Storm charge structure

1. Storm charge structure • Dipole/tripole structure • Vertically separated, oppositely charged regions/layers • Typical charge pattern has negative charge sandwiched between upper and lower positive charge • Exceptions to this charge structure exist, for example the inverted tripole where positive charge is sandwiched between upper and lower negative charge; inverted dipole structures have also been documented, negative charge aloft, positive charge below. 1

2. Observations of Storm Electrification How are these charge structures inferred? Balloon-borne electric field meter observations using Coulomb’s Law applications Measuring electric field changes at the ground Location of negative charge layer is largely invariant with temperature! 2

3. Why do we care about “inverted” storms? They typically have huge flash rates (Weins, Rutledge and Tessendorf, 2005). Inverted storms are often severe, producing hail, tornadoes and damaging winds (Carey, Rutledge and Petersen, 2003). They are interesting scientifically and a challenge to understand: what are the mechanisms that promote an inverted charge structure? Weins et al. (2005)

4. Positive CG producing storms favor certain regions (Zajac and Rutledge, 2000). Positive CG’s have larger mean currents compared to negative CG’s, especially when tied to trailing stratiform precipitation in squall type MCSs (Rutledge and MacGorman, 1988). Zajac and Rutledge (2000)

5. How does these charge structures develop?

6. Any charging theory must address the following • The average duration of precipitation and electrical activity from a single convective storm is about 30 min. • Charge separation in thunderstorms results in cloud electric fields of 105 V/m. At these field values breakdown can occur where ions are ripped from the surface of ice particles allowing the medium between positive and negative charge centers to become highly conductive. A single lightning stroke may transfer upwards of 100 coulombs of charge to the Earth’s surface. Typical current associated with a cloud to ground discharge is 10 kA. We conclude the duration of a typical flash is on the other of 10-30 milliseconds. • Electrical charge is typically located between about -5ºC and -40ºC. • Electrification is closely tied to the development of mixed phase precipitation in clouds. So the mixed phase region in thunderclouds is of particular interest for explaining charge generation and separation into layers with opposite charge polarity. 4

7. Conceptual Model of charging mechanisms Two basic charging mechanisms. One involves precipitation processes, the so-called precipitation mechanism. The second mechanism involves convective air motions transporting free ions in the troposphere, the convective charging mechanism (Vonnegut 1955). Precipitation Based Air Motion Based (Convective Charging) E. Williams, Scientific American 5

8. Noninductive charging(Precipitation-based charging) • Consistent with lots of observational data that suggest strong E fields and lightning occur in clouds that develop a robust mixed-phase precipitation process involving ice particles in the presence of supercooled liquid water (in form of small droplets) 6

9. Precipitation or non-inductive mechanism Basic premise is that large and small ice particles collide and rebound in a cloud (in the presence of supercooled water), with charge of opposite sign being retained on the graupel and small ice particles respectively. Graupel charges negatively under certain conditions and positively under other conditions. Will consider the “why” of this shortly. Williams, Scientific American 7

10. Noninductive charging • Exact mechanisms for this type of charging are not completely understood • Lab experiments have shown that magnitude and sign of charge are dependent on: • Temperature (Takahashi, 1978) • Liquid water content (Takahashi, 1978; Saunders et al., 1991; Saunders and Brooks, 1992); and perhaps more importantly, effective liquid water content (Saunders and Peck, 1998). • Ice crystal size (Jayaratne et al., 1983 and Keith and Saunders, 1990) • Takahashi (1978) was one of the first to report on laboratory data to show the sign and magnitude of charge on a rod of ice (simulating graupel) whirling though a field of small ice crystals in a field of supercooled water droplets at various temperatures and liquid water contents. Takahashi’s study was only preceded by Reynolds et al. 1957. 8

11. Takahashi, 1978, JAS 10-4 esu = 33 fC Femto = 10-15 Three distinct charge regions found! Negative charging to graupel for temperatures colder than -10 C over modest to substantial liquid water contents. Positive charging to graupel at very low and extremely high liquid water contents. We will find that other studies do not conform to these results. 9

12. Examples of disagreements: Plots of various lab results indicating the sign of charge imparted on the rimer (graupel particle). There are considerable differences in the so-called charge reversal temperature. Effective liquid water content is the fraction of the cloud water that strike the rimer, which is dependent on droplet diameter and graupel velocity, which in turns defines the collision efficiency. We will explore these NIC results in more detail in our paper reviews.

