CONVECTIVE UPDRAFT AS DRIVER OF THUNDERSTORM ELECTRIFICATION: POSSIBLE STUDIES USING WP RASS P. Pradeep Kumar Department of Atmospheric & Space Sciences University of Pune

1.CHARGE STRUCTURE OF THUNDERSTORM 1.1What is a thundercloud? Lightning is associated with convective activity. Thunder (and thus lightning) is used by the professional weather observer to classify the severity of convective activity. Cumulonimbus clouds are the largest form of convective cloud and typically produce lightning. Cumulonimbus clouds with lightning activity are generally referred to as thunderclouds.

1.2When do Thunderstorms form ? If the atmosphere is sufficiently unstable, convective development can result in the thunderstorm, a cumulonimbus cloud (Cb) with lighting discharge, possibly hail, and, in extreme cases, accompanied by a tornado.

Air mass thunderstorms are thunderstorms not associated with weather fronts. They form mostly in the summer as the sun heats the surface and causes convection. There are three basic stages in the lifespan of a thunderstorm:

In the formative cumulative stage, there are updrafts at all points at the base of the cloud. In the mature thunderstorm, there will also be a strong downdraft caused by both (a) the drag of rain and hail falling through the cloud and (b) evaporative cooling as dry air is entrained into the cloud. In the dissipating stage, there are weak downdrafts throughout the cloud. The typical lifetime of an air mass thunderstorm is one hour or less.

CHARGE STRUCTURE INSIDE A THUNDERCLOUD The classical thundercloud model can be described as a positive electric dipole with a positively charged region above a negatively charged region. A weaker, positively charged region at the base of the cloud gives it more of a double-dipole structure.

The average values taken are p is +10 coulombs (C) at 2 km, N is -40 C at 5 km, and P is +40 C at 10 km. These are representative of values that can vary considerably with geography and from cloud to cloud.

1.4What are Super cell Thunderstorms ? The mesoscale organization is such that the entire storm behaves as a single entity rather than as a group of cells.

A. Anvil- The Anvil is one of the most impressive features of a severe storm due to its aerial coverage and icy texture. Within a severe storm, moisture is transported from the lower troposphere to deep into the upper troposphere.

 B. Overshooting Top- The core of the updraft has the strongest convective upward vertical velocity. This core of rapidly rising air will only slow down and stop when it encounters a very stable layer in the atmosphere.  This very stable layer is the tropopause.

 C. Mammatus- Mammatus are pouched shaped clouds that protrude downward from the thunderstorm's anvil. They form as negatively buoyant moisture laden air sinks.  The cloud remains visible until the air sinks enough that the relative humidity falls below 100%.

Charge structure based on charge analyses of 49 soundings through thunderstorms.(by Stolzenburg, Marshall, and Rust; published in the Journal of Geophysical Research, Vol. D103, 1998, pp ).

2.THE LIGHTNING FLASH  2.1Why does lightning occur?  The charge buildup in thunderclouds are unstable. When electric fields generated by the charge buildup become too strong (typically 3-4 kilovolt/cm at the altitude of the negative charge region of the cloud) electrical breakdown of the air occurs and charge is exchanged within the cloud or to the ground. Charge is exchanged by a lightning flash.

Intracloud (IC) flashes, redistributing the charge within the cloud, account for over half the lightning flashes in northern latitudes. Cloud-to-cloud and cloud-to-air flashes are less common. Aside from aviation, these three types of flashes have little effect on people.

The negative cloud-to-ground lightning flash can be broken down into three stages. The stepped leader, the return stroke, and the dart leader.  The stepped leader is a small packet of negative charge that descends from the cloud to the ground along the path of least resistance.

 In its path, the leader leaves a trail of ionized gas. It moves in steps, each typically tens of meters in length and microseconds in duration.  After a step, the leader pauses for about 50 microseconds, then takes its next step.  The leader charge packet sometimes breaks up to follow different paths, giving lightning its forked appearance.

When the downward moving leader connects with a surface corona discharge, a continuous path between the cloud and the ground is established and a powerful return stroke is triggered. The return stroke rapidly moves as a wave upwards into the cloud following the ionized trail of the stepped leader, stripping the electrons from its path.

After the return stroke, the lightning flash may end, or, if enough charge in the cloud is collected, a dart leader may come down from the cloud following a direct path to the surface. In turn, the dart leader triggers a second return stroke.

A single lightning flash can comprise several return strokes. The average number of return strokes in a lightning flash is 3 or 4, each stroke typically separated by 40 to 80 milliseconds.

The path of the electrical current is only a few cm wide but the stroke heats the air to almost 30,000 degrees C (five times the surface temperature of the sun). The resulting shock wave (from the rapid expansion) of the air) is what we know as thunder.

2.4Are lightning flashes positive or negative? Coming from higher altitudes in the cloud, positive flashes make up about 10% of all lightning flashes. They are usually composed of a single stroke, and have longer, continuing currents. From the forestry perspective, positive flashes are of greater concern because the longer currents are more likely to start fires.

