenic aerosol radiative forcing for climate modeling studies. MACv2-SP has been designed to be easy to implement, change and use, and thereby enable studies exploring the climatic effects of different patterns of aerosol radiative forcing, including a Twomey effect. MACv2-SP is formulated in terms of nine spatial plumes associated with different major anthropogenic source regions. The shape of the plumes is fit to the Max Planck Institute Aerosol Climatology, version 2, whose present-day (2005) distribution is anchored by surface-based observations. Two types of plumes are considered: one predominantly associated with biomass burning, the other with industrial emissions. These differ in the prescription of their annual cycle and in their optical properties, thereby implicitly accounting for different contributions of absorbing aerosol to the different plumes. A Twomey effect for each plume is prescribed as a change in the host model's background cloud-droplet population density using relationships derived from satellite data. Year-to-year variations in the amplitude of the plumes over the historical period (1850–2016) are derived by scaling the plumes with associated national emission sources of SO2 and NH3. Experiments using MACv2-SP are performed with the Max Planck Institute Earth System Model. The globally and annually averaged instantaneous and effective aerosol radiative forcings are estimated to be −0.6 and −0.5 W m−2, respectively. Forcing from aerosol–cloud interactions (the Twomey effect) offsets the reduction of clear-sky forcing by clouds, so that the net effect of clouds on the aerosol forcing is small; hence, the clear-sky forcing, which is more readily measurable, provides a good estimate of the total aerosol forcing.

Download Full-text

Challenges in constraining anthropogenic aerosol effects on cloud radiative forcing using present-day spatiotemporal variability

Proceedings of the National Academy of Sciences ◽

10.1073/pnas.1514036113 ◽

2016 ◽

Vol 113 (21) ◽

pp. 5804-5811 ◽

Cited By ~ 81

Author(s):

Steven Ghan ◽

Minghuai Wang ◽

Shipeng Zhang ◽

Sylvaine Ferrachat ◽

Andrew Gettelman ◽

...

Keyword(s):

Optical Depth ◽

Temporal Variability ◽

Radiative Forcing ◽

Spatiotemporal Variability ◽

Anthropogenic Change ◽

Anthropogenic Aerosol ◽

Cloud Optical Depth ◽

Cloud Radiative Forcing ◽

Aerosol Effects ◽

The Relationship

A large number of processes are involved in the chain from emissions of aerosol precursor gases and primary particles to impacts on cloud radiative forcing. Those processes are manifest in a number of relationships that can be expressed as factors dlnX/dlnY driving aerosol effects on cloud radiative forcing. These factors include the relationships between cloud condensation nuclei (CCN) concentration and emissions, droplet number and CCN concentration, cloud fraction and droplet number, cloud optical depth and droplet number, and cloud radiative forcing and cloud optical depth. The relationship between cloud optical depth and droplet number can be further decomposed into the sum of two terms involving the relationship of droplet effective radius and cloud liquid water path with droplet number. These relationships can be constrained using observations of recent spatial and temporal variability of these quantities. However, we are most interested in the radiative forcing since the preindustrial era. Because few relevant measurements are available from that era, relationships from recent variability have been assumed to be applicable to the preindustrial to present-day change. Our analysis of Aerosol Comparisons between Observations and Models (AeroCom) model simulations suggests that estimates of relationships from recent variability are poor constraints on relationships from anthropogenic change for some terms, with even the sign of some relationships differing in many regions. Proxies connecting recent spatial/temporal variability to anthropogenic change, or sustained measurements in regions where emissions have changed, are needed to constrain estimates of anthropogenic aerosol impacts on cloud radiative forcing.

