GG3069   Climate and Weather Systems

Lecture 6: Thunderstorms and Tornadoes  

 

Thunderstorms and tornadoes illustrate many of the physical principles discussed so far in the course. In this lecture we will look at the formation of thurderstorms (including tornadic thuderstorms), invoking the principles of energy transfer, buoyancy, moist processes, and forces acting on winds. We will also see the usefulness of Tephigrams in describing these processes.

Thunderstorms

Thunderstorms are a violent expression of deep, moist convection. Two conditions are required for such vigorous convection:

• conditional instability

• a trigger mechanism

Conditional instability

Under conditions of absolute instability, convection can dissipate energy building up at ground levelby gentle uplift. For a violent thunderstorm to develop, instability must be suppressed until a large amount of energy has accumulated. Conditions where this can occur are shown on the diagram below.

In the diagram (modified from Stull, 2000), the environmental curve is shown in bold, and shows the kind of thermal structure that encourages thunderstorm development. Now, consider what happens when air at ground level is heated (Point A). Warmed air will rise, and follow a dry adiabat as its pressure decreases. (In other words, its temperature decreases at the dry adiabatic lapse rate, but its potential temperature remains constant, since the only influence on its temperature during uplift is the pressure) The air will rise until it reaches height Zi. At this point, rising air will find itself cooler than the surrounding air, and uplift will be inhibited. Thus the atmosphere will only exhibit instability below Zi: in more concrete terms, this means that warming at the ground feeds dry thermals with limited vertical development. We can now examine what will happen if for any reason air can be lifted above Zi. Eventually, the air will cool to the dew point temperature, at which the air is saturated and condensation of cloud droplets occurs. This level is known as the Lifting Condensation Level (LCL), and here the rising air will follow a saturated adiabat. In other words, release of latent heat during condensation slows the rate of cooling of the rising air, and it is now cooling at the non-linear Saturated Adiabatic Lapse Rate. Now, recall that above height Zi, uplift cannot be maintained by internal processes alone: we have had to imagine what would happen IF uplift could occur. At a certain point, however, our rising air parcel crosses the environmental curve. Here, uplift will be self-sustaining: this is known as the Level of Free Convection (LFC). Uplift and cumulus formation will thus follow if air can be lifted to the LFC, and uplift will continue until the path curve (saturated adiabat) recrosses the environment curve at the Limit of Convection (LOC).This particular atmosphere is therefore conditionally unstable: instability is conditional upon air being uplifted to the LFC by some mechanism.

Trigger mechanisms

The conditions described above are necessary, but insufficient for a thunderstorm to occur. We also need some external factor that will trigger the initial uplift to the LFC. Several triggersa re possible:

  1. forced uplift over mountains
  2. localised regions of excess surface heating
  3. fronts (boundaries between air masses with different thermal properties)
  4. gust fronts (cold gusts from other thunderstorms)
  5. Drylines (boundaries between air masses with different moisture contents)

Thunderstorms arising from cases (1) and (2) are known as air mass thunderstorms, because they occur within a single air mass. We will examine these first, then go on to look at the generally more severe thunderstorms associated with fronts.

Air mass thunderstorms

Air mass thunderstorms occur when air is destabilised by heating at the ground or by forced ascent over mountains. In the case of heating at the ground, the input of energy (longwave radiation from a heated ground surface) increases the potential temperature in the lower atmosphere. Eventually, dry then moist thermals will warm the cooler 'lid' of stable air (between Zi and LFC on the diagram above): the atmosphere will have evolved from a state of conditional to unconditional instability due to the input of energy to the system. Air mass thunderstorms of this type form in the British Isles (and elsewhere) in hot, humid weather, and usually occur mid-afternoon following intense heating at the ground. Forced uplift over mountains can occur where winds blow against mountain masses and undergo uplift. Recall that air is often warmer over mountains than at the same altitude over the valleys (in daytime, and in the absence of snow or ice cover). This can lead to uplift of air over the mountains and upslope breezes, which can evolve into thunderstorms. Storms of this type are common in the Alps (and other mountain ranges of modertae altitude) in summer.

