1. Article Review Jennie Bukowski ATS-740 07-Mar-2017

2. Introduction
Most of charge separation occurs in mixed-phase region of thunderstorms (Simpson & Scrase, 1937; Simpson & Robinson, 1942)
T < -20°C: +24 C
T ≈ -7°C: -20 C
T ≈ 0° C: +4 C
Several laboratory and observational studies of hail / graupel electrification at the time
Cannot agree on the sign of electrification (Table 1)
May depend on CWC, temperature, droplet size distribution
Here, Takahashi investigates charging process when growing graupel collide with ice particles in a laboratory setting

4. Experiment Setup Electric Field Mill (top) Rotating Riming Probes
Chamber Schematic
Experiment Setup
Electric Field Mill (top)
Rotating Riming Probes

5. Experiment Setup – Riming Probes
Collision efficiency of roughly 80% between ice particles on probes and water droplets for riming

6. Soot Plate – records number of smaller ice particles in second stage
Columnar Ice Crystals
Two-Stage Particle Impactor – measures charge of particles
Formvar solution – records number of largest ice particles in first stage
Soot Plate – records number of smaller ice particles in second stage
Rimed ice quantities measured by moisture on methylene blue dyed paper

7. Experiment Setup
Drop Size Distributions
Funnel Setup for Part C

8. Results No electrification: High electrification:
Water and ice particles but no rotation
Rotation with only ice or only water
High electrification:
Rotation with ice particles and supercooled water droplets
Magnitude and sign of charge depend on cloud water content (CWC) and temperature
T > -10 °C: positive charge regardless of CWC
T < -10 °C: positive charge at very high and very low CWC
T < -10 °C: negative charge in-between CWC extremes

9. Magnitude and Sign of Charge During Riming (A & B)
Open circles (+)
Solid circles (-)
Crosses (uncharged)
Charge of rime per ice crystal collision [10^-4 esu]

10. Sign and Relationship Between Rime Charge and Particle Charge
Positive Rime
Positive Rime
Negative Rime
Negative Particles
Negative Particles
Positive Particles

11. Rimer Surface and Particle Shapes
Ice crystal shape is highly sensitive to supersaturation and temperature
Brittle; splinters
Hard hexagonal plates; unbroken
Broad branches
Presence of water layer on rimer in (c) with high CWC?

12. Broken ice branches will be negatively charged
Physical Mechanisms
Broken ice branches will be negatively charged
Rime interior positively charged
Free protons move in direction of temperature gradient
Latent heat released during riming increases temperature of surface ice branches

13. Positively charged ice is unbroken
Physical Mechanisms
Negatively charged rime occurs when cool ice particles hit the relatively warmer rime
Positively charged ice is unbroken
Charge separation can occur when the two collide
Existing temperature difference between ice and rime determines sign

14. High CWC can produce a water film around rimer surface
Physical Mechanisms
High CWC can produce a water film around rimer surface
Ions are more abundant in water
OH- remains in water and H+ moves toward ice
Collisions rip off negatively charged ions

15. Conclusions
Magnitude and sign of electrification due to collisions between forming graupel and ice particles depends on temperature and cloud water content
Low CWC ( + graupel)  few ice crystals  cannot support electrification
1 < CWC < 2 g/m³  graupel highly charged
T > -10 °C  positive graupel charge
T < -10 °C  negative graupel charge
Terminal velocity differences between graupel and ice particles leads to lower positive and upper negative charge structure of thunderstorms
Shear of ice particles leads to positive slant with height