Positive flashes are more common in winter storms. The apparent cause of this is the lower freezing level, which places the positive charge center closer to the ground, thus increasing the likelihood of a flash. A popular theory is that horizontal wind shears force a tilting of the dipole axis providing a route for the positive flash, but this has yet to be shown conclusively.

Red sprites, blue jets and elves are upper atmospheric optical phenomena associated with thunderstorms that have only recently been documented by using low light level television technology.

Red sprites are large but weak luminous flashes that appear directly above an active thunderstorm system and are coincident with powerful positive cloud-to-ground lightning strokes. Their spatial structures range from small single or multiple vertically elongated spots, to bright groupings which extend from above the cloud tops to altitudes up to almost 60 miles (about Their spatial structures range from small single or multiple vertically elongated spots, to bright groupings which extend from above the cloud tops to altitudes up to almost 60 miles (about 95 km.) Sprites are predominantly red and they usually last no more than a few milliseconds. 95 km.) Sprites are predominantly red and they usually last no more than a few milliseconds.

Blue jets are a second high altitude optical phenomenon, distinct from sprites and first documented in 1994 (although pilots had earlier reported similar sightings). Blue jets are optical ejections from the top of the electrically active core regions of thunderstorms, but not directly associated with cloud-to-ground lightning.

Blue starters differ from blue jets in that the are brighter but shorter (reaching to only about 12 miles altitude). These were reported to occur over regions where large hailstones were falling.

Elves are rapidly expanding (up to 300 miles across) disk-shaped regions of luminosity, lasting less than a thousandth of a second, which occur high above energetic cloud-to-ground lightning of positive or negative polarity. Elves most likely result when an energetic electromagnetic pulse (EMP) propagates into the ionosphere.

2.6 Is there an association between lightning activity and radar echoes? There is a general association between radar reflectivity and negatively charged lightning flashes. Lightning discharge sources are located near, but not necessarily within, the area of highest reflectivity

3.CLOUD ELECTRIFICATION MECHANISMS

There are two general theories to explain the charge buildup required to electrify a thundercloud. (a)The convective theory proposes that free ions in the atmosphere are captured by cloud droplets and then are moved by the convective currents in the cloud to produce the charged regions. (B)The gravitational theory assumes that negatively charged particles are heavier and are separated from lighter positively charged particles by gravitational settling.

For the gravitational theory to work, there must be some charge exchange process between particles of different sizes. Charge can be exchanged between particles in various states by inductive and non-inductive processes. The most promising is the non inductive exchange between ice crystals and hailstones, referred to as the ice-ice process.

The effectiveness of the ice-ice process lies in the thermo-electric properties of ice. The mobility of the (OH3)+ defect in ice is greater than the (OH)- defect and the number of defects increase with temperature. When warm and cold ice particles come in contact, the positive defect flows faster from the warmer to the colder particles than the converse, giving the colder particles a net positive charge. Therefore in the typical scenario, a warm hailstone or snow pellet will acquire a net negative charge as it falls through a region of cold ice crystals.

There are three steps in the process of Ice-Ice (Graupel or hail) collision in the presence of supercooled droplets. Collisions between hail or graupel and supercooled water droplets causes freezing of the previously liquid water. The release of latent heat maintains the hail or graupel at a high temperature than the ice particles.

Collision, but not accretion, between the colder ice particles and the warmer hail or graupel causes a charge separation and the transfer of a positive ion from the warmer object to the colder one. Updrafts in the cloud carry the small, positively charged, ice particles to the upper part of the cloud while the heavier, negatively charged, hail and graupel migrate to lower levels.

WHAT NEEDS TO BE DONE USING WP/ RASS Linking the microphysics and convective updrafts to charge development and subsequent lightning discharges needs to be understood. Such an understanding would improve the way electrical activity is incorporated into weather forecasting problems.

There is ample evidence that there is link between convective updrafts and lightning flash rates. However, their dependence on local regimes and environmental conditions are still not well known.

The magnitude and location of charge destroyed in lightning flash can be determined by measuring the electric field change during lightning flash at spatially separated locations. Such measurements have been done over Pune using a network of electric field stations.

Some observations in other climatological regions have reported taller storms that produced no lightning. One reason that storm height threshold sometimes falsely identifies storms as thunderstorms is that tall storms can have updrafts with insufficient speeds or spatial extend to drive electrification mechanisms to the point of causing lightning.

Some observations show that flash rates tend to be large on days with large CAPE (Convective Available Potential Energy). Others have noted the reverse, that many storms with low flash rates occurred on days with large CAPE. So there are some other factors other than CAPE which contribute to the electrification.

The current status shows that there is considerable uncertainty in understanding the proper linkages to thunderstorm electrification.

The role of bulk water properties like ice/hail/supercooled droplets have to be properly CAPE, Convective updrafts and understood. Such an understanding would be helpful in linking the thunderstorm electrical activity and lightning to operational weather forecasting.

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