A laboratory investigation into the aggregation efficiency of small ice crystals

Connolly, Paul; Emersic, Christopher; Field, Paul R.

doi:10.5194/acp-12-2055-2012

Cited by 105 publications

(90 citation statements)

References 51 publications

(80 reference statements)

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“…Recently, Connolly et al (2012) showed that large aggregation efficiencies were associated with ice crystals that had quite complex geometry. In their study dendrites and stellar crystal habits had much higher aggregation efficiencies than simple hexagonal plates or columnar crystals.…”

Section: Discussionmentioning

confidence: 99%

Observations and modelling of microphysical variability, aggregation and sedimentation in tropical anvil cirrus outflow regions

et al. 2012

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Abstract. Aircraft measurements of the microphysics of a tropical convective anvil (at temperatures ∼ −60 • C) forming above the Hector storm, over the Tiwi Islands, Northern Australia, have been conducted with a view to determining ice crystal aggregation efficiencies from in situ measurements. The observed microphysics have been compared to an explicit bin-microphysical model of the anvil region, which includes crystal growth by vapour diffusion and aggregation and the process of differential sedimentation.It has been found in flights made using straight and level runs perpendicular to the storm that the number of ice crystals initially decreased with distance from the storm as aggregation took place resulting in larger crystals, followed by their loss from the cloud layer due to sedimentation. The net result was that the mass (i.e. Ice Water Content) in the anvil Ci cloud decreased, but also that the average particle size (weighted by number) remained relatively constant along the length of the anvil outflow. Comparisons with the explicit microphysics model showed that the changes in the shapes of the ice crystal spectra as a function of distance from the storm could be explained by the model if the aggregation efficiency was set to values of E agg ∼0.5 and higher. This result is supported by recent literature on aggregation efficiencies for complex ice particles and suggests that either the mechanism of particle interlocking is important to the aggregation process, or that other effects are occuring, such as enhancement of ice-aggregation by high electric fields that arise as a consequence of charge separation within the storm.It is noteworthy that this value of the ice crystal aggregation efficiency is much larger than values used in cloud resolving models at these temperatures, which typically use E ∼ 0.0016. These results are important to understanding how cold clouds evolve in time and for the treatment of the evolution of tropical Ci in numerical models.

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Section: Discussionmentioning

confidence: 99%

Observations and modelling of microphysical variability, aggregation and sedimentation in tropical anvil cirrus outflow regions

et al. 2012

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“…This was done in some former studies (see, e.g., Passarelli, 1978;Lin et al, 1983;Mitchell, 1988;Levkov et al, 1992;Ferrier, 1994;Lawson et al, 1998;Field and Heymsfield, 2003;Morrison et al, 2005). However, the main focus in all these model studies as well as in most observational studies (see, e.g., Field et al, 2006;Connolly et al, 2012) is on the "high" temperature regime, i.e. T > −30 • C, where precipitation is mainly formed via the ice phase.…”

Section: Introductionmentioning

confidence: 99%

Formulation and test of an ice aggregation scheme for two-moment bulk microphysics schemes

2013

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A simple formulation of aggregation for two-moment bulk microphysical models is derived. The solution involves the evaluation of a double integral of the collection kernel weighted with the crystal size (or mass) distribution. This quantity is to be inserted into the differential equation for the crystal number concentration which has classical Smoluchowski form. The double integrals are evaluated numerically for log-normal size distributions over a large range of geometric mean masses. A polynomial fit of the results is given that yields good accuracy. Various tests of the new parameterisation are described: aggregation as stand-alone process, in a box-model, and in 2-D simulations of a cirrostratus cloud. These tests suggest that aggregation can become important for warm cirrus, leading even to higher and longer-lasting in-cloud supersaturation. Cold cirrus clouds are hardly affected by aggregation. The collection efficiency is taken from a parameterisation that assumes a dependence on temperature, a situation that might be improved when reliable measurements from cloud chambers suggests the necessary constraints for the choice of this parameter

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“…The Manchester Ice Cloud Chamber (MICC) is a 10 m high stainless-steel tube of 1 m diameter which is contained in three cold rooms (spanning over three floors ;Connolly et al, 2012;Emersic et al, 2015). The cold rooms can be temperature-controlled from room temperature to approximately −50 • C. Two scroll pumps are used to evacuate the chamber in order to form clouds in these experiments.…”

Section: Manchester Ice Cloud Chamber (Micc)mentioning

confidence: 99%

“…The Aerosol-Cloud and Precipitation Interactions Model (ACPIM; Connolly et al, 2012) was chosen for testing of the experimental data of the ammonium sulfate control runs. The model is adapted to be used with chamber measurement data such as the ones reported here.…”

Section: Model Comparison For Ammonium Sulfate Control Experimentsmentioning

confidence: 99%

The efficiency of secondary organic aerosol particles acting as ice-nucleating particles under mixed-phase cloud conditions

Frey

Dorsey

et al. 2018

Atmos. Chem. Phys.

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Abstract. Secondary organic aerosol (SOA) particles have been found to be efficient ice-nucleating particles under the cold conditions of (tropical) upper-tropospheric cirrus clouds. Whether they also are efficient at initiating freezing under slightly warmer conditions as found in mixedphase clouds remains undetermined. Here, we study the icenucleating ability of photochemically produced SOA particles with the combination of the Manchester Aerosol Chamber and Manchester Ice Cloud Chamber. Three SOA systems were tested resembling biogenic and anthropogenic particles as well as particles of different phase state. These are namely α-pinene, heptadecane, and 1,3,5-trimethylbenzene. After the aerosol particles were formed, they were transferred into the cloud chamber, where subsequent quasi-adiabatic cloud activation experiments were performed. Additionally, the ice-forming abilities of ammonium sulfate and kaolinite were investigated as a reference to test the experimental setup.Clouds were formed in the temperature range of −20 to −28.6 • C. Only the reference experiment using dust particles showed evidence of ice nucleation. No ice particles were observed in any other experiment. Thus, we conclude that SOA particles produced under the conditions of the reported experiments are not efficient ice-nucleating particles starting at liquid saturation under mixed-phase cloud conditions.

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A laboratory investigation into the aggregation efficiency of small ice crystals

Cited by 105 publications

References 51 publications

Observations and modelling of microphysical variability, aggregation and sedimentation in tropical anvil cirrus outflow regions

Observations and modelling of microphysical variability, aggregation and sedimentation in tropical anvil cirrus outflow regions

Formulation and test of an ice aggregation scheme for two-moment bulk microphysics schemes

The efficiency of secondary organic aerosol particles acting as ice-nucleating particles under mixed-phase cloud conditions

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