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Tornadogenesis: How tornadoes form

A tornado can be a very fascinating weather phenomenon to observe if you are a weather enthusiast like myself and many others.  If you are not like me and have a fascination with severe weather, tornadoes can be one of the most terrifying weather disasters to experience in your life.  Spring is almost here so i figured I would write this article.  In it we will discuss how tornadoes form starting with the basic and general concepts and move into the more in depth look at current research surround the formation of tornadoes.  To start off, let’s accurately call tornado formation by its scientific name, TORNADOGENESIS, since you will read it quite frequently as we go along.

Cheyenne-Wells-Tornado-May-2015 How tornadoes form

Cheyenne Wells CO tornado – May 2015

So, how are tornadoes formed?
Let’s start with a general conception of how tornadoes form.  As your knowledge of tornadoes increases, be sure you click the links to tornado research papers written by scientists and learn more about how tornadoes form.

For the longest time, it has been taught that tornadoes form when warm, moist air from the gulf meets cool dry air from the west.  These two airmasses collide and BOOM….tornadoes.  As the scientific research around tornadoes has blossomed, it has become very apparent that this description of tornado formation is inaccurate.  While there is some truth to it, this truth does not pertain to tornado formation but rather to supercell thunderstorm environmental characteristics.  In other words, when warm, moist gulf air meets drier air from the west, we now call this a “dry-line” and it is a supercell thunderstorm hot spot when other conditions are in place.

First off, in its most basic definition, a tornado is a violent column of spinning air that extends from the surface to the base of a cumulonimbus (thunderstorm) cloud.  When tornadoes form, their funnel extends from the base of the convective cloud down to the surface where debris and dirt are lofted into the air.  This surface can be earthen ground or even water, such as the ocean.

The most damaging tornadoes, also the most studied tornadoes, typically form from supercell thunderstorms and follow an almost typical life cycle.

Tornadoes can also form in every part of the world and have been documented in places like Italy, Alaska, New Zealand, Australia, and Africa.  However, one of the most common places for tornado occurrences is in the United States in an area known as Tornado Alley.

Tornado_Alley How tornadoes form Tornado Activity in the United States[/caption]

Most, but not all, tornadoes occur with supercell thunderstorms.

What is a supercell thunderstorm then?
In short, a supercell is a rotating thunderstorm.  To expand a little, a supercell thunderstorm is a thunderstorm that has a persistent rotating updraft that are caused by the storm developing within areas of strong vertical windshear.  What is windshear?  The National Weather Service defines windshear as “a change in wind speed and/or direction with height”.  When I forecast my target area for potential tornadoes, I am looking for a type of vertical and horizontal wind shear that storm chasers and the National Weather Service refer to as veering winds.  That is, the winds at the surface are coming from one direction (most of the time out of the southeast) while winds aloft (or at altitude) are going another direction (typically out of the west or southwest).

directionalshear How tornadoes form

Directional Shear – a change in wind direction with height.

speedshear How tornadoes form

Speed Shear – A change in wind speed with height

Mesocyclone (Meso):

This rotating air, created by speed shear and directional shear, are lifted into the convective storm to develop a mesocyclone, which is a cyclonic flowing column of air within the thunderstorm.  That is, counter-clockwise flowing air (in the northern hemisphere) being lifted into the storm cause the the storm to take on supercell characteristics.  A thunderstorm with a persistent mesocyclone is considered a supercell thunderstorm.  Sometimes, a supercell thunderstorm takes on very amazing features, like this tornado warned supercell near West Point, Nebraska in 2013.

Once the supercell storm is matured, meaning that precipitation is reaching the ground, a few things start to take place.  First, , as the precipitation begins to fall to the surface, the momentum of the falling rain drops begin to cause the air from higher up in the storm updraft to accelerate downward along with the rain.  As this rain reaches the earths surface bringing down the air from aloft, a baroclonic boundary is formed, known by storm chasers and scientists as downdraft.  You may even start to see lightning in the storm, too.

Supercell thunderstorms have two types of downdrafts without any sort of gap in between them.  Rear-Flank Downdraft (RFD), located at the rear or western side of a supercell, and Forward-Flank Downdraft (FFD), usually north and north east of the mesocyclone updraft.  They are part of the same descending air but the names are derived from their location relative to the storm updraft.  This falling/sinking air is thought to have a significant role in tornadogenesis.  In most cases, the FFD now causes the rotating storm to ingest cooler and drier air.  In turn, as this inflow air is ingested into the base of the rotating updraft, the ambient temperature(s) are cooled to the dew point.  This is often when the “wall cloud” becomes visible.

