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T. G. Owe Berg
,
Thomas A. Gaukler
, and
Urte Vaughan

Abstract

Collision efficiencies E have been determined from particle trajectories for the case of a 1-mm glass sphere and 6–20 μ spherical glass particles falling in still air. An empirical formula for the dependence of E upon scavenger size, scavenger velocity, and particle terminal velocity has been derived.

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T. G. Owe Berg
,
Robert J. Trainor Jr.
, and
Urte Vaughan

Abstract

Experiments with charged water droplets show the existence of the metastable states predicted by Cahn as well as the well-known unstable state predicted by Lord Rayleigh. The charge lost at metastability is completely recovered with time, whereas the charge lost at instability is only partially recovered. The recovery of charge may be a space-charge effect, or it may be a result of electrification that accompanies the exchange of water vapor.

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T. G. Owe Berg
,
Thomas A. Gaukler
, and
Urte Vaughan

Abstract

The collision of a falling drop With a small particle has been studied by high-speed photography. The trajectory of the particle relative to the center of the drop and relative to a fixed point has been determined under various conditions. The effect of electrostatic charges on drop and particle has been studied.

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Vaughan T. J. Phillips
,
Jun-Ichi Yano
,
Marco Formenton
,
Eyal Ilotoviz
,
Vijay Kanawade
,
Innocent Kudzotsa
,
Jiming Sun
,
Aaron Bansemer
,
Andrew G. Detwiler
,
Alexander Khain
, and
Sarah A. Tessendorf

Abstract

In Part I of this two-part paper, a formulation was developed to treat fragmentation in ice–ice collisions. In the present Part II, the formulation is implemented in two microphysically advanced cloud models simulating a convective line observed over the U.S. high plains. One model is 2D with a spectral bin microphysics scheme. The other has a hybrid bin–two-moment bulk microphysics scheme in 3D. The case consists of cumulonimbus cells with cold cloud bases (near 0°C) in a dry troposphere.

Only with breakup included in the simulation are aircraft observations of particles with maximum dimensions >0.2 mm in the storm adequately predicted by both models. In fact, breakup in ice–ice collisions is by far the most prolific process of ice initiation in the simulated clouds (95%–98% of all nonhomogeneous ice), apart from homogeneous freezing of droplets. Inclusion of breakup in the cloud-resolving model (CRM) simulations increased, by between about one and two orders of magnitude, the average concentration of ice between about 0° and −30°C. Most of the breakup is due to collisions of snow with graupel/hail. It is broadly consistent with the theoretical result in Part I about an explosive tendency for ice multiplication.

Breakup in collisions of snow (crystals >~1 mm and aggregates) with denser graupel/hail was the main pathway for collisional breakup and initiated about 60%–90% of all ice particles not from homogeneous freezing, in the simulations by both models. Breakup is predicted to reduce accumulated surface precipitation in the simulated storm by about 20%–40%.

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Sachin Patade
,
Vaughan T. J. Phillips
,
Pierre Amato
,
Heinz G. Bingemer
,
Susannah M. Burrows
,
Paul J. DeMott
,
Fabio L. T. Goncalves
,
Daniel A. Knopf
,
Cindy E. Morris
,
Carl Alwmark
,
Paulo Artaxo
,
Christopher Pöhlker
,
Jann Schrod
, and
Bettina Weber

Abstract

To resolve the various types of biological ice nuclei (IN) with atmospheric models, an extension of the empirical parameterization (EP) is proposed to predict the active IN from multiple groups of primary biological aerosol particles (PBAPs). Our approach is to utilize coincident observations of PBAP sizes, concentrations, biological composition, and ice nucleating ability. The parameterization organizes PBAPs into five basic groups: 1) fungal spores, 2) bacteria, 3) pollen, 4) viral particles, plant/animal detritus, 5) algae, and their respective fragments. This new biological component of the EP was constructed by fitting predicted concentrations of PBAP IN to those observed at the Amazon Tall Tower Observatory (ATTO) site located in the central Amazon. The fitting parameters for pollen and viral particles and plant/animal detritus, which are much less active as IN than fungal and bacterial groups, are constrained based on their ice nucleation activity from the literature. The parameterization has empirically derived dependencies on the surface area of each group (except algae), and the effects of variability in their mean sizes and number concentrations are represented via their influences on surface area. The concentration of active algal IN is estimated from literature-based measurements. Predictions of this new biological component of the EP are consistent with previous laboratory and field observations not used in its construction. The EP scheme was implemented in a 0D parcel model. It confirms that biological IN account for most of the total IN activation at temperatures warmer than −20°C and at colder temperatures dust and soot become increasingly more important to ice nucleation.

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Vaughan T. J. Phillips
,
Marco Formenton
,
Vijay P. Kanawade
,
Linus R. Karlsson
,
Sachin Patade
,
Jiming Sun
,
Christelle Barthe
,
Jean-Pierre Pinty
,
Andrew G. Detwiler
,
Weitao Lyu
, and
Sarah A. Tessendorf

Abstract

In this two-part paper, influences from environmental factors on lightning in a convective storm are assessed with a model. In Part I, an electrical component is described and applied in the Aerosol–Cloud model (AC). AC treats many types of secondary (e.g., breakup in ice–ice collisions, raindrop-freezing fragmentation, rime splintering) and primary (heterogeneous, homogeneous freezing) ice initiation. AC represents lightning flashes with a statistical treatment of branching from a fractal law constrained by video imagery.

The storm simulated is from the Severe Thunderstorm Electrification and Precipitation Study (STEPS; 19/20 June 2000). The simulation was validated microphysically [e.g., ice/droplet concentrations and mean sizes, liquid water content (LWC), reflectivity, surface precipitation] and dynamically (e.g., ascent) in our 2017 paper. Predicted ice concentrations (~10 L−1) agreed—to within a factor of about 2—with aircraft data at flight levels (−10° to −15°C). Here, electrical statistics of the same simulation are compared with observations. Flash rates (to within a factor of 2), triggering altitudes and polarity of flashes, and electric fields, all agree with the coincident STEPS observations.

The “normal” tripole of charge structure observed during an electrical balloon sounding is reproduced by AC. It is related to reversal of polarity of noninductive charging in ice–ice collisions seen in laboratory experiments when temperature or LWC are varied. Positively charged graupel and negatively charged snow at most midlevels, charged away from the fastest updrafts, is predicted to cause the normal tripole. Total charge separated in the simulated storm is dominated by collisions involving secondary ice from fragmentation in graupel–snow collisions.

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