The importance of vertical velocity variability for estimates of the indirect aerosol effects

Abstract. The activation of aerosols to form cloud droplets is dependent upon vertical velocities whose local variability is not typically resolved at the GCM grid scale. Consequently, it is necessary to represent the sub-grid-scale variability of vertical velocity in the calculation of cloud droplet number concentration. This study uses the UK Chemistry and Aerosols community model (UKCA) within the Hadley Centre Global Environmental Model (HadGEM3), coupled for the first time to an explicit aerosol activation parameterisation, and hence known as UKCA-Activate. We explore the range of uncertainty in estimates of the indirect aerosol effects attributable to the choice of parameterisation of the sub-grid-scale variability of vertical velocity in HadGEM-UKCA. Results of simulations demonstrate that the use of a characteristic vertical velocity cannot replicate results derived with a distribution of vertical velocities, and is to be discouraged in GCMs. This study focuses on the effect of the variance (σw2) of a Gaussian pdf of vertical velocity. Fixed values of σw2 (spanning the range measured in situ by nine flight campaigns found in the literature) and a configuration in which σw2 depends on turbulent kinetic energy are tested. Results from the mid-range fixed σw2 and TKE-based configurations both compare well with observed vertical velocity distributions and cloud droplet number concentrations. The radiative flux perturbation due to the total effects of anthropogenic aerosol is estimated at −1.4 W m−2 with σw2 = 0.1 m s−1, −1.7 W m−2 with σw2 derived from TKE, −1.9 W m−2 with σw = 0.4 m s−1 and −2.0 W m−2 with σw = 0.7 m s−1. The breadth of this range (0.6 W m−2) corresponds to almost a third of the total estimate of −1.9 W m−2, obtained with the mid-range value of σw = 0.4 m s−1, and is comparable to the total diversity of current aerosol forcing estimates. Reducing the uncertainty in the parameterisation of σw would therefore be an important step towards reducing the uncertainty in estimates of the indirect aerosol effects. Detailed examination of regional radiative flux perturbations reveals that aerosol microphysics can be responsible for some climate-relevant radiative effects, highlighting the importance of including microphysical aerosol processes in GCMs.

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The importance of vertical velocity variability for estimates of the indirect aerosol effects

Atmospheric Chemistry and Physics ◽

10.5194/acp-14-6369-2014 ◽

2014 ◽

Vol 14 (12) ◽

pp. 6369-6393 ◽

Cited By ~ 38

Author(s):

R. E. L. West ◽

P. Stier ◽

A. Jones ◽

C. E. Johnson ◽

G. W. Mann ◽

...

Keyword(s):

Vertical Velocity ◽

Cloud Droplet ◽

Detailed Examination ◽

Radiative Flux ◽

Subgrid Scale ◽

Anthropogenic Aerosol ◽

Aerosol Forcing ◽

Hadley Centre ◽

The Uk ◽

Aerosol Effects

Abstract. The activation of aerosols to form cloud droplets is dependent upon vertical velocities whose local variability is not typically resolved at the GCM grid scale. Consequently, it is necessary to represent the subgrid-scale variability of vertical velocity in the calculation of cloud droplet number concentration. This study uses the UK Chemistry and Aerosols community model (UKCA) within the Hadley Centre Global Environmental Model (HadGEM3), coupled for the first time to an explicit aerosol activation parameterisation, and hence known as UKCA-Activate. We explore the range of uncertainty in estimates of the indirect aerosol effects attributable to the choice of parameterisation of the subgrid-scale variability of vertical velocity in HadGEM-UKCA. Results of simulations demonstrate that the use of a characteristic vertical velocity cannot replicate results derived with a distribution of vertical velocities, and is to be discouraged in GCMs. This study focuses on the effect of the variance (σw2) of a Gaussian pdf (probability density function) of vertical velocity. Fixed values of σw (spanning the range measured in situ by nine flight campaigns found in the literature) and a configuration in which σw depends on turbulent kinetic energy are tested. Results from the mid-range fixed σw and TKE-based configurations both compare well with observed vertical velocity distributions and cloud droplet number concentrations. The radiative flux perturbation due to the total effects of anthropogenic aerosol is estimated at −1.9 W m−2 with σw = 0.1 m s−1, −2.1 W m−2 with σw derived from TKE, −2.25 W m−2 with σw = 0.4 m s−1, and −2.3 W m−2 with σw = 0.7 m s−1. The breadth of this range is 0.4 W m−2, which is comparable to a substantial fraction of the total diversity of current aerosol forcing estimates. Reducing the uncertainty in the parameterisation of σw would therefore be an important step towards reducing the uncertainty in estimates of the indirect aerosol effects. Detailed examination of regional radiative flux perturbations reveals that aerosol microphysics can be responsible for some climate-relevant radiative effects, highlighting the importance of including microphysical aerosol processes in GCMs.

