Quantifying the aerosol effect on droplet size distribution at cloud top

Abstract. This work uses the number concentration-effective diameter phase-space to test cloud sensitivity to variations in the aerosol population characteristics, such as the aerosol size distribution, number concentration and hygroscopicity. It is based on the information from the top of a cloud simulated by a bin-microphysics single-column model, for initial conditions typical of the Amazon. It is shown that the cloud-top evolution can be very sensitive to aerosol properties, but the relative importance of each parameter is variable. The sensitivity to each aerosol characteristic varies as a function of the tested parameter and is conditioned by the base values of the other parameters. The median radius of the aerosols showed the largest influence on this sensitivity. We show that all aerosol properties can have significant impacts on cloud microphysics, especially if the median radius of the aerosol size distribution is smaller than 0.05 μm.

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Influence of Microphysical Variability on Stochastic Condensation in a Turbulent Laboratory Cloud

Journal of the Atmospheric Sciences ◽

10.1175/jas-d-17-0158.1 ◽

2018 ◽

Vol 75 (1) ◽

pp. 189-201 ◽

Cited By ~ 16

Author(s):

N. Desai ◽

K. K. Chandrakar ◽

K. Chang ◽

W. Cantrell ◽

R. A. Shaw

Keyword(s):

Relaxation Time ◽

Size Distribution ◽

Droplet Size ◽

Droplet Size Distribution ◽

Number Concentration ◽

Droplet Growth ◽

Diffusional Growth ◽

Phase Relaxation ◽

Distribution Width ◽

Phase Relaxation Time

Diffusional growth of droplets by stochastic condensation and a resulting broadening of the size distribution has been considered as a mechanism for bridging the cloud droplet growth gap between condensation and collision–coalescence. Recent studies have shown that supersaturation fluctuations can lead to a broadening of the droplet size distribution at the condensational stage of droplet growth. However, most studies using stochastic models assume the phase relaxation time of a cloud parcel to be constant. In this paper, two questions are asked: how variability in droplet number concentration and radius influence the phase relaxation time and what effect it has on the droplet size distributions. To answer these questions, steady-state cloud conditions are created in the laboratory and digital inline holography is used to directly observe the variations in local number concentration and droplet size distribution and, thereby, the integral radius. Stochastic equations are also extended to account for fluctuations in integral radius and obtain new terms that are compared with the laboratory observations. It is found that the variability in integral radius is primarily driven by variations in the droplet number concentration and not the droplet radius. This variability does not contribute significantly to the mean droplet growth rate but does contribute significantly to the rate of increase of the size distribution width.

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Impact of cloud base turbulence on CCN activation: Single size CCN

Journal of the Atmospheric Sciences ◽

10.1175/jas-d-21-0184.1 ◽

2021 ◽

Keyword(s):

Droplet Size ◽

Initial Conditions ◽

Cloud Condensation Nuclei ◽

Droplet Size Distribution ◽

Spectral Width ◽

Cloud Base ◽

Computational Domain ◽

Critical Supersaturation ◽

The Impact ◽

Single Size

Abstract This paper examines the impact of cloud-base turbulence on activation of cloud condensation nuclei (CCN). Following our previous studies, we contrast activation within a non-turbulent adiabatic parcel and an adiabatic parcel filled with turbulence. The latter is simulated by applying a forced implicit large eddy simulation within a triply periodic computational domain of 643 m3. We consider two monodisperse CCN. Small CCN have a dry radius of 0.01 micron and a corresponding activation (critical) radius and critical supersaturation of 0.6 micron and 1.3%, respectively. Large CCN have a dry radius of 0.2 micron and feature activation radius of 5.4 micron and critical supersaturation 0.15 %. CCN are assumed in 200 cm−3 concentration in all cases. Mean cloud base updraft velocities of 0.33, 1, and 3 m s−1 are considered. In the non-turbulent parcel, all CCN are activated and lead to a monodisperse droplet size distribution above the cloud base, with practically the same droplet size in all simulations. In contrast, turbulence can lead to activation of only a fraction of all CCN with a non-zero spectral width above the cloud base, of the order of 1 micron, especially in the case of small CCN and weak mean cloud base ascent. We compare our results to studies of the turbulent single-size CCN activation in the Pi chamber. Sensitivity simulations that apply a smaller turbulence intensity, smaller computational domain, and modified initial conditions document the impact of specific modeling assumptions. The simulations call for a more realistic high-resolution modeling of turbulent cloud base activation.