How can you find a mesocyclone?  Typically, the best detection of mesocyclone formation is Doppler radar that shows wind velocities within a thunderstorm.

Radar-algorithme_eng How tornadoes form

Storm Relative Velocity with highlighted mesocyclones.

Now the stage is set for tornadogenesis.  Keep in mind that fewer than 20% of all supercells actually develop tornadoes so just because you can find a mesocyclone on radar or you see a wall cloud while you are out spotting or chasing, does not mean that you will actually see a tornado.

If all conditions are achieved and no outside disturbances occur, a tornado can likely develop from the above conditions.  The actual process of tornadogenesis is still, for the most part, unknown.  However, what we do know at this point in time is that a supercell feature known as rear-flank downdraft (RFD) has a lot to do with tornado formation than previously thought.

What does a tornado look like on radar?  It looks like this.

05june-rapiddow-wide-2 How tornadoes form

Tornado on Doppler radar

This was a radar loop of the June 5, 2009 Goshen County, WY tornado showing Base Reflectivity on the left and Relative Velocity on the right and was documented by a Doppler on Wheels (DOW)

Rear-Flank and Forward-Flank Downdrafts:

As this ingestion of air into the rotating updraft intensifies with the help of the RFD, a low pressure area is created at the surface.  This lower pressure allows the lowering of the of a fully condensed funnel.  As the funnel descends and reaches the ground, the RFD, which consists of mainly warmer dry air, also reaches the ground and creates a type of gust front (known in the storm chaser talk as “the ghost train” or “RFD inflow”).  This RFD inflow creates a surge  that helps to tilt the horizontal vorticity into vertical vorticity and enhance the vorticity of the air being ingested to aid in tornadogenesis.

Supercell thunderstorms have two types of downdrafts.  The Forward-Flank Downdraft (FFD), which forms first and has associated precipitation, is located ahead of the updrafts direction of travel. The Rear-Flank Downdraft, forms second at the rear of the storms updraft, hence the name.  They are part of the same descending air but the names are derived from their location relative to the storm updraft and both are thought to have significant impact on the development of tornadoes. As the supercell continues in its life-cycle, the RFD wraps itself around the mesocyclone. The outward moving air from the RFD creates an outflow boundary and is known as the RFD gust front.

The RFD can sometimes be noted as a clear slot that wraps around the area where the tornado develops. As the RFD advances and completely wraps around the mesocycone, an occlusion occurs and, typically, a tornado forms near this point of occlusion shortly after. This occlusion is also a good indicator that a new meso could be forming further east along the RFD bulge.

The VORTEX Program theorized that once a mesocyclone develops, the tornadogenesis is related to the temperature differences across the edge of the downdraft air wrapping around the mesocyclone. This process is now known as the Occlusion Downdraft. However, it has also been observed in computer model studies along with temperature observations near the destructive May 3rd 1999 tornadoes that tornadogenesis can occur without this temperature difference near the downdraft boundary.

In fact, Ted Fujita hypothesized in 1975 that downdraft air is recycled into a developing tornado.  This recycled downward moving air full of vorticity then adds to the intensification of the developing tornado and can be noted by the “clear slot” or “RFD cut” that happens just prior to tornado development.  It has been documented that within a few minutes after the RFD reaches the ground, a tornado usually touches down and starts creating damage.

supercell How tornadoes form

Supercell diagram – Lee Grenci

Leigh Orf (Central Michigan University) gave an exciting look at supercell and tornadogenesis computer simulations.  Check out the video below and make sure to visit the website of Leigh Orf and read/watch his latest research.  In his research videos, it is clear that there is some major tornadogenesis influence from both the FFD and the RFD surge as it begins its occlusion.  Visual simulations show how the inflow vorticity is stretched into vertical vorticity into the storms updraft.  It’s pretty amazing.

Conclusion: 
Pretty cool stuff, huh?  While we understand far more about how tornadoes form than we ever have before, there are still instances when tornadoes form and dissipate without any warnings being issued at all.  Furthermore, forecasts for tornado severity and strength along with duration are still problematic in that we just don’t know enough, yet.

Science is always trying to unravel the mysteries of tornadoes and how they form.  Currently, VORTEX-SE is undergoing their latest research to understand environmental effects of tornado development in the southeastern US.  There is current research being conducted by the National Severe Storms Laboratory (NSSL) and the National Center for Atmospheric Research (NCAR), too, so the information we obtain and learn about will inevitably help to inform and warn people better in the future.

Read through this infographic and learn a little more about how tornadoes form and what to do if you live in a tornado threat area.

sftornadoinfographicfinal How tornadoes form

Improve your tornado IQ

Infographic design byMegan Heck

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