Two environmental factors influence how thunderstorms behave and evolve: instability and wind shear.
- Instability: determines how intense the thunderstorm will become. High to extreme instability is the ingredient which makes severe thunderstorms more likely.
- Wind shear: is the critical ingredient. It shapes the form of the thunderstorm. It also influences how the updraft and downdraft in the thunderstorm interact. Also, wind shear critically infulences how thunderstorms move and interact with each other.
The importance of wind shear
Wind shear is a very important factor in thunderstorms! It is difficult to overemphasize the role of shear in the behaviour of thunderstorms. It influences their type, their potential for severe weather and their interaction. As we have already discussed: wind shear is the change in speed and/or direction of wind with altitude. If the wind direction stays the same and only the speed changes, we call it unidirectional or speed shear. If the wind direction changes, we call it directional shear. Both the absence or presence, and the type of wind shear is very important.
Wind shear pushes the downdraft, the cooled air and preciptation (rain and hail), away from the updraft. It essentially keeps the updraft free and undisturbed.
The thunderstorm spectrum
Depending on the amount of wind shear there are three basic types of thunderstorms: single cell, multicell and supercell thunderstorms.
Single-cell / pulse thunderstorms
Single cell or pulse thunderstorms are by far the most common type of thunderstorms. They form in unstable environments with no or very little shear. As the name implies, they are essentially a pulse of warm, moist air rising up. As there is no wind shear, the cooled air and precipitation is not offset from the updraft – rather, it falls back into the updraft, essentially choking off, or collapsing the thunderstorm. A typical pulse thunderstorm lasts 20-30 minutes. As the outflow spreads out it can trigger the formation of new pulse thunderstorms.
Pulse thunderstorms are typically not severe. They usually produce locally heavy, but brief rainfall and small hail. Pulse thunderstorms can become severe in case of high to extreme instability: they can produce large hail and severe straight line winds (downburst). Very rarely do the prouce a weak tornado. In particular, pulse thunderstorms in high to extreme instability produce intense lightning.
The possibility of downbursts presents a significant threat to aviation.
Multicell thunderstorm is a fairly broad term encompassing thunderstorms in moderately sheared environments. Their common characteristic is that they form in sufficient shear for the downdraft to be offset from the updraft. I.e. the downdraft does not inhibit the updraft like in pulse thunderstorms, rather, the downdraft initiates new storm cells, producing a multicell.
A multicell thunderstorm conists of a group of cells, moving as a single unit. Each cell is at a different stage in its life cycle. As the older cells dissipate, newer cells form upwind (upshear) of the multicell.
There are two basic types of multicell thunderstorms: multicell clusters and multicell squall lines.
A multicell cluster consits of a group of single cells, or pulses, in different lifetime stages. The cluster moves with the mean wind, with new storm cells developing upwind.
Multicell cluster characteristics
- A cluster of storm cells in different stages, moving in the same general direction.
- Severe weather threat is low to moderate.
- Main threats include severe straight line winds (downbursts).
- Also threats of small to marginally large hail.
- Slow-moving clusters can produce persistent heavy rainfall and flash flooding.
A squall line is a form of multicell thunderstorm. It is a long line of separate thunderstorms, which usually develops ahead of a cold front. In squall lines multiple downdrafts from individual storm cells merge to form a long and more or less continuous gust front. Due to the linear nature of the system, squall lines are often referred to as QLCS = Quasi-Linear Convective Systems.
Squall line characteristics
- Multicell squall lines form in environments with more instability and more shear than multicell clusters.
- Severe weather threat is moderate to high.
- Many closely spaced storms produce a storm structure different from a multicell cluster.
- A line of storms with a continuous, well-developed gust front at the leading edge of the line.
Squall line severe weather threats
- Main severe weather threat in squall lines is severe straight line wind. Some squall lines develop very severe straight line winds; the most severe exceed 200 km/h. Types of squall lines frequently associated with severe winds are bow echoes and derechos.
- Other severe weather threats in squall lines include heavy rainfall, moderately large to large hail and weak tornadoes. In particular, slow moving or stationary squall lines can produce persistent torrential rainfall which may cause flash flooding.
Bow echos and derechos
The final type of thunderstorm is a supercell. Supercells are the infamous type of thunderstorm, well-known for producing violent tornadoes and destructive hail, particularly in Tornado alley in the US.
Supercells are the rarest type of thunderstorm. Supercells are popularly associated with severe weather, most notably with severe hailstorms and tornadoes. Not all supercells are severe, however, a large percentage of severe thunderstorms are supercells.
