But, it was a supercell that produced a tornado that did damage to the Georgia Dome in Atlanta, while an SEC Tournament basketball game was in progress. The formation of a wall cloud, however, does not guarantee that a tornado will form, and some tornadoes form without being preceded by a prominent wall cloud. The FFD is also where most of the heavy precipitation is located within a supercell. Occasionally, a "tail cloud" traces the rain-cooled air from the forward-flank downdraft region into the updraft. To help you visualize and locate the various parts of a supercell, including the updraft, mesocyclone, RFD, and FFD, check out the short video "tour" of a supercell below (2:47).

To start, we're going to build off the basic model of a supercell that you learned previously. Forecasters, fortunately, were able to detect rotation in this storm using Doppler velocities, and issued a timely tornado warning for downtown Atlanta.

Supercell thunderstorms are usually when you'll hear a tornado …

Increasing warning lead time and reducing the number of tornado warnings that are false alarms are certainly major goals in the meteorological community because of the incredible danger and potential for damage associated with tornadoes. Some reports suggest that the deluge on 26 July 2005 in Mumbai, India was caused by a supercell when there was a cloud formation 15 kilometres (9.3 mi) high over the city. As a result, precipitation particles fall earthward well northeast (or east-northeast) of the updraft typically (note that the highest reflectivity is displaced from the updraft and mesocyclone). Still, a wall cloud is visual clue that a supercell might spawn a tornado, and it gives a storm observer on the ground an idea of where a … The rainfall coincided with a high tide, which exacerbated conditions. The site editor may also be contacted with questions or comments about this Open Educational Resource. Well, some falling precipitation actually gets caught in the mesocyclone's circulation and wraps around to form the hook echo on radar. A classic supercell displays a hook echo on images of radar reflectivity, which occurs as precipitation wraps around the mesocyclone (the rotating updraft), and if a tornado forms, it does so within the hook echo. Read on.

But, many supercells actually have two distinct areas of precipitation, and therefore, two distinct downdrafts, as demonstrated by this radar cross-section of a supercell (from a supercell near Rapid City, South Dakota on July 13, 2009). On this day 944 mm (37.2 in) of rain fell over the city, of which 700 mm (28 in) fell in just four hours.

Still, a wall cloud is visual clue that a supercell might spawn a tornado, and it gives a storm observer on the ground an idea of where a tornado may form in a supercell. Up next, we'll turn our attention to the dangers of tornadoes and cover tips (and myths) about tornado safety. Specifically, you should be able to describe the forward-flank downdraft, rear-flank downdraft, updraft region, mesocyclone, and tornado (if applicable). Tornadoes certainly challenge weather forecasters because it's not always clear which supercells will produce a tornado and which supercells won't. After all, supercells produce most tornadoes (and nearly all strong tornadoes), are responsible for nearly all reports of hail at least two inches in diameter, and nearly all supercells produce damaging straight-line winds.

But, supercells have unique characteristics when it comes to their updrafts and downdrafts compared to other types of thunderstorms, thanks in large part to strong vertical wind shear.

Wall clouds form when air from the forward-flank downdraft region of the storm, which has been cooled via evaporation, gets drawn back into the updraft. The most destructive and deadly tornadoes occur from supercells --which are rotating thunderstorms with a well-defined radar circulation called a mesocyclone. How can supercells have two distinct areas of precipitation? The RFD develops when dry winds in the middle and upper troposphere (typically southwesterly or westerly) encounter the back side of the updraft, where that precipitation is wrapping around the mesocyclone. If only it were that simple!

They are most common during the spring across the central United States when moderate-to-strong atmospheric wind fields, vertical wind shear (change in wind direction and/or speed with height), and instability are present. High intensity tornadoes form from supercell thunderstorms, a storm that has a " deep rotating mesocyclone." On radar imagery, forecasters look for the classic hook echo to spot the area of a supercell where a tornado may form.

Credit: Used with permission, Gibson Ridge Software / National Weather Service, ‹ Putting a Spin on Supercells and Tornadoes, classic supercell displays a hook echo on images of radar reflectivity, highest reflectivity is displaced from the updraft and mesocyclone, tornado that did damage to the Georgia Dome in Atlanta, Lesson 2: The Global Ledger of Heat Energy, Lesson 3: Global and Local Controllers of Temperature, Lesson 5: Remote Sensing of the Atmosphere, Lesson 6: Surface Patterns of Pressure and Wind, Lesson 8: The Role of Stability in Thunderstorm Formation, Putting a Spin on Supercells and Tornadoes, Lesson 10: The Human Impact on Weather and Climate, Lesson 11: Patterns of Wind, Water, and Weather in the Tropics, Lesson 13: Becoming a Savvy Weather Consumer, Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License, Department of Energy and Mineral Engineering, Department of Materials Science and Engineering, Department of Meteorology and Atmospheric Science, Earth and Environmental Systems Institute, iMPS in Renewable Energy and Sustainability Policy Program Office, BA in Energy and Sustainability Policy Program Office, 2217 Earth and Engineering Sciences Building, University Park, Pennsylvania 16802. [Supercells can also produce damaging hail, severe non-tornadic winds, unusually frequent lightning, and flash floods.]

