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

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

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)

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)

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

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

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

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

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

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 10-15 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.

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 - +

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

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

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?

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