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Long-term observations of cloud condensation nuclei in the Amazon rain forest – Part 1: Aerosol size distribution, hygroscopicity, and new model parametrizations for CCN prediction

Atmospheric Chemistry and Physics ◽

10.5194/acp-16-15709-2016 ◽

2016 ◽

Vol 16 (24) ◽

pp. 15709-15740 ◽

Cited By ~ 45

Author(s):

Mira L. Pöhlker ◽

Christopher Pöhlker ◽

Florian Ditas ◽

Thomas Klimach ◽

Isabella Hrabe de Angelis ◽

...

Keyword(s):

Size Distribution ◽

Aerosol Particle ◽

Rain Forest ◽

Seasonal Cycle ◽

Cloud Condensation Nuclei ◽

Number Concentration ◽

Aerosol Size Distribution ◽

Condensation Nuclei ◽

Central Amazon

Abstract. Size-resolved long-term measurements of atmospheric aerosol and cloud condensation nuclei (CCN) concentrations and hygroscopicity were conducted at the remote Amazon Tall Tower Observatory (ATTO) in the central Amazon Basin over a 1-year period and full seasonal cycle (March 2014–February 2015). The measurements provide a climatology of CCN properties characteristic of a remote central Amazonian rain forest site.The CCN measurements were continuously cycled through 10 levels of supersaturation (S  =  0.11 to 1.10 %) and span the aerosol particle size range from 20 to 245 nm. The mean critical diameters of CCN activation range from 43 nm at S  =  1.10 % to 172 nm at S  =  0.11 %. The particle hygroscopicity exhibits a pronounced size dependence with lower values for the Aitken mode (κAit  =  0.14 ± 0.03), higher values for the accumulation mode (κAcc  =  0.22 ± 0.05), and an overall mean value of κmean  =  0.17 ± 0.06, consistent with high fractions of organic aerosol.The hygroscopicity parameter, κ, exhibits remarkably little temporal variability: no pronounced diurnal cycles, only weak seasonal trends, and few short-term variations during long-range transport events. In contrast, the CCN number concentrations exhibit a pronounced seasonal cycle, tracking the pollution-related seasonality in total aerosol concentration. We find that the variability in the CCN concentrations in the central Amazon is mostly driven by aerosol particle number concentration and size distribution, while variations in aerosol hygroscopicity and chemical composition matter only during a few episodes.For modeling purposes, we compare different approaches of predicting CCN number concentration and present a novel parametrization, which allows accurate CCN predictions based on a small set of input data.

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A synthesis of cloud condensation nuclei counter (CCNC) measurements within the EUCAARI network

Atmospheric Chemistry and Physics ◽

10.5194/acp-15-12211-2015 ◽

2015 ◽

Vol 15 (21) ◽

pp. 12211-12229 ◽

Cited By ~ 21

Author(s):

M. Paramonov ◽

V.-M. Kerminen ◽

M. Gysel ◽

P. P. Aalto ◽

M. O. Andreae ◽

...

Keyword(s):

