Supercell Formation

This is a primer on the conditions necessary to produce supercells and tornadoes. It includes an explanation of terms often used by guides when they are forecasting the day’s possible intercepts.

Weather on our planet is caused by the uneven solar heating of the atmosphere. It causes temperature differences that results in wind or the movement of warmer air to areas where the air is not as warm. In essence, the heat mixes the air, producing rain, snow, clouds, storms, changes in air pressure etc.

The redistribution of heat can occur either vertically or horizontally. Movement of heat from the tropics to the cooler latitudes is horizontal redistribution. Vertical redistribution is what causes thunderstorms and supercells.

The mechanism behind the formation of a tornado is not completely known, but the range of conditions that exist when one forms is fairly well known. This is what forecasters look for when predicting tornado formation.

The Ingredients

A supercell requires four ingredients to form – Shear, Life, Instability and Moisture (SLIM)

A supercell has it origins in a thunderstorm which begins as a parcel of air that is warmer than the air around it. Different land and water surfaces can cause the air above it to warm faster than a neighbouring parcels. For example, air above a tilled field is warmer than a grass covered one. The air above land is warmer than the air above a body of water in the day but it cools over land faster at night. Mountains will absorb more heat than the valleys in daytime but lose it faster at night.

The uneven heating will produce localized winds. The warm air can roll over the cooler air, leading to horizontal rotation of the air parcel, like a straw rolling across a table.

As daytime heating continues, the warmer air expands and is less dense than the cooler air surrounding it. Then, like a hot air balloon, the air will begin to rise (the straw is lifted vertically off the table). The cooler air will move in below it and is subsequently warmed and rises.

There are four natural forces that can produce the LIFT necessary:

• convection, as noted above, is lift from daytime heating warming the air, making it less dense and allowing it to rise;
• orographic lifting is due to physical barriers such as mountains in which the air will flow over it;
• frontal lifting is warmer air rising over cooler, denser air, and;
• convergence lifting is when air moving near the surface interacts with and is pushed up as the air parcels collide.

With the presence of MOISTURE, that warm air will manifest itself into cloud formations like cumulus and cumulonimbus. Without moisture, the warmer air will remain aloft, unseen except perhaps by the birds riding those thermals.

This vertical transport of a warm parcel of the air described above is called convection and the weather it produces is called convective weather.

There are a number of different phenomena that can be caused by convection besides thunderstorms including rainstorms, squalls, dust devils, tropical cyclones/typhoons/hurricanes etc.

A good example of convection is a boiling pot of water. As the water warms at the base, it rises, forcing the colder water above to move down, closer to the base of the pot where the element warms it, perpetuating the cycle.

For a thunderstorm to form, that rising parcel of warm air, called an updraft, must stay warmer than the surrounding air. As the updraft rises, it will cool because the air is cooler at higher altitudes. If it cools to its dewpoint, it saturates the air (relative humidity of 100%) and condenses into liquid water.

In order for the water vapour to go from a gas to a liquid, it will form around free floating particles in the air like dust, pollen, salt or smoke. These condensation nuclei are so small and light, it doesn’t take a lot to keep them aloft.  This usually occurs about a mile above the surface, forming visible clouds as it ascends.

If the updraft loses strength, the cloud stops growing.

An updraft with more strength will rise to a level called the tropopause, just short of the stratosphere. This is a layer of warm air called the cap.

Here, at the equilibrium level, the updraft will stop moving up, and if the upper winds are strong enough (SHEAR), it will start spreading the cloud out horizontally, forming an anvil cloud (right).

With enough INSTABILITY, a storm could be strong enough to punch through the cap. In doing this, the storm would have the potential to become a supercell.

Instability is defined as the behaviour of a pocket of air when it moves. Stable air will move in the opposite direction in which it was moved (ex. air forced up a mountainside will go so far and then slide back down). Unstable air will continue to move in the direction in which it was moved. In the case of a storm, the pocket of air (updraft) will continue to climb.

Measuring instability involves two variables – CAPE and CIN.

The measure of the approximate strength of the updraft or the amount of energy available for convection is called Convective Available Potential Energy (CAPE). It is a value assigned to the maximum potential vertical speed within an updraft. The higher the number, generally above 2000, the greater likelihood that the updraft can punch through the warm layer and produce a strong storm. (See Lapse Rate below).

In order for the system to become a strong storm, the updraft needs the strength to break through the cap or that layer of warm air just short of the stratosphere. The air in the updraft has moved a significant vertical distance and has cooled as it increased in altitude. The layer of warm air at the stratosphere then prevents or delays further development because the updraft is cooler than the cap layer. Once the updraft can warm enough to break through the cap, explosive thunderstorm development is possible. The cloud that breaks through is called an overshooting top.

The strength of the cap is measured by convective inhibition (CIN). The CIN is a measure of the amount of energy needed to inhibit the ability of the updraft to rise. A weak cap is 1-50 J/kg. A moderate one is 51-199 and a strong cap is 200 and above.

A weak cap will tend to permit storms to form early in the day, before the daytime heating can produce enough instability. It usually forms weak, disorganized storms. A strong cap can prevent the storm from growing any further or even from forming at all. The moderate cap is ideal, allowing a storm to form and form later in the day when conditions are better for a supercell.

Meanwhile, the condensation nuclei that the updraft has kept aloft will collide with other particles, forming large rain droplets or ice particles within the cloud (coalescence). Some of this moisture will evaporate in the middle and upper portion of the cumulonimbus cloud and this produces a parcel of colder, denser air which will begin to sink. This is a downdraft.

