Monday Feb. 16, 2015

We'll devote today's class and most of Wednesday to qualitative descriptions of cloud electrification processes. 

The picture below is from the supplementary lecture on cold cloud/ thunderstorm cloud structure.  The tripolar charge structure is what a viable electrification process needs to explain.


The main charge centers are first an upper positive charge center at 1(a) and smaller lower positive charge centers (1c).  There is more of a layer of negative charge at (1a) that seems always to be found at temperatures between -10o and -30o C.

Screening layers are found at the top and sides of the cloud (2a and 2b in the figure).  These form because of the abrupt drop in conductivity as you move from outside the cloud into the cloud aren't part of the electrification process.

The figure below presents some of the evidence supporting the idea that the negative charge is found between -10o and -30o C in thunderstorm clouds.

 

The figure shows the locations of negative charge neutralized during cloud-to-ground and intracloud discharges in clouds in Florida, New Mexico, and Japan.  We'll look at how this is done later in the week.  In each case the negative charge is found in the same temperature range despite differences in cloud heights, cloud base altitudes, and cloud thicknesses.  (source: Krehbiel, Paul R., "The Electrical Structure of Thunderstorms," Ch. 8 in The Earth's Electrical Environment, National Academy Press, Washington, 1986)




And just so you don't think you now have everything figured out, here is a figure that demonstrates that the distribution of charge in some thunderstorms can be much more complex than the simple tripolar model discussed above.  The figure shows charge in the convective part of a mesoscale convective system (source: Stolzenburg, M., W.D. Rust, and T.C. Marshall, "Electrical Structure in Thunderstorm Convective Regions 3. Synthesis," J. Geophys. Res., 103, 14097-14108, 1998).

Convective process
We'll start with the convective process of cloud electrification. 

The convective theory starts with the positive charge found in the air above the ground during fair weather (Fig. A).  In Fig. B an updraft and a cloud have started to form and positive charge is swept upward into the cloud where the charge attaches to cloud particles.  Negative charge carriers in the surrounding air are drawn to the positive charge in the top of the cloud and form a screening layer.  Cloud edge motions shown in Fig. C then begin to carry this negative charge down and into the middle center of the cloud to form the main negative charge center.  The electric field at the ground intensifies because of the close proximity of negative charge in the cloud.  Objects on the ground go into corona discharge and "spray" positive charge into the air (Fig. D).  This is one way of accelerating the charging process.

While cloud motions can clearly have an effect on the distribution of charge inside a thunderstorm, the convective theory is generally not considered to be a viable mechanism for the initial electrification of thunderstorms.  It is hard to understand how cloud motions would always form the main layer of negative charge in the -10o and -30o C  region of the cloud, for example.

Inductive processes
In the inductive process an existing (initially fair weather) field induces charges in precipitation particles (in the same way charges were induced on a conducting sphere in a uniform field).



The downward pointing fair weather E field induces equal amounts of negative and positive charge on the tops and bottoms of different sized precipitation particles.  Because the particles are moving in different directions or at different speeds, they collide.  During the collision some of the charge on each particle is neutralized.  Each particle is left with net charge following the collision.  Positive charge moves upward, negative charge downward.

The direction of charge motions is consistent with the direction and strengthening of in-cloud electric fields.


The field at the ground under a thunderstorm points upward, the opposite of the fair weather field.  The direction of the in cloud field is the same as the fair weather field.  Thus the directions of charge movement in the inductive process is consistent with thunderstorm charge distribution. 


There are a variety of types of particles that can collide.
water - water
 ice - ice
water - ice
 ice - riming ice

When two water droplets collide, they will often stick together.  If make just a glancing collision, there won't be any charge neutralization because there isn't any charge induced at the particle's equators.  Thus a collision between two water droplets would not seem like it could separate significant amounts of charge.

Ice particle - ice particle collisions don't neutralize much charge because the particles don't remain in contact for very long and the lower electrical conductivity of ice means charge doesn't flow as readily as it does through water.

Water - ice particle and ice - riming particle collisions might work, but the general feeling still is that the inductive process is not able to start with a fair weather E field and turn it into a thunderstorm strength field. 