Size Distribution ◽

Cloud Condensation Nuclei ◽

Number Concentration ◽

West Germany ◽

Aerosol Size Distribution ◽

Spatial And Temporal Variability ◽

Condensation Nuclei ◽

Aerosol Cloud ◽

Hygroscopic Properties ◽

South West

Abstract. Cloud condensation nuclei counter (CCNC) measurements performed at 14 locations around the world within the European Integrated project on Aerosol Cloud Climate and Air Quality interactions (EUCAARI) framework have been analysed and discussed with respect to the cloud condensation nuclei (CCN) activation and hygroscopic properties of the atmospheric aerosol. The annual mean ratio of activated cloud condensation nuclei (NCCN) to the total number concentration of particles (NCN), known as the activated fraction A, shows a similar functional dependence on supersaturation S at many locations – exceptions to this being certain marine locations, a free troposphere site and background sites in south-west Germany and northern Finland. The use of total number concentration of particles above 50 and 100 nm diameter when calculating the activated fractions (A50 and A100, respectively) renders a much more stable dependence of A on S; A50 and A100 also reveal the effect of the size distribution on CCN activation. With respect to chemical composition, it was found that the hygroscopicity of aerosol particles as a function of size differs among locations. The hygroscopicity parameter κ decreased with an increasing size at a continental site in south-west Germany and fluctuated without any particular size dependence across the observed size range in the remote tropical North Atlantic and rural central Hungary. At all other locations κ increased with size. In fact, in Hyytiälä, Vavihill, Jungfraujoch and Pallas the difference in hygroscopicity between Aitken and accumulation mode aerosol was statistically significant at the 5 % significance level. In a boreal environment the assumption of a size-independent κ can lead to a potentially substantial overestimation of NCCN at S levels above 0.6 %. The same is true for other locations where κ was found to increase with size. While detailed information about aerosol hygroscopicity can significantly improve the prediction of NCCN, total aerosol number concentration and aerosol size distribution remain more important parameters. The seasonal and diurnal patterns of CCN activation and hygroscopic properties vary among three long-term locations, highlighting the spatial and temporal variability of potential aerosol–cloud interactions in various environments.

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Initiation of coalescence in a cumulus cloud: a beneficial influence of entrainment and mixing

Atmospheric Chemistry and Physics Discussions ◽

10.5194/acpd-11-10557-2011 ◽

2011 ◽

Vol 11 (4) ◽

pp. 10557-10613 ◽

Cited By ~ 7

Author(s):

W. A. Cooper ◽

S. G. Lasher-Trapp ◽

A. M. Blyth

Keyword(s):

Size Distribution ◽

Droplet Size ◽

Cloud Condensation Nuclei ◽

Droplet Size Distribution ◽

Main Peak ◽

Cumulus Cloud ◽

Cumulus Clouds ◽

Warm Rain ◽

Short Time ◽

Giant Nuclei

Abstract. Although rain has been observed to form in warm cumulus clouds within about twenty minutes, calculations that represent condensation and coalescence accurately in such clouds have had difficulty producing rainfall in such a short time except via processes involving giant cloud condensation nuclei (with diameters larger than 2 μm). This model-based study explores a different possible mechanism for accelerating the production of warm rain, one that depends on the variability in droplet trajectories arriving at a given location and time in a cumulus cloud. In the presence of entrainment such droplets experience different growth histories, and the result is broadening of the droplet size distribution. That broadening favours coalescence, leading to embryos that grow to raindrops. These calculations do lead to production of rain that is within the lower range of observations for clouds of Florida, USA, the location on which the input conditions were based. The process emphasized in this study, the formation of drizzle via collisions among droplets in the main peak of the droplet size distribution, complements the growth of precipitation on giant nuclei, which is also an important source of the first rain in the case studied. The results indicate that the mechanism developed here should be considered an important influence on the formation of rain in warm clouds.

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The role of the particle size distribution in assessing aerosol composition effects on simulated droplet activation

Atmospheric Chemistry and Physics Discussions ◽

10.5194/acpd-10-4189-2010 ◽

2010 ◽

Vol 10 (2) ◽

pp. 4189-4223 ◽

Cited By ~ 1

Author(s):

D. S. Ward ◽

T. Eidhammer ◽

W. R. Cotton ◽

S. M. Kreidenweis

Keyword(s):

