Supercooling
Water drops can collide and coalesce at temperatures well below freezing: Raindrops can supercool as far as -20 degrees without freezing. This phenomenon, which proves to be fundamental to charge separation within a cloud, can be detected by aircraft measurements and, indeed, is amply demonstrated by aircraft that ice at these temperatures. Radar can also distinguish liquid water from ice, since raindrops are flattened by air resistance as they fall, changing the radar reflectivity ratio between vertically and horizontally polarized radar beams.
When a mass of air penetrates above the 0-degree-temperature level in a cloud, initiation of the ice phase becomes increasingly likely with decreasing temperature. Freezing is initiated by the relatively rare aerosol particles (fewer than one in 1,000) whose structure bears some similarity to the lattice of an ice crystal. Such particles are of mineral origin (often clay), and are lofted into the atmosphere and sometimes carried thousands of kilometers from their place of origin by prevailing winds. Ice particles appear in significant but highly variable concentrations (a few per liter, on average) at temperatures below about -10 degrees. Their crystalline form depends on temperature. Once they become larger than a few hundred micrometers and fall at speeds greater than 20 meters per second, the crystals capture supercooled cloud drops that freeze as near hemispheres on their surface. As these build up, a porous structure known as soft hail or graupel forms (see Figure 5). Graupel has a density about two-tenths that of solid ice.
During experiments done in 1973 in the laboratory at CSIRO in Sidney, Australia, with S. C. Mossop, we discovered that when graupel grows, a very interesting phenomenon takes place. At between -3 and -10 degrees, depending on the number and size of cloud drops present, small ice splinters grow--initially directly from vapor and ultimately by accretion--to produce even more crystals. These secondary crystals result from the accretion and freezing of a special class of cloud droplet that happens to impact on droplets that were collected and frozen earlier in a rather irregular way--a bit like the structure of soot. Such droplets freeze symmetrically from top and bottom as they lose heat to the substrate and the air, giving rise to an increase of pressure in the entrapped water--somewhat like the bursting of freezing water pipes. If the temperature is too cold (below about -10 degrees), the process does not work because the initial ice formed is too defective and deforms under the increasing pressure; if the temperature is too warm (above about -3 degrees), the drop spreads out on impact and fails to form a sufficiently thick ice shell to sustain the pressure. The exact mechanism of ice ejection is still unclear, but crystals collected in flight under these conditions are in the form of ice columns, some 20 micrometers in diameter and 100 micrometers long.
This process leads to an exponential growth in the concentration of ice in a cloud until all the water in the downshear region takes on the ice phase, called glaciation. Later experiments suggested that this is a possible origin of charge separation. Simulated graupel moving through a cloud of supercooled drops and ice crystals charges significantly, with the sign and rate of the charge depending on the amount of cloud water and the temperature. Crystals bounce from the growing graupel, leading to a separation of charge by a process that is still far from clear.
The surface of a growing graupel particle is quite a complex terrain, as it is covered in an irregular way by accreted droplets in various stages of freezing--a process that takes some one-hundredth to one-tenth of second, depending on the droplet size. During the freezing process, the droplet maintains a temperature near 0 degrees because of the release of the latent heat of freezing; therefore it has a higher water-vapor pressure. The falling graupel particle itself is heated above ambient temperature and approaches 0 degrees at a sufficiently high accretion rate. Thus the local surface is bathed in vapor in a very complex pattern of local supersaturation, leading to a surface structure of great complexity on a molecular scale--the graupel particle is growing from the vapor in some places and evaporating in others. The impacting crystals bounce under a wide spread of conditions, and it is still a bit of mystery which conditions are best for charge transfer--or, indeed, whether the same charge is always transferred under a given set of environmental circumstances.
In any event, the graupel particles, which have one charge, fall faster than smaller ice crystals with the opposite charge, giving rise to a dipole--a slanted one, if the updraft were formed in a wind shear. (Other types of charge separations, including tripoles and multilayered charge distributions, can develop but are beyond the scope of this article.)
How Much Charge is Needed?
By measuring the radiation field of a discharge, we can accurately determine a lightning stroke’s current and the way it changes with time. Currents can range from several thousand to several hundred thousand amperes delivered in times from microseconds to hundreds of microseconds; the charge transferred per flash is a few to tens of coulombs.
