Dependence of marine stratocumulus formation on aerosols

Hegg, D́ean A.

doi:10.1029/1999gl900268

Cited by 13 publications

(15 citation statements)

References 32 publications

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“…Figure 9 makes the point that MBL regions associated with little or no overhead cloudiness (the cell cores) are also associated with low accumulation mode concentrations. It also excites interest in connections between droplet nuclei abundance and cloud top heights, perhaps along the lines of Hegg [1999].…”

Section: Resultsmentioning

confidence: 99%

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Accumulation mode aerosol, pockets of open cells, and particle nucleation in the remote subtropical Pacific marine boundary layer

Petters¹,

Snider²,

Stevens³

et al. 2006

J. Geophys. Res.

106

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[1] We analyze a marine boundary layer cloud field encountered during the second research flight of the second Dynamics and Chemistry of Marine Stratocumulus Experiment. The cloud field is distinguished by the presence of pockets of open cells. Differences between the pockets and the surrounding stratocumulus clouds are studied utilizing in situ and satellite data. The pockets are characterized as regions where cloud radar echo tops are unusually variable, accumulation mode aerosol concentrations are low, and Aitken mode particles with a mode diameter at 0.02 mm dominate aerosol number concentration. The Aitken mode particles are thought to be generated by a nucleation event which occurred within the marine boundary layer. The low accumulation mode concentrations associated with the pockets are proposed to be necessary for their maintenance.

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Section: Resultsmentioning

confidence: 99%

“…Cloud condensation nuclei (CCN) play a critical role in the formation [ Hegg , 1999], lifetime [ Albrecht , 1989], and dissipation [ Ackerman et al , 1993] of marine stratocumulus clouds. All of these studies focus on connections between CCN, the generation of drizzle, and CCN scavenging.…”

Section: Introductionmentioning

confidence: 99%

Accumulation mode aerosol, pockets of open cells, and particle nucleation in the remote subtropical Pacific marine boundary layer

Petters¹,

Snider²,

Stevens³

et al. 2006

J. Geophys. Res.

106

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“…In most earlier studies of cloud droplet formation, the number concentration of aerosol particles did not exceed 10 4 cm −3 (e.g. Hjelmfelt et al, 1978;Hegg, 1999;Nenes et al, 2001;Feingold, 2003;Lance et al, 2004;Lohmann et al, 2004;Ervens et al, 2005;Segal and Khain, 2006;Kivekas et al, 2008;Cubison et al, 2008;Altaratz et al, 2008). This is realistic for regions with low or moderate air pollution, but in biomass burning plumes the aerosol particle number concentrations can reach up to ∼10 5 cm −3 Reid et al, 2005;Janhäll et al, 2009).…”

Section: Introductionmentioning

confidence: 87%

Aerosol- and updraft-limited regimes of cloud droplet formation: influence of particle number, size and hygroscopicity on the activation of cloud condensation nuclei (CCN)

et al. 2009

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Abstract.We have investigated the formation of cloud droplets under pyro-convective conditions using a cloud parcel model with detailed spectral microphysics and with the κ-Köhler model approach for efficient and realistic description of the cloud condensation nucleus (CCN) activity of aerosol particles. Assuming a typical biomass burning aerosol size distribution (accumulation mode centred at 120 nm), we have calculated initial cloud droplet number concentrations (N CD ) for a wide range of updraft velocities (w=0.25-20 m s −1 ) and aerosol particle number concentrations (N CN =200-10 5 cm −3 ) at the cloud base. Depending on the ratio between updraft velocity and particle number concentration (w/N CN ), we found three distinctly different regimes of CCN activation and cloud droplet formation:(1) An aerosol-limited regime that is characterized by high w/N CN ratios (>≈10 −3 m s −1 cm 3 ), high maximum values of water vapour supersaturation (S max >≈0.5%), and high activated fractions of aerosol particles (N CD /N CN >≈90%). In this regime N CD is directly proportional to N CN and practically independent of w.(2) An updraft-limited regime that is characterized by low w/N CN ratios (<≈10 −4 m s −1 cm 3 ), low maximum values of water vapour supersaturation (S max <≈0.2%), and low activated fractions of aerosol particles (N CD /N CN <≈20%). In this regime N CD is directly proportional to w and practically independent of N CN .Correspondence to: H. Su (hsu@mpch-mainz.mpg.de) (3) An aerosol-and updraft-sensitive regime (transitional regime), which is characterized by parameter values in between the two other regimes and covers most of the conditions relevant for pyro-convection. In this regime N CD depends non-linearly on both N CN and w.In sensitivity studies we have tested the influence of aerosol particle size distribution and hygroscopicity on N CD . Within the range of effective hygroscopicity parameters that is characteristic for continental atmospheric aerosols (κ≈0.05-0.6), we found that N CD depends rather weakly on the actual value of κ. A compensation of changes in κ and S max leads to an effective buffering of N CD . Only for aerosols with very low hygroscopicity (κ<0.05) and also in the updraft-limited regime for aerosols with higher than average hygroscopicity (κ>0.3) did the relative sensitivities ∂lnN CD /∂lnκ≈ ( N CD /N CD )/( κ/κ) exceed values of ∼0.2, indicating that a 50% difference in κ would change N CD by more than 10%.The influence of changing size distribution parameters was stronger than that of particle hygroscopicity. Nevertheless, similar regimes of CCN activation were observed in simulations with varying types of size distributions (polluted and pristine continental and marine aerosols with different proportions of nucleation, Aitken, accumulation, and coarse mode particles). In general, the different regimes can be discriminated with regard to the relative sensitivities of N CD against w and N CN (∂lnN CD /∂lnw and ∂lnN CD /∂lnN CN ). We propose to separate the different regimes by rel...

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“…Cloud LWP's inferred from satellite measurements vary greatly, tending to cluster in the 50-300 g m ÿ2 range [17,18]. Drizzle rate from stratocumulus clouds could be as small as 0:01 mm h ÿ1 and as large as 1 mm h ÿ1 [19,20]. Thus observable rates of drizzle formation, assuming a radius of 100 m for the collected droplets, require J 10 ÿ6 to 10 ÿ5 cm ÿ3 s ÿ1 .…”

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confidence: 97%

Kinetic Potential and Barrier Crossing: A Model for Warm Cloud Drizzle Formation

McGraw

Liu

2003

Phys. Rev. Lett.

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The kinetic potential of nucleation theory is used to describe droplet growth processes in a cloud. Drizzle formation is identified as a statistical barrier-crossing phenomenon that transforms cloud droplets to drizzle size with a rate dependent on turbulent diffusion, droplet collection, and size distribution. Steady-state and transient drizzle rates are calculated for typical cloud conditions. We find drizzle more likely under transient conditions. The model quantifies an important indirect effect of aerosols on climate-drizzle suppression in clouds of higher droplet concentration.

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Dependence of marine stratocumulus formation on aerosols

Cited by 13 publications

References 32 publications

Accumulation mode aerosol, pockets of open cells, and particle nucleation in the remote subtropical Pacific marine boundary layer

Accumulation mode aerosol, pockets of open cells, and particle nucleation in the remote subtropical Pacific marine boundary layer

Aerosol- and updraft-limited regimes of cloud droplet formation: influence of particle number, size and hygroscopicity on the activation of cloud condensation nuclei (CCN)

Kinetic Potential and Barrier Crossing: A Model for Warm Cloud Drizzle Formation

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