Size Distribution ◽

Cloud Condensation Nuclei ◽

Atmospheric Modeling ◽

Number Concentration ◽

Aerosol Size Distribution ◽

Detailed Knowledge ◽

Aerosol Composition ◽

Droplet Activation ◽

The Impact ◽

Aerosol Hygroscopicity

Abstract. Variations in the chemical composition of atmospheric aerosols alter their hygroscopicity and can lead to changes in the cloud-active fraction of the aerosols, or cloud condensation nuclei (CCN) number concentration. To investigate the importance of this effect under different atmospheric conditions, cloud droplet formation was simulated with a Lagrangian parcel model. Initial values of updraft speed and temperature were systematically varied along with aerosol number concentration, size and hygroscopicity (represented by the hygroscopicity parameter, κ). A previous study classifies the sensitivity of CCN activity to compositional changes based on the supersaturation reached in the parcel model. We found that these classifications could not be generalized to a range of aerosol size distribution median radii. Instead, variations in sensitivity with size depend on the location of the dry critical radius for droplet activation relative to the size distribution median radius. The parcel model output was used to construct droplet activation lookup tables based on κ that were implemented in the Regional Atmospheric Modeling System (RAMS) microphysical scheme. As a first application of this system, aerosol hygroscopicity and size were varied in a series of RAMS mesoscale simulations designed to investigate the sensitivity of a mixed-phase orographic cloud case to the parameter variations. Observations from a recent field campaign in northwestern Colorado provided the basis for the aerosol field initializations. Model results show moderate sensitivity in the distribution of total case precipitation to extreme changes in κ, and minimal sensitivity to observed changes in estimated κ. The impact of varying aerosol hygroscopicity diminished with increasing median radius, as expected from the parcel model results. The conclusions drawn from these simulations could simplify similar research in other cloud regimes by defining the need, or lack of need, for detailed knowledge of aerosol composition.

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A New Mechanism of Droplet Size Distribution Broadening during Diffusional Growth

Journal of the Atmospheric Sciences ◽

10.1175/jas-d-12-0182.1 ◽

2013 ◽

Vol 70 (7) ◽

pp. 2051-2071 ◽

Cited By ~ 22

Author(s):

Alexei Korolev ◽

Mark Pinsky ◽

Alex Khain

Keyword(s):

Size Distribution ◽

Droplet Size ◽

Initial Conditions ◽

Droplet Size Distribution ◽

Cloud Base ◽

Diffusional Growth ◽

Cloud Droplets ◽

Droplet Concentration ◽

Droplet Sizes ◽

Precipitation Formation

Abstract A new mechanism has been developed for size distribution broadening toward large droplet sizes. This mechanism may explain the rapid formation of large cloud droplets, which may subsequently trigger precipitation formation through the collision–coalescence process. The essence of the new mechanism consists of a sequence of mixing events between ascending and descending parcels. When adiabatically ascending and descending parcels having the same initial conditions at the cloud base arrive at the same level, they will have different droplet sizes and temperatures, as well as different supersaturations. Isobaric mixing between such parcels followed by further ascents and descents enables the enhanced growth of large droplets. The numerical simulation of this process suggests that the formation of large 30–40-μm droplets may occur within 20–30 min inside a shallow adiabatic stratiform layer. The dependencies of the rate of the droplet size distribution broadening on the intensity of the vertical fluctuations, their spatial amplitude, rate of mixing, droplet concentration, and other parameters are considered here. The effectiveness of this mechanism in different types of clouds is discussed.

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The role of the particle size distribution in assessing aerosol composition effects on simulated droplet activation

Atmospheric Chemistry and Physics ◽

10.5194/acp-10-5435-2010 ◽

2010 ◽

Vol 10 (12) ◽

pp. 5435-5447 ◽

Cited By ~ 30

Author(s):

D. S. Ward ◽

T. Eidhammer ◽

W. R. Cotton ◽

S. M. Kreidenweis

Keyword(s):