When the downdraft reaches the ground, the spreads out as a strong, cool wind known as outflow. If the updraft is unable to break through the cap, the downdraft collapses on top of the updraft, killing the storm by robbing the updraft of its fuel of warm air. (As well, the updraft and downdrafts of other storms that are too close to each other can interfere with the storm’s development. This is why chasers prefer storm systems that are alone rather than linear systems which have multiple storms side-by-side interacting with each other.)

To prevent a storm’s downdraft from killing development, the wind shear needs to push the moisture away from the updraft, creating the anvil cloud. As a result, rain and hail will fall away from the updraft, forming a forward-flank downdraft (FFD).

With the downdraft falling away from the updraft, the updraft can strengthen and is better able to keep the larger particles of ice and rain aloft. The longer it stays aloft, the larger the hail can become. Larger hail indicates a stronger storm.

Shear at lower levels can also aid in the formation of tornadoes, and at mid-level, it can support the rotating updraft (which appear as striations as indicated by the arrows in the photo below).

These mid-level winds can be forced downward by the strong updraft which is acting as a barrier. The dry winds will move towards the ground and warm as it falls. This is called the rear-flanking downdraft (RFD). Being warm and dry, it can cause a clear slot on the back of the storm.

On a radar, the presents as a hook echo.

Both the FFD and the RFD are necessary for tornado formation.

If the updraft and forward-flanking downdraft are in balance and the cap is broken, the storm has the potential to become a supercell or a long-lived thunderstorm. Supercells, called that due to their duration not size, can last one to four hours or more. The sustained rotating updraft is called a mesocyclone.

There are three types of supercells.

High precipitation supercells tend to hide tornadic activity in precipitation around the rear-flanking downdraft. (Rain wrapped tornadoes). These storms look like a kidney bean on radar.

Low precipitation supercells have less precipitation but the weaker rear-flanking downdraft is less likely to produce a tornado.

Classic supercell conditions are in between the HP and LP types with a strong rear-flanking downdraft and less precipitation. It typically shows the “flying eagle” return on radar. This has the classic hook on the back.

Some of the characteristics of a supercell include:

Wall cloud – the lowering of a section of cloud to the rain-free base of the storm. It is a sign that a thunderstorm has become a supercell as it indicates the updraft and downdraft are separated. It will appear about ten to twenty minutes prior to a potential tornado.

Inflow bands – Bands of low cumulus clouds that extend from the storm towards the south or southeast. They usually indicate that the storm is pulling in air from some distance outside the storm.

Beaver tail – a type of warm, humid inflow band. It is a smooth, flat cloud that extends from the eastern edge of the rain-free base to the east or northeast. It can swing around the southern edge of the precipitation region, and while it doesn’t rotate, it can be indicative of rotation within the storm.

Base – The base is the lowest visible cloud underwhich a wall cloud can form. A storm in which the dewpoint and temperature at the surface are further apart will produce a higher base. That means it does not have enough moisture and will form an elevated base about eight thousand feet or move above the ground. The greater the distance, the greater the evaporation of rainfall before it reaches the ground. These storms are more likely to produce dry microbursts and are less likely to produce tornadoes.

Mammatus – Mammatus clouds are rounded pouch-like clouds that can be seen on the underside of clouds including cirrus, stratocumulus and cumulonimbus clouds.

 

 

 

Other Terminology

Lapse Rate

The atmospheric lapse rate is the change in temperature with height and is measured by weather balloons. The value is dependent on the amount of water vapour in the air.

Dry adiabatic lapse rate – Dry air cools at about 10ºC per kilometer.

Moist adiabatic lapse rate – Moist air cools at less than 6ºC per kilometer.

Adiabatic means there is no outside heat involved in the warming or cooling of parcels of air.

The air cools as it rises and condenses. In order to condense, the water molecules release heat which decreases the cooling that is causing the condensation in the first place. (In other words, the heat is being generated by the process of condensation, not by external factors).

As a result, dry air will cool faster than moist air (as there’s less condensation producing heat).

In terms of supercell formation, the lapse rate refers to instability. If the lapse rate is below 6.5ºC/km, the atmosphere is considered stable and there is no upward cloud growth.

A lapse rate at 6.5ºC/km is considered conditionally stable.

A lapse rate above 6.5ºC/km means clouds can grow vertically with no external mechanism to aid it. The higher the lapse rate, the more unstable the atmosphere is and the faster the cloud can grow vertically. The max lapse rate is usually about 9.8ºC/km (the dry adiabatic lapse rate). Very strong heating at the surface or cold air aloft arriving very quickly can produce lapse rates above 10ºC/km.

The amount of moisture can affect the level of instability. Higher dewpoints at the surface or lower levels will increase instability while higher moisture at higher levels will decrease the lapse rate and instability.

 

Dewpoint is the temperature at which the relative humidity would be at 100%. So a temperature of 15º and a dewpoint of 15º means relative humidity is 100%. The closer the two temperatures are, the more saturated the air. The further apart they are, the drier the air.

For strong storm formation, you want a dewpoint to be at least 60ºF and the temperature no more than 20º above that (dewpoint depression). At thigher elevations, the dewpoint can be in the 50s and a lower surface temperature at elevation can help minimize the dewpoint depression.

 

Relative humidity is the amount of water vapour in the air divided by the maximum capacity that same parcel of air has to hold water vapour. The warmer the air, the more water vapour it can hold.

When that air cools, it’s relative humidity will go up as, in relative terms, the cooler air has less capacity to hold the same moisture. So, air at 20º has more capacity to hold moisture than air at 10º. Therefore, the same amount of water vapour would come closer to saturating the air at 10º than at 20º.