Download Full-text

Impact of different definitions of clear-sky flux on the determination of longwave cloud radiative forcing: NICAM simulation results

Atmospheric Chemistry and Physics Discussions ◽

10.5194/acpd-10-22093-2010 ◽

2010 ◽

Vol 10 (9) ◽

pp. 22093-22107

Author(s):

B. J. Sohn ◽

T. Nakajima ◽

M. Satoh ◽

H.-S. Jang

Keyword(s):

Radiative Forcing ◽

Atmospheric Model ◽

Satellite Measurements ◽

Cloud Radiative Forcing ◽

Cloud Forcing ◽

Clear Sky ◽

Atmospheric State ◽

The Impact ◽

Mean State

Abstract. Using one month of the cloud-resolving Nonhydrostatic Icosahedral Atmospheric Model (NICAM) simulations, we examined the impact of different definitions of clear-sky flux on the determination of longwave cloud radiative forcing (CRF). Because the satellite-like cloud-free composite preferentially samples drier conditions relative to the all-sky mean state, the conventional clear-sky flux calculation using the all-sky mean state in the model may represent a more humid atmospheric state in comparison to the cloud-free state. The drier bias is evident for the cloud-free composite in the NICAM simulations, causing an overestimation of the longwave CRF by about 10% compared to the NICAM simulated longwave CRF. Overall, water vapor contributions of up to 10% of the total longwave CRF should be added to make the NICAM-generated cloud forcing comparable to the satellite measurements.

Download Full-text

Impact of different definitions of clear-sky flux on the determination of longwave cloud radiative forcing: NICAM simulation results

Atmospheric Chemistry and Physics ◽

10.5194/acp-10-11641-2010 ◽

2010 ◽

Vol 10 (23) ◽

pp. 11641-11646 ◽

Cited By ~ 25

Author(s):

B. J. Sohn ◽

T. Nakajima ◽

M. Satoh ◽

H.-S. Jang

Keyword(s):

Radiative Forcing ◽

Atmospheric Model ◽

Satellite Measurements ◽

Cloud Radiative Forcing ◽

Cloud Forcing ◽

Clear Sky ◽

Atmospheric State ◽

The Impact ◽

Mean State

Abstract. Using one month of the cloud-resolving Nonhydrostatic Icosahedral Atmospheric Model (NICAM) simulations, we examined the impact of different definitions of clear-sky flux on the determination of longwave cloud radiative forcing (CRF). Because the satellite-like cloud-free composite preferentially samples drier conditions relative to the all-sky mean state, the conventional clear-sky flux calculation using the all-sky mean state in the model may represent a more humid atmospheric state in comparison to the cloud-free state. The drier bias is evident for the cloud-free composite in the NICAM simulations, causing an overestimation of the longwave CRF by about 10% compared to the NICAM simulated longwave CRF. Overall, water vapor contributions of up to 10% of the total longwave CRF should be taken account for making model-generated cloud forcing comparable to the satellite measurements.

Download Full-text

A new approach to simulate aerosol effects on cirrus clouds in EMAC v2.54

10.5194/gmd-2019-212 ◽

2019 ◽

Author(s):

Mattia Righi ◽

Johannes Hendricks ◽

Ulrike Lohmann ◽

Christof Gerhard Beer ◽

Valerian Hahn ◽

...

Keyword(s):

