Thunderstorms come in many variations: some are relatively light, with some rain and lightning here and there, while others are really severe.
Did you know that thunderstorms can be classified into a category depending on storm type – namely, single cell storms, multicell storms and supercell storms? And that each of those categories has different storm characteristics, including severity?
In this article, we’ll take a look at these three storm types. We’ll first look into the differences between the three types, and subsequently cover why the difference occurs. Let’s go!
Wind shear and storm type
Before we can cover the storm types that a thunderstorm can be classified into, we must take a look at a meteorological concept first – the concept of wind shear.
It lies at the basis of the storm type, as you shall see when we discuss the individual types next.
What’s important to understand first is that wind speed is not equal with altitude. While near the ground, the wind may be blowing at e.g. 40 kilometers per hour, it may be 80 kilometers per hour at 2 kilometers altitude and 120 or 60 at 5 kilometers. The wind pattern with altitude is entirely dependent on the weather setting and the presence of macro-weather phenomena such as the position of the jet stream, and low and high pressure areas.
However, it’s important to discern two scenarios, which can occur together:
- The scenario where wind speed alters with height, often in an increasing way. This is called speed shear.
- The scenario where wind direction alters with height, called directional shear.
As we shall see now, with significant speed and directional shear present, storm can become very severe and hence dangerous.
Three storm types
We’ll now take a look at three storm types a storm can be classified into – single-cell storms, multicell storms and supercell storms. We also describe which one emerges given what type of wind shear, for both speed and directional shear.
If you’re a bit into the weather, you know that storm cells are composed of an updraft of hot, humid air and a downdraft of cold air. This is true for each storm cell, at the minimum.
And if there is just one such combination – that is, one updraft with one corresponding downdraft – we talk about single-cell storms. Single-cell storms form in highly unstable environments (visible as high CAPE values) where both speed shear and directional shear are low. As such:
- The updraft and downdraft are not separated well.
- Once rain falls and the downdraft is thus active, it will eventually cut off the updraft – often, within 30 to 45 minutes from the storm’s appearance.
- The storm then dies.
Low speed shear often occurs in an environment where wind speed is low in general. That’s why severe single-cell storms are often capable of producing flash floods, together with some lightning activity, while the risk of hail and more dynamic phenomena like tornadoes is relatively low.
In a high-speed-shear environment with lower directional shear, single cells tend to align themselves in lines (as can be seen on the radar image below). Sometimes, they grow into mesoscale convective complexes which are another way of describing a cluster of storm cells that move together.
When storms show these characteristics, we call them multicellular storms or multicells.
Higher speed shear ensures that storms can align themselves in lines because they tend to make updrafts more resistent against the downdraft cutoff described above. This occurs because with increased speed shear, the downdraft occurs further away from the downdraft – since the updraft is tilted horizontally when altitude increases.
As such, it takes longer for the storm’s outflow to lift the updraft and cut it off from the storm’s source of energy. What’s more, in systems like those, storms can often produce new storm cells in front of themselves because of the same outflow boundary, which pushes up new air and hence causes a new thunderstorm to form. In settings like those, storms often survive for hours in a row.
Due to the more dynamic setting, multicellular lines often tend to produce severe wind gusts, and are capable of producing severe hail. Mesoscale convective complexes do often tend to produce extremely high lightning activities and increased risk of flash flooding.
When besides speed shear directional shear is also high, thunderstorm initiation often precedes growth into the most severe storm cell that is known on Earth: the supercell.
Supercells are visible on radar by means of a so-called hook echo, which can be seen in the radar image below.
While supercells benefit from the same properties as multicells with respect to thunderstorm duration, they also form what is known as a mesocyclone because of the increased directional shear. In those cases, the storm’s updraft rotates around its own axis, indicating the possibility of tornadogenesis i.e. tornado formation, as well as significant hail and severe wind gusts.
Supercell. (2003, June 12). Wikipedia, the free encyclopedia. Retrieved August 24, 2020, from https://en.wikipedia.org/wiki/Supercell
Thunderstorm. (2002, August 12). Wikipedia, the free encyclopedia. Retrieved August 24, 2020, from https://en.wikipedia.org/wiki/Thunderstorm#Multicell_lines