Air mass thunderstorms typically follow life cycles over 1-2 hours. The storm begins as uplifting warm air crosses the Level of Free Convection, and latent heat is released as the freely rising air forms cumulus cloud. The rising air and the release of latent heat form a positive feedback, leading to very rapid uplift. The role of latent heat is of crucial importance: it is latent heat (i.e. energy originally absorbed during the evaporation of water) that provides most of the energy of the storm. Typical updraught speeds are 5-10 m per second, and can exceed 30 m per second for large storms.

Precipitation is formed mainly by the Bergeron-Findeisen mechanism (i.e. the formation of ice crystals high in the cloud), and is initially kept in suspension by updraughts. Rain development requires convection depths of 1-2 km over water and 4-5 km over land, and times of 15 and 30 minutes respectively. Differences appear to reflect the differing availability and size of cloud condensation nuclei (CCNs) over land and water. Glaciation in thunderclouds can be seen as wispy edges to the upper cloud. Hail in thunderstorms can reach very large dimensions, due to recycling of hail by updraughts. Hailstones have a concentric structure that reflects varying temperature and humidity at different elevations within the storm. Hailstones may reach >5cm across, and can cause considerable damage and danger to life.

Cumulus towers will climb until arrested by a change in thermal structure of air: i.e. when they reach a pont where the saturated adiabat crosses the environmental curve. This Limit of Convection can be associated with the boundary between the troposphere and the stratosphere (tropopause) in the largest systems, especially in the tropics, but in the mid-latitudes may be a stable layer in the mid-troposhere. Cloud masses may overshoot the base of the stable layer due to the momentum of the ascending air. The momentum of the storm, and the degree of overshoot, can be calculated from the shaded area in the diagram above.

Anvil formation: flat cloud layers are formed by the lateral spreading of cloud at the base of the stable layer at the top of a storm. The anvil is often elongated in one particular direction by upper level winds. Mammatus cloud may form at the base of anvils: this consists of pendant, "breast-shaped" cloud masses which form as air in the anvil chills and sinks due to the evaporation of cloud droplets and associated absorption of latent heat and air chilling.

As the storm develops, downdraughts form within the cloud. These consist of falling air, and are experienced on the ground as cold, gusty winds associated with precipitation. Downdraughts are formed by (1) chilling of air by evaporating precipitation (latent heat consumption); and (2) frictional drag by falling precipitation. During the life of an air-mass thunderstorm, owndraughts gradually gain in power, and eventually overwhelm updraughts and the storm dissipates. Downdraughts consitute a major hazard to aircraft in regions prone to thunderstorms, such as the Mid-west of the USA.

Frontal thunderstorms

In atmospheres with a suitable thermal structure, hunderstorms can be triggered by the convergence of air masses with contrasting temperature or humidity. Squall-line thunderstorms develop along cold fronts, where cool air meets warmer air. The cool air is denser than the warm, so tends to undercut it, wedging the warm air aloft and initiating uplift. This can elevate warm air to above the Level of Free Convection. Storms typically occur in groups aligned along the front, and then move along approximately parallel to the direction of travel of the front. As each storm advances, precipitation released from updraughts falls to earth through the colder trailing edge, creating a separate downdraught area: this is different from the situation in air mass thunderstorms, in which the downdraughts mix with the updraughts. Consequently, squall-line thunderstorms tend not to dissipate as quickly as air mass storms. Part of downdraught travels under the updraught, creating a gust front, which perpetuates undercutting and triggering the uplift of more warm air. Thus updraught and downdraught are complementary, not in opposition as in air mass thunderstorm. A crucial factor here is differing wind velocities or directions at different altitudes, or wind shear. Wind shear ensures the separation of updraughts and downdraughts, and hence extends the life of a storm or series of storms.

Gust fronts, or the spreading fronts of downdraughts that have reached the ground, can play an important role in triggering a sequence of storms. Such Multicell Thunderstorms consist of a series of cells which develop in sequence. Three dimensional airflow patterns cause downdraughts to spread over the ground to the right of the updraught area (relative to the direction of movement of the storm). The downdraughts wedge warm air aloft, thus initiating renewed convective activity to the right of the original cell. This new cell grows as the original cell wanes, and the sequence is repeated, sometimes for several hours. Storms thus propagate through time, each cell helping to generate subsequent cells: hence the name ‘multicell thunderstorm’.