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Impact of the modal aerosol scheme GLOMAP-mode on aerosol forcing in the Hadley Centre Global Environmental Model

Atmospheric Chemistry and Physics ◽

10.5194/acp-13-3027-2013 ◽

2013 ◽

Vol 13 (6) ◽

pp. 3027-3044 ◽

Cited By ~ 71

Author(s):

N. Bellouin ◽

G. W. Mann ◽

M. T. Woodhouse ◽

C. Johnson ◽

K. S. Carslaw ◽

...

Keyword(s):

Residence Time ◽

Large Scale ◽

Cloud Condensation Nuclei ◽

Cloud Droplet ◽

Environmental Model ◽

Anthropogenic Aerosol ◽

Microphysics Scheme ◽

Aerosol Forcing ◽

Global Environmental ◽

Hadley Centre

Abstract. The Hadley Centre Global Environmental Model (HadGEM) includes two aerosol schemes: the Coupled Large-scale Aerosol Simulator for Studies in Climate (CLASSIC), and the new Global Model of Aerosol Processes (GLOMAP-mode). GLOMAP-mode is a modal aerosol microphysics scheme that simulates not only aerosol mass but also aerosol number, represents internally-mixed particles, and includes aerosol microphysical processes such as nucleation. In this study, both schemes provide hindcast simulations of natural and anthropogenic aerosol species for the period 2000–2006. HadGEM simulations of the aerosol optical depth using GLOMAP-mode compare better than CLASSIC against a data-assimilated aerosol re-analysis and aerosol ground-based observations. Because of differences in wet deposition rates, GLOMAP-mode sulphate aerosol residence time is two days longer than CLASSIC sulphate aerosols, whereas black carbon residence time is much shorter. As a result, CLASSIC underestimates aerosol optical depths in continental regions of the Northern Hemisphere and likely overestimates absorption in remote regions. Aerosol direct and first indirect radiative forcings are computed from simulations of aerosols with emissions for the year 1850 and 2000. In 1850, GLOMAP-mode predicts lower aerosol optical depths and higher cloud droplet number concentrations than CLASSIC. Consequently, simulated clouds are much less susceptible to natural and anthropogenic aerosol changes when the microphysical scheme is used. In particular, the response of cloud condensation nuclei to an increase in dimethyl sulphide emissions becomes a factor of four smaller. The combined effect of different 1850 baselines, residence times, and abilities to affect cloud droplet number, leads to substantial differences in the aerosol forcings simulated by the two schemes. GLOMAP-mode finds a present-day direct aerosol forcing of −0.49 W m−2 on a global average, 72% stronger than the corresponding forcing from CLASSIC. This difference is compensated by changes in first indirect aerosol forcing: the forcing of −1.17 W m−2 obtained with GLOMAP-mode is 20% weaker than with CLASSIC. Results suggest that mass-based schemes such as CLASSIC lack the necessary sophistication to provide realistic input to aerosol-cloud interaction schemes. Furthermore, the importance of the 1850 baseline highlights how model skill in predicting present-day aerosol does not guarantee reliable forcing estimates. Those findings suggest that the more complex representation of aerosol processes in microphysical schemes improves the fidelity of simulated aerosol forcings.

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Impact of the modal aerosol scheme GLOMAP-mode on aerosol forcing in the Hadley Centre Global Environmental Model

Atmospheric Chemistry and Physics Discussions ◽

10.5194/acpd-12-21437-2012 ◽

2012 ◽

Vol 12 (8) ◽

pp. 21437-21479 ◽

Cited By ~ 4

Author(s):

N. Bellouin ◽

G. W. Mann ◽

M. T. Woodhouse ◽

C. Johnson ◽

K. S. Carslaw ◽

...