Please send comments or suggestions on accessibility to the site editor. The visual clue is called a wall cloud (credit: NOAA Photo Library), which is a local lowering of the cloud base in the mesocyclone. Because the wall cloud is part of a mesocyclone, it often visually displays rotation, and if enough vertical stretching occurs, a funnel cloud may start to descend from the wall cloud.

The John A. Dutton e-Education Institute is the learning design unit of the College of Earth and Mineral Sciences at The Pennsylvania State University. Some portions adapted from original course materials by David Babb and Lee M. Grenci. In addition to the FFD, there's another downdraft associated with precipitation called the rear-flank downdraft (RFD). This cascade of precipitation particles promotes a downdraft (falling precipitation particles drag air down with them) called the forward-flank downdraft (FFD). Some other complex factors (beyond the scope of the course) also contribute to the formation of the RFD, but the bottom line is that a separate rear-flank downdraft exists in supercells. The formation of a wall cloud, however, does not guarantee that a tornado will form, and some tornadoes form without being preceded by a prominent wall cloud. The College of Earth and Mineral Sciences is committed to making its websites accessible to all users, and welcomes comments or suggestions on access improvements.

Precipitation particles swept upward in the rotating updraft are rapidly carried downstream away from the updraft by strong upper-level winds (air movement within the storm is traced out by tan arrows on the cross-section of a supercell below). Although it lacks an obvious hook echo, the thunderstorm in this radar reflectivity image from 0136Z on March 15, 2008 is a tornadic supercell, which caused damage to the Georgia Dome in Atlanta, Georgia.
To further complicate matters, some tornadoes form in supercells that don't display a hook echo at all.

Wall clouds hang below the base of a supercell's cumulonimbus cloud, and form as rain-cooled air from the forward-flank downdraft region gets drawn into the updraft. Supercell thunderstorms are perhaps the most violent of all thunderstorm types, and are capable of producing damaging winds, large hail, and weak-to-violent tornadoes. So, supercells are a triple threat when it comes to severe weather.

Precipitation that gets swept up in the rotating updraft (the mesocyclone) gets rapidly carried downstream away from the updraft by strong upper-level winds. The Pennsylvania State University © 2020. But, the presence of a hook echo does not guarantee that a tornado will form.

The RFD even comes with its own gust front, as the leading edge of rain-cooled air spreads out laterally from the splashdown point of the RFD. We're going to continue with a closer look at supercells since they're such prolific severe weather producers.
Course Author: Steven Seman (Assistant Teaching Professor, Department of Meteorology and Atmospheric Science, College of Earth and Mineral Sciences, If and when a tornado is going to form, it forms out of the mesocyclone, and sometimes a supercell will give off a visual warning that a tornado may form, even before the appearance of a funnel cloud.

Formally, the FFD is the main region of downdraft in the forward (leading) part of a supercell. You should also be able to describe the cloud formation (a wall cloud) that's often a precursor to a funnel cloud or tornado formation, and identify the location on radar reflectivity where a tornado might form in a classic supercell. Because the precipitation particles have been swept away from the main updraft thanks to strong vertical wind shear, the FFD does not interfere with the updraft, and since updraft and the FFD are separated, the stage is set for supercells to be long-lived.

This storm doesn't display any obvious hook echo. For example, check out the radar reflectivity image from Warner Robbins, Georgia at 0136Z on March 15, 2008 (on the left).

As a supercell grows in size, the vortex in the middle will begin to tilt, siphoning warm air and moisture … Because of its high relative humidity, this air cools to the point of net condensation at a lower altitude than other environmental air being drawn into the updraft. In the video, I discuss where these various parts of a supercell appear on idealized radar reflectivity, but also show that not all supercells take on a "classic" look with a hook echo. This courseware module is part of Penn State's College of Earth and Mineral Sciences' OER Initiative. Because this air has been cooled by evaporation, its relative humidity is already fairly high, so when it rises into the updraft (and cools further), net condensation occurs more quickly than it does in surrounding air parcels, causing the cloud base to form at a lower altitude (as seen in the schematic below).

In fact, there's really nothing about its appearance that clearly indicates it was a supercell. Tornadoes also challenge public readiness because pinpointing the location where a tornado may form is rarely possible more than 20 or 30 minutes in advance, and sometimes its much less than that! A supercell, unlike a regular ole thunderstorm, contains the most-important ingredient in the making of a tornado: a spinning column of air at its center, called a vortex. Contact Us, Privacy & Legal Statements | Copyright Information

At the end of this section, you should be familiar with the anatomy of a supercell.

The interaction with this dry air promotes evaporation and associated cooling, which, in turn, promotes negative buoyancy and downward accelerations.

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