Drylines are sharp horizontal boundaries between air masses of similar temperature but contrasting humidity. The dew point temperature (at which saturation occurs) may differ by as much as 9° C on either side of a dryline. Drylines are especially common in Texas, Oklahoma and Kansas, where warm dry air originating in the arid Southwest meets warm moist air originating over the Gulf of Mexico. During convergence of the dry and moist air, some of the dry air flowing from the high, western deserts can over-ride the moist air, forming a stable 'lid' over the low-level moist air. This can then allow the build-up of energy at low levels. Convergence can lead to thunderstorm development, and once started, multicell thunderstorms can be perpetuated by the downdraughts.

Surface conditions associated with the formation of a dryline and severe thunderstorms in the USA.

Supercell Thunderstorms are exceptionally large thunderstorms, which consist of single, well-organised airflow patterns which may persist for many hours. They are often, but not always, associated with very destructive tornadoes. Why this should be is still not fully understood, and is a question of obvious interest to weather forecasters.

Tornadoes

Tornadoes are rapidly spinning vortices of air, formed in association with severe convective storms. The exact conditions necessary for tornado formation are not fully understood, but huge progress has been made in the last two decades due to the developemnt of Doppler radar, which allows measurement of the velocity fields in tornadoes and the associated storms, and sophisticated mathematical modelling. A good review paper is: Klemp, J.B. 1987. Dynamics of Tormadic Thunderstorms. Annual Review of Fluid Mechanics 19, 369 - 402, from which the following account and diagrams are drawn.

A crucial factor in tornado formation (in addition to conditional instability and a suitable trigger mechanism) is strong wind shear. This can occur during convergence of air masses (such as along a cold front or dryline), when wind directions at ground level can be opposite to those aloft. Wind shear can set up rotors, or rotating cylinders of air about horizontal axes aligned at right angles to the wind direction (see Figure below). The rotating air is said to have horizontal voricity. Now, if uplift occurs over such a rotor, the rotor is deformed upwards into a loop, forming two counter-rotating vortices with vertical axes of rotation. Thus, horizontal vorticity has been transformed into vertical vorticity beneath and within the gathering storm:

The Figure shows the usual situation, in which the vortex on the south of the storm (to the right relative to the direction of storm motion) is rotating cyclonically (anticlockwise in the northern hemisphere example shown here), and the vortex on the north is rotating anticyclonically. The plus and minus signs in the vortices refer to positive (cyclonic) and negative (anticyclonic) vorticity, respectively. The downdraught between the vortices can split the storm into two, but this does not always happen. Wind dynamics within the storm tend to strengthen the cyclonic vortex and weaken the anticyclonic vortex, and the cyclonic vortex develops into a large rotating element within the storm, known as a mesocyclone. Vorticity within the mesocyclone can be increased by vertical uplift and stretching of the vortex: this causes horizontal shrinking and convergence, and a consequent increase in rate of rotation (to conserve angular momentum). Ultimately, the inner part of the vortex can spin up into a funnel cloud, which can then propagate towards the ground to form a tornado. Most tornadoes have positive or cyclonic vorticity (that is, they rotate in the same direction as large storm systems: anticlockwise in the northern hemisphere), but some have been known to have anticyclonic vorticity.

In the Figure above, the hachured lines at the base of the storm are the gust fronts from spreading downdraughts: these are funnelling warm air into the vortex. Note that the cyclonic vortex has become dominant and has spawned a tornado, whereas the anticyclonic vortex has weakened.

The pressure drop in the centre of a tornado may be as much as 200-250 hPa: it is this that makes the funnel visible by causing air entering vortex to reach saturation, and condensation to occur. (The funnel also contains dust and other debris carried upward)

Tornadoes move with an overall direction and velocity (translational speed) determined by air movement in the lower troposphere. The winds felt on the ground are a combination of the translational speed and the rotational speed.For the most common, cyclonically rotating tornadoes, wind speeds on the ground are greatest on the right flank of the tornado (to the left as seen from in front, looking at the approaching tornado), where rotational and translational speeds are added. Large tornadoes may contain subsidiary vortices, which travelaround the main vortex, creating a complex pattern of destruction. The vortex of a tornado is usually only a few hundreds of metres in diameter. The fastest winds can reach 50-100 m per second.

