1. The Effect on Thunderstorm Charging of the Rate of Rime Accretion by Graupel
Trent Davis
9 March 2017

2. Background
Reynolds et al. (1957) first to note liquid water requirement for electrification from ice crystal collisions with riming targets
Takahashi (1978) found sign was dependent on cloud temperature and liquid water content (LWC)
Jayaratne et al. (1983) in agreement with Takahashi
(-) charging of graupel at low temperatures
(+) charging of ice crystals carried in updrafts
Descending graupel below Reversal Temperature can form (+) charge region

3. Background
Saunders et al. (1991) (SKM) note charge dependence on rime accretion
Effective Liquid Water Content (EW)-portion of cloud LWC captured by rimer, also effected by collision efficiency.
Traditionally, models inherently use LWC=EW
Wojcik (1995) found using EW  more realistic charge distributions
Using EW allows (-) graupel charging for higher LWC
EW similar to Manchester group’s work (in terms of velocity importance)
Rime accretion rate on graupel dependent on velocity
Accretion increases ≈ EW increases, both  (+) rimer charging according to Brooks (1993)

4. Velocity on Rime Accretion
Charge Transfer with Velocity per Crystal Separation Event
Velocity has 2 effects on charge transfer
Increasing ice crystal velocity of impact increases charge transfer
Increasing impact velocity increases rate of rime accretion, which affects charge transfer by LH release
Keith and Saunders (1990) reduced LWC to measure velocity independently of rime accretion rate (kept constant)

5. Velocity on Rime Accretion
These results from Fig. 2 led SKM to conclude sign of charge dependence on temperature and EW are independent of velocity.
However, rime accretion favors positive rimers according to previous studies
Rime Accretion Rate: RAR = EWxV (g m-2s-1)
Increasing V  positive riming
Increasing LWC (EW)  positive riming
Sign of Charge Transfer to Riming Target From Ice Crystal Collisions at 3 m s-1
Sign of Charge Transfer to Riming Target Using RAR

6. Experimental Tests of Velocity
Used Manchester cloud chamber
Drew seeded cloud past riming rod, then weighed rime accretion for EW. Current measured from rod.
Compared charge transfers at 3 and 9 m s-1 to investigate discrepancy with Manchester and Takahashi (1978)
Determined critical EW values for charge sign reversal, similar results for 6 and 10 m s-1 at -10 and -15°C
High EW  charged positively
Low EW  charged negatively

7. Experimental Tests of Velocity
Higher EW required for charge reversal at 3 than 9 m s-1, while RAR remains constant.
Further tests on velocity effects on charge transfer with constant ambient conditions were performed, results in Fig. 4.
“…large graupel falling at several meters per second with a high droplet collision efficiency will be able to charge negatively only in regions of cloud with low EW.”
“…small graupel falling at velocities less than 3 m s-1 will charge negatively at higher values of EW than indicated in Fig. 1.”
Collisions Start

8. Revised Charge Transfer Equations
SKM gave charge transfer to riming target per ice crystal separation as:
Revised equations for RAR:
Positive charging:
Negative charging:

9. Model Specifications 1-D with active and non-active cloud masses
4 hydrometeor types: cloud droplets, rain drops, ice crystals, and graupel
Autoconversion to form rain drops, collision-coalescence to grow them
Ice crystals from IN activation, grow by deposition and riming
Freezing of rain drops to form graupel, hail growth allowed
For ice crystals and cloud droplets, Vt≈0 m s-1
Marshall-Palmer distribution for rain drops and graupel
Charge generated by rebounding ice crystal-to-graupel collisions
Typically, ~30% ice crystals involved in graupel collisions
Net charge summed over graupel and ice crystals
Ice crystals and graupel act as charge carriers

10. Case Study
July 19, 1981 small, isolated storm during CCOPE experiment in Montana
Initialized with 8 thermals of 2 km radii,
Initially at cloud base, w=3 m s-1 and T=1°C
Max cloud top height to 11.6 km at 24 min. (only ~1 km too high)
Dynamics and microphysics were comparable
Growth occurred slightly too quickly
Max w of 17 m s-1 comparable to m s-1 observations.
Max LWC of 1.4 g m-3 at 5.8 km MSL, while 2.5 g m-3 measured at 7 km.

11. Simulation Results - - + +
Charge Density in Active Regions (nC m-3) + -
Initially, graupel charge positively below charge reversal level and widely dist. below main (-) charge region (weak, inverted dipole)
Expected though as collisions below this level
Later, normal dipole forms
Ice crystals appear to have carried (+) charge aloft
Graupel the main carrier of (-) charge in thermals later in simulation
Non-Active Regions - +

12. Simulation Results
(-) Charge Density on Ice Crystals in Active Regions (nC m-3)
Initially, ice crystals carry (-) charge in rising thermals after collisions with graupel below Reversal Temperature.
Later, they carry (+) charge after collisions begin to occur above Reversal Temperature.
Conclude that charge carried by graupel dominates lower charge density, usually being (-)
Same as a) but for (+) Charge Density

13. Discussion
Charge transfer measurements at constant EW and RAR aren’t synonymous  RAR importance
LH release from freezing cloud droplets may influence rimer temperature and control charge transfer.
Temp. has been ruled out in previous studies though
Riming influence on charging directly related to QLL on rimer surface, as RAR considers.
Can increase from EW and/or V
Droplet freezing to rimer warms environment and reduces vapor diffusion to rimer
At warmer temp.s or high EW, collected droplets take longer to freeze and bathe rimer in water vapor  charges positively
At cooler temp.s or low EW, less water vapor is available to diffuse to rimer  charges negatively

14. More Discussion
Saunders and Brooks (1992) showed that the sign of charge transfer to the rimer depends on the relative growth rates of ice crystals, not the rimer itself.
Other factors:
Ventilation of rimer affects sublimation
Distribution of cloud droplets and ice crystals because the rimer’s fall speed and size will change along with the collection efficiency.
Were not able to test charge sign reversal at extremely low EW values due to cloud chamber controls, hence the change of Fig. 1 to Fig. 3.
LWC required for charge sign reversal decreases as velocity increases according to RAR equation
Takahashi (1978) may have overestimated LWC values because ice crystal and cloud droplets were used to compute LWC
Drop LWC of 3.5 g m-3 to 1.7 g m-3 according to new tests?

15. Conclusions Rime accretion rate defined as RAR=EWxV
Compilation of many similar studies
Revised equations for (-) and (+) charging created for temperature-EW charge reversal curve.
Montana storm modeled to test these
(-) charge region formed from -10 to -30°C initially with charge transfer collisions occurring below the Reversal Temperature
Normal dipole formed later
Ice crystals carrying (+) charge to upper region, and graupel main carriers of (-) charge.
Realistic simulation overall, thus RAR should be used rather than EW alone to determine charge sign reversal temperatures