Cloud Electrification (Charge Separation)

Science 123		Spring 2020
	Cloud Electrification (Charge Separation)

Observations:
- all clouds are electrified to some degree, but only in clouds with vigorous convection is sufficient charge separation attained (~ 1 mV per meter)
- strong cloud electrification follows the development of ice crystals in the cloud
- lightning and maximum precipitation radar echo are co-located within the cloud (Illingworth and Lees, 1992)
- graupel is required for lightning to occur (Illingworth and Lees, 1992)
- warm cloud constituents (graupel, cloud droplets, or rain drops) carry a negative charge
- cold cloud constituents (ice crystals, snow flakes) carry a positive charge
- Baker et al. (1995) derived the following relationship relating lightning frequency $f$ to cloud radius $R$, updraft speed $w$, and ice volume within the cloud $\bar{V}_i$:
  
  $f \approx R \cdot w^6 \cdot \bar{V}_i$
- Lightning frequency is much greater over land surfaces than water surfaces

Atmospheric Ions
- There are, primarily, two classes of ions in the atmosphere: small and large ions.
- The ionization in the lower atmosphere is mostly caused by natural radioactivity (responsible for about 50%) and cosmic rays (about 50%). Cosmic ray sources are either solar (solar flares) or other galactic structures such as supernovas and exploding stars.
- An ionizing particle will create an ion pair when it strikes an atmospheric molecule. The liberated electron is negatively charged, and the nucleus of the molecule from which the electron is liberated forms a heavier, positive ion.
- Molecules such as NO and NO₂ are believed to dominate the negative small ion population while oxonium (an oxygen ion) and water are believed to make up the majority of positive small ions in the atmosphere.
- positive ions are the predominant type near the earth's surface, the ratio of positive to negative ions is approximately 1.2

Charge Separation Theory.

Any viable theory, or model, must explain the qualitative features of the average thundercloud as well as meet the quantitative requirements (e.g. creating sufficient charge separation within the typical time frame).

The numerous theories put forth on charge separation mechanisms can be grouped into three main categories:

Convective Charging

positive ions are the predominant type near the earth's surface, the ratio of positive to negative ions is approximately 1.2, so the first panel shows a collection of positive ions (a net positive charge) near the surface of the earth that is swept up into the developing cumulus cloud in the early development stages
convective updraft carries the net positive charge into the forming cloud and, eventually, lifts the positive ions to the very top of the cloud
the positive area within the cloud's updraft causes a negative ion "curtain" to form on the top and side boundaries of the cumulus cloud, as shown in the second figure
entrainment of the net negatively charge environmental parcels form much of the cloud's downdraft, therefore creating the negatively charged lower portion of the cloud
many investigators have found qualitative and quantitative discrepancies between this theory and observation (Chiu and Klett, 1976; Latham, 1981; Williams et al., 1989)

Inductive Charging (has at least 3 sub-categories, two outlined below)

Selective Ion Capture

earth electric field induces charge separation within the individual cloud constituents-- negative on the top and positive on the bottom
as the cloud constituent falls through the cloud, it attracts free negative ions to the bottom surface, but positive ions can not "catch up" to the constituent to attach to the top
the net negatively charged cloud constituents migrate to the lower, warmer region of the cloud
removing the attached negative ions from the higher, colder region of the cloud leaves a net positive charge near the top of the cloud
theory produces the correct polarity, but is found to be effective only under weak electric fields, so is quantitatively unrealistic

Rebound of Particles

charge separation by charge migration within individual cloud particles due to the earth's electric field
negative charge on the top, positive charge on the bottom
larger, faster falling particle and a smaller particle collide
net positive charge transferred to the smaller particle because the larger particle has greater surface area, net negative to the larger, warmer particle
heavier, negative particle falls to the bottom of the cloud, positive, lighter particle is lifted to the top of the cloud
one problem (of several) with this theory is the assumption that the collision angle is such that positive and negative surface regions of the respective hydrometeors actually collide

Non-inductive Charging (has at least 7 sub-categories, two outlined below)

Thermoelectric Effect
- laboratory experiments show that when a hail pellet collides with an ice crystal having a colder temperature, the hail/graupel becomes charged
- caused by the temperature gradient at the point of contact
- H⁺ ions travel faster in the ice lattice than the OH^- ions, so a net positive charge transfer to the colder ice particle
- this process requires impact velocities and temperature differences that are unlikely in cloud environments

Riming Electrification

substantial electric charge can be deposited on riming graupel in a collision with an ice crystal in a cloud with super-cooled water vapor present
Magnitude and sign of the electrification are a function of temperature and cloud water content (CWC). Collision of graupel (G) and ice crystals (IC) at temperatures less than -20 degrees C results in a negative charge transfer to the graupel (G$^-$) that falls to lower regions of the cloud. The lighter, positive charged ice crystals (IC$^+$) remain in the higher reaches of the cloud. In warmer environments (middle cloud height), the collision of graupel and ice crystals result in a positive charge transfer to the graupel (G$^+$) that ultimately fall to lower levels in the cloud. The negatively charged ice crystals (IC$^-$) remain in the mid-altitude of the cloud to reinforce the overall negative cloud charge in this region. The lower portion of the cloud is populated by both positively and negatively charged graupel, thus explaining the tripole pattern observed in many electrified clouds.
The magnitude of charge transfer decreases in higher moisture environments. This explains why more heavily moisture-laiden ocean based clouds seem to produce fewer lightning strokes
considered by many researchers to be the major charging mechanism
promising theory, but it is not known why the collisions between ice crystals and graupel result in the charging behavior characterisitics

Summary paper by Clive Saunders - charge separation mechanisms