Wednesday Feb. 18, 2015

The 3rd homework assignment was collected today.  A new assignment was also handed out.  It is due next Wednesday (Feb. 25).  It contains an extra credit problem that appeared on the 2005 midterm exam for this course.  I don't remember ever having been able to solve the question and am hoping one of you might.

 The following information was copied from the www.SnowCrystals.com website.   This is an information packed website and you should definitely have a look at it if only to view the photomicrographs of snow crystals.  The information below is longer available.  It briefly discusses the quasi-liquid layer that is found at the boundary between ice and air. 



The quasi liquid layer seems to play an important role in charging in the non inductive process.


The two colliding particles in the non-inductive process are each thought to be surrounded by a quasi liquid layer.  The particle that is growing faster will have a thicker quasi liquid layer and will transfer mass to the other particle at the time of the collision.  The particle that gains mass ends up with negative charge.

Now 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). 

Here is my interpretation of what happens in regions A and B.  The temperature is low in both cases.  The difference is the liquid water content, the concentration of supercooled water droplets.


Region A
Supercooled droplets are colliding with and sticking to the graupel particle at a relatively low rate.  That is because of the low effective liquid water content.  The droplets warm as they release latent heat and try to freeze.  The droplets warm but there isn't enough latent heat energy release to warm the graupel particle itself.  So both the graupel and the ice crystal are cold and the rates of deposition of water vapor to the graupel and the ice crystal are about equal (deposition is the water vapor to ice phase change).  The graupel gets some additional deposition from the warm water droplets on its surface (the thin purple arrows from the droplets to the graupel particle in the figure).  As a result the graupel is growing more quickly and has the thicker quasi liquid layer.  Mass transfer during a collision with an ice crystal is to the ice crystal.  The ice crystal ends up with negative charge, the graupel particle with positive charge.


Region B
The effective liquid water content is higher and there are more supercooled water droplets colliding with and sticking to the graupel particle.  Now as they release latent heat and try to freeze they are able to warm the graupel particle.  The net rate of deposition from the surroundings to the graupel particle is reduced (2 arrows are shown in B versus 3 arrows in A).  Even with the deposition from the droplets on the surface of the graupel particle, the ice crystal grows more quickly and has the thicker quasi liquid layer.  Mass transfer during a collision goes to the graupel particle and the graupel particle ends up with negative charge.

Region C
Both the ice crystal and the graupel particle are in warmer surroundings and the rate of deposition to each is reduced.  The graupel is warmer than it was in Region B.  Now however the deposition from the accreted water droplets is increased because it takes them longer to freeze.  The graupel particle grows faster and has the thicker quasi liquid layer.  Mass transfer is to the ice crystal again and it ends up with negative charge.

Most thunderstorms have the dipolar (or tripolar) charge distribution as shown below at left.  The majority of cloud to ground (CG) discharges from these types of storms carry negative charge to ground.  Just a few percent of the CG discharges are positively charged (we'll see though that these positive CG discharges sometimes have very large peak currents).

A much larger percentage (50% or more) of positive CG discharges have been observed coming from some Central Plains storms in recent field experiments.  This would suggest the clouds might have had an inverted charge distribution like that shown below at right.  And one might wonder whether a different cloud electrification mechanism is at work (different from the Reynolds, Brook, Gourley process discussed last Thursday).


Other than the high incidence of positive CG discharge, E field soundings in thunderstorms have provided evidence of the inverted charge structure.  This is discussed in a fairly recent paper that you can find here.  The two figures below illustrate E field soundings in "normal" clouds and in clouds with inverted charge distributions.  These are a little more detailed than was shown in class.


As you move upward from the bottom toward the top of the cloud you encounter a positively charged screening layer, the main negative and positive charge centers, and a positive screening layer at the top of the cloud.


Just the opposite situation was observed in the inverted polarity clouds.  At first glance these clouds would seem to invalidate the Reynolds, Brook, Gourley process we have been studying or at least suggest there might be another process that sometimes is at work. 

Follow up studies, however, seem to indicate that the non-inductive electrication mechanism  is capable of explaining clouds with inverted charge distributions.  This is shown on the figure below


In the center of the figure is the graph that we examined in some detail earlier in this lecture.  It shows the polarity of charge acquired by a graupel particle colliding with ice crystals in a laboratory simulation of a cloud environment containing supercooled water droplets.  The liquid water content on the vertical axis is a measure of the supercooled droplet concentration.

Liquid water content in a typical cloud would be found near the level of the lower dotted line on the graph.  The tripolar cloud charge distribution in a typical cloud is shown at left.  High in the cloud where the temperature is cold, the graupel particle acquires negative charge and the ice crystals positive charge (region B on the graph).  The ice crystals are carried upward and form the main positive charge center.  The heavier graupel particle descends to form the main negative charge center.  Somewhat lower in the cloud where temperatures are warmer than TR (the reversal temperature) but still below freezing the polarity of the charging changes (region C on the graph).  The graupel ends up with positive charge and the ice crystals with negative charge.  The positively charged graupel form the lower positive charge centers.

It seems that the unusual Central Plains storms have very high liquid water contents.  If we look at the level of the upper dotted line on the charging graph in the center of the figure we see that the graupel particle always ends up with positive charge in this high LWC environment.  There is no charge reversal temperature.  The Reynolds, Brook, Gourley mechanism can account for clouds with inverted charge distributions.  That would seem to be further validation of this process (and of the laboratory results from UMIST).