13. Convective Charging Theory-involving convective air motions -Normal fair-weather E field establishes + charge concentration in lower troposphere (via corona processes at Earth’s surface), which when carried by updrafts to the top of storms, attracts negative ions, which are then carried down by downdrafts on cloud edges -Charge is separated by the up- and downdrafts -Chiu and Klett (1976) argued that this method is unlikely to produce sufficient cloud charging to generate lightning and this mechanism has been dismissed Saunders 2008 11

14. How do hydrometeors get “charged” via the noninductive mechanism?

15. Electrical Properties of Water Lets also recall the structure of a water molecule. Water has a permanent dipole moment. H is positive, O is negative. - - - + • Quasi-liquid layer • Dash (1989), Golecki and Jaccard (1978) and Elbaum et al. (1992) showed the presence of a QLL, an ultra-thin phase transition between vapor and ice to temperatures as cold as -30ºC • The QLL may be about 10 molecular layers in thickness or less • The orientation of the water molecules in the surface layer is non-random. + - + - + + - - + - - - - - - + - - - - + - - - 13

16. Electrical properties of water Air • Electric double-layer (Faraday layer; a dipole layer just inside the vapor-ice interface, within the QLL, the phase transition layer.) • Faraday studied this problem and argued that water molecules are oriented with their negative end pointing to the vapor and their positive ends pointing inward. This arrangement in the QLL is ambiguous. But most studies have argued for negative charge situated outside of positive charge. There is much controversy about the EDL and its true configuration. • Charging resulting from this electrical double layer configuration • There are various ways the EDL can cause charge separation. For example, when two particles are rubbed together where one particle shears more charge from the outer layer of the other particle compared to the lower layer, a net charge could be transferred from the outer layer, leaving one particle with net negative charge and the other particle net positive charge. Baker and Dash (1994) suggested that for two particles with different quasi-liquid layer thicknesses, upon contact, mass (negative charge) will flow from the thicker QLL to the thinner QLL. When the particles rebound, the particle with the thicker QLL will have net positive charge. The particle with the thinner QLL will have net negative charge. - - Water or ice particle. The proximity of the charges is not shown to scale. - + + - - + + + - Initial - + - Vapor + - - + + + + + Solid 14

17. Williams et al. 1994, JGR Showed that regions of deposition and sublimation are linked to the sign of charge found in Takahashi’s lab study. Consider two ice particles each growing by deposition, larger particle will be growing faster (since dm/dt ~ r) and will therefore have a thicker QLL. Negative charge will therefore be transferred from the large particle (thick QLL) to the small particle (thin QLL). Large particle is positive and small particle is negative after rebounding. This process occurs at relatively low supercooled water contents. Now consider a large ice particle growing by riming contacts numerous small ice particles. Due to surface heating (owing to riming) the large particle can be in a state of sublimation providedsupercooled liquid water contents are sufficiently large. It has been argued, but has not been not proven, that the large particle undergoing sublimation will have a thinner QLL compared to the small ice crystals which are in a depositional growth mode. Therefore the large particle will acquire negative charge and the small particle will be positively charged after the particles contact and rebound. For water soaked graupel (hail), wet growth occurs. Takahashi argues that in this case negative charge is preferentially lost as water is shed from hail, leaving hail with net positive charge. This result is very controversial. Wet growth may not be necessary for positive charging of graupel/hail. The larger graupel particle falls and the small ice particles are carried upward leading to ordinary dipole charge configuration of negative charge underlying positive charge. Lower pocket of positive charge in the ordinary tripole configuration may then form as graupel switches back to a depositional state at lower water contents or undergoes melting, which can reverse the sign of charge on that particle. The papers we will review in class will broaden our understanding of the sign of charge related to non-inductive charging. Caption is wrong, open circles denote positive charging on graupel, filled circles indicate negative charging of graupel. 15

18. - - - - - - + + + + + + Inductive Charging • General overview: • Electric field induces charge on surface of hydrometeors • Hydrometeor will acquire charges of opposite polarity on opposite surfaces of hydrometeor. This charge is “induced” by the ambient electric field. • When particles collide: large ‘precipitation particle’acquires negative charge and falls relative to the updraft, smaller particle gets positive charge and is carried upwards by updraft E 17

19. Inductive Charging • Problem: Cannot explain initial intensification of electric field • Thus, inductive charging is thought to be responsible for further strengthening of the in-cloud electric field that was generated by the non-inductive mechanism. • Eventually the in-cloud E field gets so strong that breakdown occurs resulting in a lightning discharge. Breakdown fields are on the order of 105 V/m (100 kV/m). 18

20. A neat example of inductive charging NSF G-V aircraft getting ready to penetrate a thin anvil cloud attached to a thunderstorm during the DC3 field campaign in 2012 Documentation of FDA’s, frozen drop aggregates consisting of chains of small, frozen droplets in the core of the anvil. The droplets likely froze via homogeneous freezing and then subsequently aggregated due to inductive charging in the thunderstorm updraft. The normal graupel based charging mechanism produced strong electric fields in the storm updraft region that allowed inductive charging to build the FDA’s. Stith et al. (2014) 19