Radiative Forcing ◽

Climate Model ◽

Cirrus Clouds ◽

Coupled Models ◽

Cloud Droplets ◽

Anthropogenic Aerosol ◽

New Model ◽

Aerosol Radiative Forcing ◽

Stratiform Clouds ◽

The Impact

Abstract. A new cloud microphysical scheme including a detailed parameterization for aerosol-driven ice formation in cirrus clouds is implemented in the global chemistry climate model EMAC and coupled to the aerosol submodel MADE3. The new scheme is able to consistently simulate three regimes of stratiform clouds (liquid, mixed- and ice-phase (cirrus) clouds), considering the impact of aerosol on the activation of cloud droplets and the nucleation of ice crystals. In the cirrus regime, it accounts for the competition between homogeneous and heterogeneous freezing for the available supersaturated water vapor, taking into account different types of ice-nucleating particles, whose specific ice-nucleating properties can be flexibly varied in the model setup. The new model configuration was tuned using satellite data to find the optimal set of parameters that reproduces the observations. A detailed evaluation is also performed comparing the model results for standard cloud and radiation variables with a comprehensive set of observations from satellite retrievals and in situ measurements. The performance of EMAC-MADE3 in this new coupled configuration is in line with similar global coupled models and with other global aerosol models featuring ice cloud parameterizations. Some remaining discrepancies, especially with regard to ice crystal number concentrations in cirrus, which are a common problem of this kind of models, need to be the subject of future investigations. To further demonstrate the readiness of the new model system for application studies, an estimate of the global anthropogenic aerosol radiative forcing is provided and discussed in the context of the CMIP5 results for the IPCC.

Download Full-text

Correction for Ghan et al., Challenges in constraining anthropogenic aerosol effects on cloud radiative forcing using present-day spatiotemporal variability

Proceedings of the National Academy of Sciences ◽

10.1073/pnas.1604888113 ◽

2016 ◽

Vol 113 (21) ◽

pp. E3049-E3049

Keyword(s):

Radiative Forcing ◽

Spatiotemporal Variability ◽

Anthropogenic Aerosol ◽

Cloud Radiative Forcing ◽

Aerosol Effects

Download Full-text

A global study of hygroscopicity-driven light scattering enhancement in the context of other in-situ aerosol optical properties

10.5194/acp-2020-1250 ◽

2020 ◽

Author(s):

Gloria Titos ◽

María A. Burgos ◽

Paul Zieger ◽

Lucas Alados-Arboledas ◽

Urs Baltensperger ◽

...

Keyword(s):

Light Scattering ◽

Radiative Forcing ◽

Incident Light ◽

Single Scattering ◽

Single Scattering Albedo ◽

Opposite Pattern ◽

Aerosol Radiative Forcing ◽

Dry Conditions ◽

The Difference ◽

The Impact

Abstract. The scattering and backscattering enhancement factors (f(RH) and fb(RH)) describe how aerosol particle light scattering and backscattering, respectively, change with relative humidity (RH). They are important parameters in estimating direct aerosol radiative forcing (DARF). In this study we use a dataset presented in Burgos et al. (2019) that compiles f(RH) and fb(RH) measurements at three wavelengths (i.e. 450, 550 and 700 nm) performed with tandem nephelometer systems at multiple sites around the world. We present an overview of f(RH) and fb(RH) based on both long-term and campaign observations from 23 sites representing a range of aerosol types. The scattering enhancement shows a strong variability from site to site, with no clear pattern with respect to total scattering coefficient. In general, higher f(RH) is observed at Arctic and marine sites while lower values are found at urban and desert sites, although a consistent pattern as a function of site type is not observed. The backscattering enhancement fb(RH) is consistently lower tan f(RH) at all sites, with the difference between f(RH) and fb(RH) increasing for aerosol with higher f(RH). This is consistent with Mie theory which predicts higher enhancement of the light-scattering in the forward than in the backward direction as the particle takes up water. Our results show that the scattering enhancement is higher for PM1 than PM10 at most sites, which is also supported by theory due to the change in scattering efficiency with the size parameter that relates particle size and wavelength of incident light. At marine-influenced sites this difference is enhanced when coarse particles (likely sea salt) predominate. For most sites, f(RH) is observed to increase with increasing wavelength, except at sites with a known dust influence where the spectral dependence of f(RH) is found to be low or even exhibit the opposite pattern. The impact of RH on aerosol properties used to calculate radiative forcing (e.g., single scattering albedo, ω0, and backscattered fraction, b) is evaluated. The single scattering albedo generally increases with RH while b decreases. The net effect of aerosol hygroscopicity on radiative forcing efficiency (RFE) is an increase in the absolute forcing effect (negative sign) by a factor of up to 4 at RH = 90 % compared to dry conditions (RH

Download Full-text