While supercell thunderstorms are mostly know from the USA, they form elsewhere also, for example: India, western Australia, plains of Argentina and Brazil, South Africa and in many places in Europe.
ANATOMY OF A SUPERCELL THUNDERSTORM
Supercells are by far the most complex and highly organized type of thunderstorms. The anatomy of a supercell will take a bit more time and effort to understand, however, …
The main distinguishing characteristic of a supercell is a rotating updraft, called a mesocyclone.
The rotation is a direct result of wind shear. Wind shear produces horizontal rolling motion of the air, called vorticity. At any time air in a strongly sheared environment is full of small rolling vortices. As the warm, moist air convectivelly rises into an updraft, this rolling motion is stretched vertically, producing a rotating updraft.
MOGOČE TU PRIMER VORTICITY S KELVIN-HELMHOLTZ INSTABILITY CLOUDS
Another distinguishing characteristic is the separation of donwdrafts. A supercell develops two downdrafts: a Forward flank downdraft (FFD) and a Rear flank downdraft.
- Forward flank downdraft (FFD): is the main downdraft of the supercell, located ahead of the mesocyclone. The forward flank downdraft is air that was lifted with the updraft, pushed upwind (upshear) and cooled. The downdraft is additionally cooled by evaporational cooling. The forward flank downdraft spreads ahead and to the north of the motion of the supercell. Air in the forward flank downdraft is cool and moist.
- Rear flank downdraft (RFD): the rear flank downdraft is composed of relatively warm and dry air, forced down from mid levels of the storm. In some cases, rain from the FFD is wrapped around the mesocyclone by its rotation and mixed into the RFD. The RFD is located to the north and trailing the mesocyclone.
Supercells are often isolated, but they can also form embedded within squall lines. In particular, supercells frequently occur at the southern end of squall lines, typically known as “Tail-end Charlie”.
Supercells form in strongly sheared environments. A rough threshold for supercell formation is 20 m/s of shear in the 0-6 km layer, although they can form with less shear (>15 m/s). Additionally, the wind must veer with altitude: it must change direction clockwise with increasing altitude (in the northern hemisphere).
Supercells are frequently produce severe weather:
- Hail of all sizes, from small to giant (current record hail was 20 cm in diameter, and it was produced by a severe supercell thunderstorm in South Dakota, USA). Slow moving supercells may cover the landscape with thick hail blankets.
- Torrential rainfall and consequent flash flooding.
- Severe straight line winds.
Low Precipitation (LP) supercells
Are characterized by relatively light precipitation, mostly hail and relatively light rain, that is well separated from the storm updraft. Their updrafts, while strong, are typically relatively slender and strongly tilted. They often display strongly striated low-level structure and corkscrew mid and upper parts of the updraft, sometimes referred to among storm chasers as a barber pole updraft. LP supercell form in environments with comparatively low moisture, thus their bases are high. Typical LP supercells are higher plateaus or in post-frontal conditions, where a strong jetstream aloft persists.
LP supercells frequently produce large or very large hail. Due to the strong updraft tilt and sparation of the precipitation, you may encounter hail falling from the sky far from the cloud base. Often with no rain. Hail is the main severe threat with LP supercells.
Other severe threats with LP supercells include strong to severe straight line winds and occasionally tornadoes. However, these are much more rare than severe hail.
LP supercells are often very photogenic, as they tend to form in environments with little low level cloud cover, rendering the storm structure highly visible.
High Precipitation (HP) supercells
HP supercells are characterized by intense precipitation, mostly very heavy rainfall. They requently produce flash floods. They can produce large to very large hail, but hail is usually smaller with HP supecells. They frequently produce severe straight line winds (downbursts). Tornadoes also occur with HP supercells.
HP supercells form in environments with lots of moisture. Shear is not as strong as with classic and LP supercells; in particular the mid-level flow is relatively weak. Thus the precipitation is not as offset from the updraft as in the other two types, and it tends to wrap around the updraft. Whereas in LP and classic supercells the RFD is dry (or mostly dry) it is usually contains heavy rainfall in HP supercells.
HP supercells are often not as isolated as classic supercells, may be embedded in squall lines and can be difficult to spot visually with little prior experience.
HP supercells often evovle into bow echoes / as the rear flank downdraft or a rear inflow jet causes the storm to accelerate outward, resulting in a bowing storm with damaging straight-line winds.
– weak mid- and upper-level winds
– weak deep-layer shear
// storm gets upright, but is not very much leaning
– high SREH