Article Review Jennie Bukowski ATS-740 07-Mar-2017 1 Introduction Most of charge separation occurs in mixed-phase region of thunderstorms (Simpson & Scrase, 1937; Simpson & Robinson, 1942) T < -20C: +24 C T -7C: -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 2 Chamber Schematic Experiment
Setup Electric Field Mill (top) Rotating Riming Probes 4 Experiment Setup Riming Probes Collision efficiency of roughly 80% between ice particles on probes and water droplets for riming
5 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 6 Experiment Setup Drop Size Distributions Funnel Setup for Part C
7 Results No electrification: a) Water and ice particles but no rotation b) Rotation with only ice or only water High electrification: a) Rotation with ice particles and supercooled water droplets
Magnitude and sign of charge depend on cloud water content (CWC) and temperature a) T > -10 C: positive charge regardless of CWC b) T < -10 C: positive charge at very high and very low CWC c) T < -10 C: negative charge in-between CWC extremes 8 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] 9 Sign and Relationship Between Rime Charge and Particle Charge Positive Rime Positive Rime Negative Rime
Negative Particles Positive Particles Negative Particles 10 Rimer Surface and Particle Shapes Ice crystal shape is highly sensitive to supersaturation and temperature
a) Brittle; splinters b) Hard hexagonal plates; unbroken c) Broad branches Presence of water layer on rimer in (c) with high CWC? 11 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 12 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 13
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
14 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/m 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 15
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