Keyword(s):

Residence Time ◽

Large Scale ◽

Cloud Condensation Nuclei ◽

Cloud Droplet ◽

Environmental Model ◽

Anthropogenic Aerosol ◽

Microphysics Scheme ◽

Aerosol Forcing ◽

Global Environmental ◽

Hadley Centre

Abstract. The Hadley Centre Global Environmental Model (HadGEM) includes two aerosol schemes: the Coupled Large-scale Aerosol Simulator for Studies in Climate (CLASSIC), and the new Global Model of Aerosol Processes (GLOMAP-mode). GLOMAP-mode is a modal aerosol microphysics scheme that simulates not only aerosol mass but also aerosol number, represents internally-mixed particles, and includes aerosol microphysical processes such as nucleation. In this study, both schemes provide hindcast simulations of natural and anthropogenic aerosol species for the period 2000–2006. HadGEM simulations using GLOMAP-mode compare better than CLASSIC against a data-assimilated aerosol re-analysis and aerosol ground-based observations. GLOMAP-mode sulphate aerosol residence time is two days longer than CLASSIC sulphate aerosols, whereas black carbon residence time is much shorter. As a result, CLASSIC underestimates aerosol optical depths in continental regions of the Northern Hemisphere and likely overestimates absorption in remote regions. Aerosol direct and first indirect radiative forcings are computed from simulations of aerosols with emissions for the year 1850 and 2000. In 1850, GLOMAP-mode predicts lower aerosol optical depths and higher cloud droplet number concentrations than CLASSIC. Consequently, simulated clouds are much less susceptible to natural and anthropogenic aerosol changes when the microphysical scheme is used. In particular, the response of cloud condensation nuclei to an increase in dimethyl sulphide emissions becomes a factor of four smaller. The combined effect of different 1850 baselines, residence times, and cloud susceptibilities, leads to substantial differences in the aerosol forcings simulated by the two schemes. GLOMAP-mode finds a present-day direct aerosol forcing of −0.49 W m−2 on a global average, 72% stronger than the corresponding forcing from CLASSIC. This difference is compensated by changes in first indirect aerosol forcing: the forcing of −1.17 W m−2 obtained with GLOMAP-mode is 20% weaker than with CLASSIC. Results suggest that mass-based schemes such as CLASSIC lack the necessary sophistication to provide realistic input to aerosol-cloud interaction schemes. Furthermore, the importance of the 1850 baseline highlights how model skill in predicting present-day aerosol does not guarantee reliable forcing estimates. Those findings suggest that the more complex representation of aerosol processes in microphysical schemes improves the fidelity of simulated aerosol forcings.

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Monte Carlo-based subgrid parameterization of vertical velocity and stratiform cloud microphysics in ECHAM5.5-HAM2

Atmospheric Chemistry and Physics Discussions ◽

10.5194/acpd-13-5477-2013 ◽

2013 ◽

Vol 13 (2) ◽

pp. 5477-5507

Author(s):

J. Tonttila ◽

P. Räisänen ◽

H. Järvinen

Keyword(s):

Monte Carlo ◽

Vertical Velocity ◽

Cloud Droplet ◽

Cloud Microphysics ◽

Radiative Effects ◽

Cloud Droplets ◽

Microphysical Properties ◽

Cloud Radiative Effects ◽

Cloud Activation ◽

The Impact

Abstract. A new method for parameterizing the subgrid variations of vertical velocity and cloud droplet number concentration (CDNC) is presented for GCMs. These parameterizations build on top of existing parameterizations that create stochastic subgrid cloud columns inside the GCM grid-cells, which can be employed by the Monte Carlo independent column approximation approach for radiative transfer. The new model version adds a description for vertical velocity in individual subgrid columns, which can be used to compute cloud activation and the subgrid distribution of the number of cloud droplets explicitly. This provides a consistent way for simulating the cloud radiative effects with two-moment cloud microphysical properties defined in subgrid-scale. The primary impact of the new parameterizations is to decrease the CDNC over polluted continents, while over the oceans the impact is smaller. This promotes changes in the global distribution of the cloud radiative effects and might thus have implications on model estimation of the indirect radiative effect of aerosols.

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Human influence strengthens the contrast between tropical wet and dry regions

10.5194/egusphere-egu2020-16966 ◽

2020 ◽

Author(s):

Andrew Schurer ◽

Gabriele Hegerl ◽

Andrew Ballinger ◽

Andrew Friedman

Keyword(s):