This imposes specific limitations: Since typical thunderstorm charge densities are on the order of 10 coulombs per cubic kilometer, the particle interactions described above must take place over sufficient time and in a large enough volume to generate sufficient charge. Further, the cloud must be deep enough to cover the range of temperatures for charge separation, and the generation process must be able to provide a flash rate in excess of 10 per minute, a frequency readily observed in more severe storms. Small clouds of a cubic kilometer or so clearly will not work; 3 to 4 kilometers horizontally and 5 to 8 kilometers vertically are the minimum dimensions.
Shear also plays an important role in the process of charge separation. Recent observations made by research aircraft flying through thunderstorms demonstrate the asymmetry of cloud vertical velocities (see Figure 7). They also show that the supercooled water drops are confined to the upshear, high-updraft region. The downshear region is composed of ice, primarily in the form of vapor-grown ice and aggregates, initiated at the highest, coldest region of the growing cloud. In between lies the most interesting region, with a transition from entirely supercooled water to cloud composed almost entirely of ice crystals, the ice particles consisting of graupel upshear, changing to graupel plus ice crystals, then to ice crystals (columns and needles) and snow downshear (see Figure 7). Charge separation can only take place where the cloud has the optimal mix of particles--a region that may extend only a few hundred meters horizontally. The boundaries of the region, however, are far from stationary. An observational aircraft suffers substantial turbulence during a transect, implying a moderate mixing rate of the ice into other cloud regions. The upshear cloud region is constantly renewed; otherwise the system would quickly cease to exist.
More Complicated Geometries
he airmass thunderstorm frequently forms from convection in moist air overlying a relatively homogeneous land surface that is heated by the sun, with storm activity beginning in the late morning or early afternoon. A similar phenomenon takes place as cold air passes over a warmer ocean, although here the diurnal influence of solar heating is minimal. The cloud that eventually grows into a thunderstorm is but one of many smaller clouds that is especially favored by some surface feature of surface heating or topography overland or a warm spot in the ocean. These systems almost always have vertical shear of the horizontal wind: The charge is usually (but not always) plus above, minus below. The life cycle of the thunderstorm phase is measured in minutes, with the number of lightning flashes ranging from one to a few tens of flashes.
As we consider thunderstorms of longer time scale and larger size, other complications arise. Once a storm has developed, the cold downdraft, enhanced by falling precipitation, descends to the surface and spreads out, initiating other storms, particularly if the cold air (or gust) front should pass beneath a neighboring cloud or collide with another gust front from another storm. A sequence of storms may develop, sometimes related to the local topography. If the anvil of a well-developed thunderstorm cloud happens to overlie the newly developing storm, falling ice will rapidly be ingested into the system and may inhibit the orderly development of the electrically charged regions of the single cell, so much that precipitation is produced with little electrical activity (see Figure 8).
Lines of thunderstorms may also develop along cold fronts and sea-breeze fronts. Here the wind direction aloft is often such that the anvil is carried away from newly developing cells, which can evolve as individuals uninfluenced by ice produced by their neighbors. The squall line of the Kansas/Oklahoma/Texas region is a special example. They generally follow a weak, inactive cold front and are fueled by water vapor drawn from the Gulf of Mexico. These systems evolve as a complex flow in three dimensions with ice aloft being rapidly removed in the anvil and the moist air from below feeding upward into an updraft with speeds that are often high enough to grow grapefruit-size hailstones from large amounts of supercooled water. The updrafts need to move at about 50 meters per second to accomplish this feat. Such updrafts carry supercooled rain and cloud drops to considerable heights, with temperatures as cold as -20 degrees.
These storms have intense electrical activity, are essentially steady state (for many hours) and travel sequentially along the moving front. The electrical activity probably evolves from a sheet of mixed particles as in the air-mass thunderstorm, but the squall line has much greater horizontal and vertical extent and is also probably highly convoluted (as suggested by radar echo). The amount of charge separated depends on the relative collision rates of graupel and crystals of different size; the larger hailstones grow with a wet surface and probably do not influence the charge separation all that much, although they may be indicative of extensive regions of smaller graupel of optimum size elsewhere.
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© American Scientist 1998