Tornado strength is measured on the Fujita scale, from F0 to F5 (strongest). F4 tornadoes are devastating, and can destry frame houses. F5's are of extreme ferocity, and can move heavy structures large distances. The National Severe Storms Forecast Center in Kansas City, Missouri, reports that the majority of tornadoes are small (F0 and F1), with about one F5 annually. Extensive damage is caused by the high winds, and the rapid pressure drop as the tornado passes over. Windows explode from buildings and vehicles, as the pressure outside is suddenly much lower than than that inside. There are even apochryphal stories of chicken feathers popping off, due to the pressure difference between the outside air and air in the base of the quills.

The average number of fatalities in the USA is c. 100 per year, usually associated with a few severe events. The worst on record was March 1925, which killed 689 people in 3 hours over 200 km in Missouri, Illinois and Indiana.

Tornadoes in Britain

Tornadoes are not normally regarded as part of the British weather scene, but in fact they are not uncommon: there was on average 14 days per year with tornadoes in the period 1960-1982. Tornadoes in Britain occur in a similar situation to North American tornadoes: in association with frontal thunderstorms, usually during the rapid passge of cold fronts. Most are minor - and never as severe as their American counterparts - but in Nov. 1982, 102 were reported in a single day during south-westerly airflow ahead of a cold front. A notable recent event occurred in Selsey in January 1998, when a tornado associated with a squall line cut a swathe of destruction through town.  

Thunder and Lightning

Thunder and lightning are two manifestations of the sudden release of electrical charge during a storm. This release happens because charge separation occurs both within the cloud and between the cloud and the ground. The electrical potential difference between oppositely charged regions builds up to a threshold level, after which it is rapidly and temporarily neutralized. Several mechanisms are involved in charge separation, but the most important factor is that large particles (e.g. hail) become negatively charged, while small particles become positively charged. Thus, as larger particles fall out of the cloud, negative charge is carried downward, leaving the upper parts of the cloud with a net positive charge. Electrical charge separation also occurs on the ground below the storm as the result of induced charges, or the accumulation of opposite charge attracted by charge in the cloud. Lightning is essentially a giant spark induced when the insulation of the air breaks down, and consists of a flow of electrons from the cloud towards the earth. The lightning stroke raises the temperature in narrow channel of air to 30,000° K. This occurs so fast, that the air has no time to expand and the air reaches 10-100 times normal atmospheric pressure. The resulting shock wave causes noise heard as thunder.  (Lucretius in 55 BC supposed that thunder was the sound of great clouds crashing together)

The delay between lightning and thunder reflects the different travel speeds of light and sound: light travels at 300,000 km per second; sound travels at 330m per second. The difference in speeds means that the sound lags behind the light by about 3 seconds per kilometre, so the distance to the lightning stroke can be estimated by counting out the seconds between the lightning flash and the associated thunder. The prolonged booming of thunder is seldom caused by echoing, but is due to the arrival at different times of sound from different parts of the lightning stroke: the stroke may be several km long, so there may be as much as 10 sec difference in arrival time from the most distant parts and the nearest parts.

Lightning conductors were invented by Benjamin Franklin, who demonstrated the electrical nature of lighning in 1752 by flying a kite in a storm. The installation of conductors has been a major life-saver. For example, lightning was a major cause of death in Royal Navy, in the days when Britannia ruled the waves and the Navy maintained a large fleet in all weathers. Men were frequently aloft in bad weather, altering sails in response to squalls and shifts of wind in thunderstorms. Lighning commonly killed men in direct strikes and also blew holes in the ship's hull below masts. In one 5 year period, the British Navy lost 70 ships to lighning. The Admiralty mistrusted conductors (being a new-fangled, American invention), and refused to install them on ships for many years. Eventually they arrived at a compromise: temporary conductors were carried on board, to be hoisted when lighning threatened. This resulted in even more deaths than before, as many men were killed while hauling conductors. Eventually permanent copper strips inroduced, vastly reducing the rate of death and injury.

Internet resources:

Want to go on a tornado-chasing tour? Or just learn more? Go to:

http://www.stormchasing.com/links.html

A good site maintained by NOAA, with lots of information and pics:

http://www.spc.noaa.gov/faq/tornado/index.html

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