Size Distribution ◽

Cloud Condensation Nuclei ◽

Atmospheric Modeling ◽

Number Concentration ◽

Aerosol Size Distribution ◽

Detailed Knowledge ◽

Aerosol Composition ◽

Droplet Activation ◽

The Impact ◽

Aerosol Hygroscopicity

Abstract. Variations in the chemical composition of atmospheric aerosols alter their hygroscopicity and can lead to changes in the cloud-active fraction of the aerosols, or cloud condensation nuclei (CCN) number concentration. To investigate the importance of this effect under different atmospheric conditions, cloud droplet formation was simulated with a Lagrangian parcel model. Initial values of updraft speed and temperature were systematically varied along with aerosol number concentration, size and hygroscopicity (represented by the hygroscopicity parameter, κ). A previous study classifies the sensitivity of CCN activity to compositional changes based on the supersaturation reached in the parcel model. We found that these classifications could not be generalized to a range of aerosol size distribution median radii. Instead, variations in sensitivity with size depend on the location of the dry critical radius for droplet activation relative to the size distribution median radius. The parcel model output was used to construct droplet activation lookup tables based on κ that were implemented in the Regional Atmospheric Modeling System (RAMS) microphysical scheme. As a first application of this system, aerosol hygroscopicity and size were varied in a series of RAMS mesoscale simulations designed to investigate the sensitivity of a mixed-phase orographic cloud case to the parameter variations. Observations from a recent field campaign in northwestern Colorado provided the basis for the aerosol field initializations. Model results show moderate sensitivity in the distribution of total case precipitation to extreme changes in κ, and minimal sensitivity to observed changes in estimated κ. The impact of varying aerosol hygroscopicity diminished with increasing median radius, as expected from the parcel model results. The conclusions drawn from these simulations could simplify similar research in other cloud regimes by defining the need, or lack of need, for detailed knowledge of aerosol composition.

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A synthesis of cloud condensation nuclei counter (CCNC) measurements within the EUCAARI network

Atmospheric Chemistry and Physics Discussions ◽

10.5194/acpd-15-15039-2015 ◽

2015 ◽

Vol 15 (11) ◽

pp. 15039-15086

Author(s):

M. Paramonov ◽

V.-M. Kerminen ◽

M. Gysel ◽

P. P. Aalto ◽

M. O. Andreae ◽

...

Keyword(s):

Size Distribution ◽

Functional Dependence ◽

Cloud Condensation Nuclei ◽

Number Concentration ◽

West Germany ◽

Aerosol Size Distribution ◽

Spatial And Temporal Variability ◽

Condensation Nuclei ◽

Hygroscopic Properties ◽

South West

Abstract. Cloud Condensation Nuclei Counter (CCNC) measurements performed at 14 locations around the world within the EUCAARI framework have been analysed and discussed with respect to the cloud condensation nuclei (CCN) activation and hygroscopic properties of the atmospheric aerosol. The annual mean ratio of activated cloud condensation nuclei (NCCN) to the total number concentration of particles (NCN), known as the activated fraction A, shows a similar functional dependence on supersaturation S at many locations; exceptions to this being certain marine locations, a free troposphere site and background sites in south-west Germany and northern Finland. The use of total number concentration of particles above 50 and 100 nm diameter when calculating the activated fractions (A50 and A100, respectively) renders a much more stable dependence of A on S; A50 and A100 also reveal the effect of the size distribution on CCN activation. With respect to chemical composition, it was found that the hygroscopicity of aerosol particles as a function of size differs among locations. The hygroscopicity parameter κ decreased with an increasing size at a continental site in south-west Germany and fluctuated without any particular size dependence across the observed size range in the remote tropical North Atlantic and rural central Hungary. At all other locations κ increased with size. In fact, in Hyytiälä, Vavihill, Jungfraujoch and Pallas the difference in hygroscopicity between Aitken and accumulation mode aerosol was statistically significant at the 5% significance level. In a boreal environment the assumption of a size-independent κ can lead to a potentially substantial overestimation of NCCN at S levels above 0.6%; similar is true for other locations where κ was found to increase with size. While detailed information about aerosol hygroscopicity can significantly improve the prediction of NCCN, total aerosol number concentration and aerosol size distribution remain more important parameters. The seasonal and diurnal patterns of CCN activation and hygroscopic properties vary among three long-term locations, highlighting the spatial and temporal variability of potential aerosol-cloud interactions in various environments.

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