Climate Models ◽

Climate Model ◽

Reanalysis Data ◽

Ensemble Member ◽

Anthropogenic Aerosol ◽

In Situ Data ◽

Aerosol Forcing ◽

External Forcings ◽

Aerosol Effects ◽

The Tropics

<p>Climate models predict a strengthening contrast between wet and dry regions in the tropics and subtropics (30&#176;S-30&#176;N), and data from the latest model intercomparison project (CMIP6) support this expectation. Rainfall in ascending regions increases, and in descending regions decreases in both climate model and reanalysis data. This strengthening contrast can be captured by tracking rainfall change each month in the wettest and driest third of the tropics and subtropics combined. Since wet and dry regions are selected individually for each model ensemble member, and the observations, and for each month, this analysis is largely unaffected by biases in location of precipitation features. Blended satellite and in situ data support the model-simulated tendency to sharpening contrasts between wet and dry regions, with rainfall in wet regions increasing substantially contrasted by a slight decrease in dry regions. These new datasets allow us to detect with more confidence the effect of external forcings on these changes, attribute them for the first time to the response to anthropogenic and natural forcings separately, and determine that the observed trends are statistically larger than the model responses. Our results show that the observed change is best explained by increasing greenhouse gases with natural forcing contributing some increase following the drop in wet region precipitation after Mount Pinatubo, while anthropogenic aerosol effects are expected to show a weak tropic-wide trend at the present time of flat global aerosol forcing. As expected from climate models, the observed signal strengthens further when focusing on the wet tail of spatial distributions in both models and data.</p>

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Anthropogenic aerosol forcing – insights from multiple estimates from aerosol-climate models with reduced complexity

Atmospheric Chemistry and Physics ◽

10.5194/acp-19-6821-2019 ◽

2019 ◽

Vol 19 (10) ◽

pp. 6821-6841 ◽

Cited By ~ 10

Author(s):

Stephanie Fiedler ◽

Stefan Kinne ◽

Wan Ting Katty Huang ◽

Petri Räisänen ◽

Declan O'Donnell ◽

...

Keyword(s):

Radiative Forcing ◽

Climate Models ◽

Coupled Model ◽

Cloud Droplet ◽

Internal Variability ◽

Anthropogenic Aerosol ◽

Aerosol Forcing ◽

Effective Radiative Forcing ◽

Absolute Magnitudes ◽

Natural Aerosols

Abstract. This study assesses the change in anthropogenic aerosol forcing from the mid-1970s to the mid-2000s. Both decades had similar global-mean anthropogenic aerosol optical depths but substantially different global distributions. For both years, we quantify (i) the forcing spread due to model-internal variability and (ii) the forcing spread among models. Our assessment is based on new ensembles of atmosphere-only simulations with five state-of-the-art Earth system models. Four of these models will be used in the sixth Coupled Model Intercomparison Project (CMIP6; Eyring et al., 2016). Here, the complexity of the anthropogenic aerosol has been reduced in the participating models. In all our simulations, we prescribe the same patterns of the anthropogenic aerosol optical properties and associated effects on the cloud droplet number concentration. We calculate the instantaneous radiative forcing (RF) and the effective radiative forcing (ERF). Their difference defines the net contribution from rapid adjustments. Our simulations show a model spread in ERF from −0.4 to −0.9 W m−2. The standard deviation in annual ERF is 0.3 W m−2, based on 180 individual estimates from each participating model. This result implies that identifying the model spread in ERF due to systematic differences requires averaging over a sufficiently large number of years. Moreover, we find almost identical ERFs for the mid-1970s and mid-2000s for individual models, although there are major model differences in natural aerosols and clouds. The model-ensemble mean ERF is −0.54 W m−2 for the pre-industrial era to the mid-1970s and −0.59 W m−2 for the pre-industrial era to the mid-2000s. Our result suggests that comparing ERF changes between two observable periods rather than absolute magnitudes relative to a poorly constrained pre-industrial state might provide a better test for a model's ability to represent transient climate changes.

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Exploring aerosol cloud interaction using VOCALS-REx aircraft measurements

10.5194/acp-2018-667 ◽

2018 ◽

Author(s):

Hailing Jia ◽

Xiaoyan Ma ◽

Yangang Liu

Keyword(s):

