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

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

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

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

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

Experiment Setup Drop Size Distributions Funnel Setup for Part C

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

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]

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

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?

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

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

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

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