Before we leave the inductive process behind a quick demonstration of an apparatus that depends on induction (and positive feedback) to produce a surprising amount of charge from tap water.  The apparatus is a Kelvin Water Dropper or Kelvin Electrostatic Generator.  A picture of the apparatus used in class is shown below at left











Water from a reservoir (at the top of the photo) travels down two sections of tubing and out of two plastic nozzles positioned near the middle of the photo.  The falling streams of water fall through two metal rings (colored red and green) and into two plastic cups at the bottom of the picture.  The cup on the left is electrically connected to the ring at right (the red ring), the cup on the right is connected to the left (green) ring.  The two cups are also electrically connected to 5 neon lamps wired together in series.  The right photo above is a close up of the neon lamps.

The operation (as I understand it) is explained below.

Let's assume that the water in the left cup (Pt. 1) contains some + charge (it is equally likely that it contains - charge).  The left cup is electrically connected to the right (red) ring, Pt. 2.  The positively charged red ring will cause (by induction) negative charge to build up at the bottom end of the falling stream of water.  The stream then breaks up into drops which carry the negative charge to the cup on the right (Pt. 3).  This is connected to the left ring at Pt. 4.  As shown in the close up at Pt. 5 positive charge is induced at the tip of the stream of water.  As the stream breaks into drops the drops carry more positive charge to the cup on the left.  Charge builds up until a spark flashes across the spark gap (or until the bank of neon lamps flashes in the apparatus used in class).

There are a lot of videos on YouTube.  This first video (from Reinhard Schumacher at Carnegie Mellon University) shows an apparatus that produces a visible spark and also contains a sensitive electroscope.  Here's a second video (from Thomas Kim) that produces a visible and audible spark (note how the sound of the dripping water also stops a second or two before the spark).

Non-inductive process
The non-inductive process, also called the Reynolds, Brook, Gourley process is generally thought to be the most viable explanation for the rapid initial electrification of thunderstorms.  It doesn't require a existing field.  The process is shown in general terms below.  We will look at some of the details later.


Basically graupel collides with a snow crystal, in the presence of supercooled water droplets, and then depending on the environmental temperature, the graupel ends up with negative or positive charge and the ice crystal ends up with the opposite polarity.  At temperatures colder than about -15 C the snow crystal ends up with positive charge and the graupel is negative.  The different sizes and fall velocities of the two types of particles means they will tend to separate after the collision.  The negatively charged graupel tends to accumulate in the middle of the cloud and the snow crystal is carried up to higher parts of the cloud.  At temperatures warmer than about -15 C the polarities reverse.  We will see that this charge reversal temperature can vary depending on the cloud liquid water content.  The letters A, B, and C show how this process can account for the three main charge centers in a thunderstorm (A & C are the upper and lower positive charge centers, respectively; B is the main negative charge center)

The following figure shows the experimental apparatus used to make the initial measurements of charging.  [source: S.E. Reynolds, M. Brook, and Mary Foulks Gourley, "Thunderstorm Charge Separation", J. Meteorology, 14, 426-436, 1957)

Two metal balls at the ends of arms mounted on a rotating shaft simulate graupel particles in a cloud (pt. 1).  These balls become electrically charged the electrical path from the balls to a sensitive electrometer has been highlighted in yellow.  A little bit wider view of the apparatus is shown below.


Temperature and liquid water content (LWC) are measured near the spinning arms (pt. 2).  Liquid water content is really just a measure of the concentration of supercooled water droplets. 

Warm water at pt. 3 is the source of water vapor.  Cooling coils (pt. 4) cool the air inside the chamber.  Cold air will sink and probably establish a vertical temperature gradient.  The spinning metal ball could be raised or lower to change the temperature of the surrounding air.  Replicas of ice crystals that form can be collected at pt. 5 for later examination.

This would not be an easy experiment to conduct.  It would be difficult to create, measure, and monitor the cloud environment in the chamber and the charging that does occur is probably very weak and a sensitive electrometer would be needed.


On Wednesday we'll look at some of the details from more recent studies of the non-inductive electrification process.  Results from some of the work done by a group at the University of Manchester Institute of Science and Technology (UMIST) are summarized on the graph below (source: Saunders, C.P.R. and S.L. Peck, "Laboratory Studies of the Influence of the Rime Accretion Rate on Charge Transfer during Crystal/Graupel Collisions," J. Geophys. Res., 103, 13949-13956, 1998).


The + and - polarities in the figure refer to the charge on the metal target that simulates a graupel particle in these laboratory experiments.  The charge depends on the environmental temperature and on the cloud liquid water content as shown in the figure above.  EW on the vertical axis is effective liquid water content and is the product of liquid water content and collision efficiency (what fraction of the supercooled water droplets colliding with the target stick and eventually freeze to the target).