Vertical Velocity ◽

Cloud Droplet ◽

Liquid Water Content ◽

Relative Dispersion ◽

Aircraft Measurements ◽

Cloud Droplets ◽

Aerosol Cloud ◽

Microphysical Properties ◽

Entrainment Zone ◽

The Difference

Abstract. In situ aircraft measurements during the VAMOS Ocean–Cloud–Atmosphere–Land Study-Regional Experiment (VOCALS-REx) field campaign are employed to study the interaction between aerosol and stratocumulus over the southeast Pacific Ocean, as well as entrainment process near the top of stratocumulus and its possible impacts on aerosol–cloud interaction. Our analysis suggest that the increase of liquid water content (LWC) is mainly contributed by cloud droplet number concentration (Nd) instead of effective radius of cloud droplets in the polluted case, in which more droplets form with smaller size, while the opposite is true in the clean case. By looking into the influences of dynamical conditions and aerosol microphysical properties on the cloud droplet formation, it is confirmed that cloud droplets are more easily to form under the conditions with large vertical velocity and aerosol size. An increase in aerosol concentration tends to increase both Nd and relative dispersion (&varepsilon;), while an increase in vertical velocity (w) often increases Nd but decreases &varepsilon;. After constraining the differences of cloud dynamics, positive correlation between &varepsilon; and Nd become stronger, implying that perturbations of w could weaken the influence of aerosol on &varepsilon;, and hence may result in an underestimation of aerosol dispersion effect. The difference of cloud microphysical properties between entrainment and non-entrainment zones confirms that the entrainment-mixing mechanism is predominantly extreme inhomogeneous in the stratocumulus that capped by a sharp inversion, namely the entrainment reduces Nd and LWC by 28.9 % and 24.8 % on average, respectively, while the size of droplets is relatively unaffected. In entrainment zone, smaller aerosols and drier air entrained from the top induce less cloud droplet with respect to total in-cloud particles (0.56 ± 0.22) than the case in non-entrainment zone (0.73 ± .0.13) by inhibiting aerosol activation and promoting cloud droplets evaporation.

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Simple Plumes: A parameterization of anthropogenic aerosol optical properties and an associated Twomey effect for climate studies

10.5194/gmd-2016-189 ◽

2016 ◽

Cited By ~ 5

Author(s):

Bjorn Stevens ◽

Stephanie Fiedler ◽

Stefan Kinne ◽

Karsten Peters ◽

Sebastian Rast ◽

...

Keyword(s):

Optical Properties ◽

Cloud Droplet ◽

Anthropogenic Source ◽

Climate Modelling ◽

Industrial Emissions ◽

Max Planck ◽

Aerosol Optical Properties ◽

Climatic Effects ◽

Anthropogenic Aerosol ◽

Aerosol Forcing

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 for the harmonized use in climate modelling studies. It has been designed to be easy to implement, change and use, and thereby enable studies exploring the climatic effects of different plausible aerosol distributions and their impact on clouds. 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, which is based on present day (2005) observations. Decadal variations in the amplitude of the plumes over the historical (post 1850) period is derived by scaling the plumes with associated national emission sources of SO2 and NH3. Two types of plumes are considered: one predominantly associated with biomass burning the other with industrial emissions. The two types of plumes 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. Experiments using the simple plume model are performed with the Max Planck Institute Earth System Model. The instantaneous and effective aerosol radiative forcings is estimated to be −0.6 W m−2 and −0.7 W m−2 respectively. Forcing from aerosol-cloud interactions (the Twomey effect) offsets the reduction of clear-sky forcing from clouds, so that the net effect of clouds on the aerosol forcing is small.

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CCN and Vertical Velocity Influences on Droplet Concentrations and Supersaturations in Clean and Polluted Stratus Clouds

Journal of the Atmospheric Sciences ◽

10.1175/jas-d-13-086.1 ◽

2013 ◽

Vol 71 (1) ◽

pp. 312-331 ◽

Cited By ~ 35

Author(s):

James G. Hudson ◽

Stephen Noble

Keyword(s):

Vertical Velocity ◽

Cloud Condensation Nuclei ◽

Cloud Droplet ◽

Cloud Microphysics ◽

Cloud Droplets ◽

Stratus Cloud ◽

Marine Stratus ◽

Lower Cloud ◽

High Concentrations ◽

Positive Correlations

Abstract Cloud microphysics and cloud condensation nuclei (CCN) measurements from two marine stratus cloud projects are presented and analyzed. Results show that the increase of cloud droplet concentrations Nc with CCN concentrations NCCN rolls off for NCCN at 1% supersaturation (S)N1% above 400 cm−3. Moreover, at such high concentrations Nc was not so well correlated with NCCN but tended to be more closely related to vertical velocity W or variations of W (σw). This changeover from predominate Nc dependence on NCCN to Nc dependence on W or σw is due to the higher slope k of CCN spectra at lower S, which is made more relevant by the lower cloud S that is forced by higher NCCN. Higher k makes greater influence of W or σw variations than NCCN variations on Nc. This changeover at high NCCN thus seems to limit the indirect aerosol effect (IAE). On the other hand, in clean-air stratus cloud S often exceeded 1% and decreased to slightly less than 0.1% in polluted conditions. This means that smaller CCN [those with higher critical S (Sc)], which are generally more numerous than larger CCN (lower Sc), are capable of producing stratus cloud droplets, especially when they are advected into clean marine air masses where they can induce IAE. Positive correlations between turbulence σw and NCCN are attributed to greater differential latent heat exchange of smaller more numerous cloud droplets that evaporate more readily. Such apparent CCN influences on cloud dynamics tend to support trends that oppose conventional IAE, that is, less rather than greater cloudiness in polluted environments.

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