![Tornadic supercell](https://upload.wikimedia.org/wikipedia/commons/6/67/Tornadic_supercell.jpg)
The rear flank downdraft (RFD) is a region of dry air wrapping around the back of a
mesocyclone
A mesocyclone is a meso-gamma mesoscale (or storm scale) region of rotation (vortex), typically around in diameter, most often noticed on radar within thunderstorms. In the northern hemisphere it is usually located in the right rear flank (back ...
in a
supercell
A supercell is a thunderstorm characterized by the presence of a mesocyclone: a deep, persistently rotating updraft. Due to this, these storms are sometimes referred to as rotating thunderstorms. Of the four classifications of thunderstorms (s ...
thunderstorm.
These areas of descending air are thought to be essential in the production of many supercellular
tornado
A tornado is a violently rotating column of air that is in contact with both the surface of the Earth and a cumulonimbus cloud or, in rare cases, the base of a cumulus cloud. It is often referred to as a twister, whirlwind or cyclone, altho ...
es. Large
hail
Hail is a form of solid precipitation. It is distinct from ice pellets (American English "sleet"), though the two are often confused. It consists of balls or irregular lumps of ice, each of which is called a hailstone. Ice pellets generally fal ...
within the rear flank downdraft often shows up brightly as a hook on
weather radar
Weather radar, also called weather surveillance radar (WSR) and Doppler weather radar, is a type of radar used to locate precipitation, calculate its motion, and estimate its type (rain, snow, hail etc.). Modern weather radars are mostly puls ...
images, producing the characteristic ''
hook echo
A hook echo is a pendant or hook-shaped weather radar signature as part of some supercell thunderstorms. It is found in the lower portions of a storm as air and precipitation flow into a mesocyclone, resulting in a curved feature of reflectivity. ...
'', which often indicates the presence of a tornado.
[
]
Formation
The rear flank downdraft can arise owing to negative buoyancy
Buoyancy (), or upthrust, is an upward force exerted by a fluid that opposes the weight of a partially or fully immersed object. In a column of fluid, pressure increases with depth as a result of the weight of the overlying fluid. Thus the p ...
, which can be generated by cold anomalies produced at the rear of the supercell thunderstorm by evaporative cooling of precipitation
In meteorology, precipitation is any product of the condensation of atmospheric water vapor that falls under gravitational pull from clouds. The main forms of precipitation include drizzle, rain, sleet, snow, ice pellets, graupel and hail. ...
or hail
Hail is a form of solid precipitation. It is distinct from ice pellets (American English "sleet"), though the two are often confused. It consists of balls or irregular lumps of ice, each of which is called a hailstone. Ice pellets generally fal ...
melting, or injection of dry and cooler air in the cloud, and by vertical perturbation pressure gradients that can arise from vertical gradients of vertical vorticity, ''stagnation'' of environmental flow at an updraft, and pressure perturbations due to vertical buoyancy variations (which are partially due to hydrostatic effects).[
Vertical ]pressure
Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country and e ...
perturbations are generated by the buildup of pressure due to the vertical buoyancy, creating a pressure perturbation gradient. The subsiding air is generally dry and as it subsides the air warms adiabatically and can form a clearing in the cloud cover called a clear slot.[ A clear slot can be observed to wrap around a tornado or form away from a tornado in the shape of a horseshoe. This clearing is most likely the formation of the hook echo region associated with tornado formation.][ An RFD originating in dry air warming adiabatically can produce warmer observations out of the RFD at the surface.
]
Thermodynamic characteristics
RFDs may present themselves as a clear slot wrapping itself at least two-thirds of the way around the tornado
A tornado is a violently rotating column of air that is in contact with both the surface of the Earth and a cumulonimbus cloud or, in rare cases, the base of a cumulus cloud. It is often referred to as a twister, whirlwind or cyclone, altho ...
, but the clear slot is not always evident in cases where an RFD is present. Many documents indicate that surface pressure excesses up to a few millibars exist within RFDs. Some findings showed that within the RFDs equivalent potential temperature Equivalent potential temperature, commonly referred to as theta-e \left( \theta_e \right), is a quantity that is conserved during changes to an air parcel's pressure (that is, during vertical motions in the Earth's atmosphere, atmosphere), even if w ...
(θe) is cold with respect to the inflow. Moreover, the lowest wet-bulb potential temperature (θw) values observed at the surface were within the RFD. There are, however, also observations of warm, high-θe air within RFDs.[
]
Difference from forward flank downdraft
Compared to the forward flank downdraft
Forward is a relative direction, the opposite of backward.
Forward may also refer to:
People
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** Sm ...
(FFD) the rear flank downdraft (RFD) consists of warm and dry air. This is because the RFD is forced down from the mid-levels of the atmosphere, resulting in compressional heating of downward moving parcels. The FFD, in contrast, is driven by precipitation loading and evaporative cooling in the precipitation core of a supercell thunderstorm, making the FFD relatively cold and wet. Both are thought to be significant in tornado formation.
Role in tornadogenesis
Association with hook echo
Rear-flank downdrafts have a well-established association with hook echoes. Firstly, the initial rear flank downdraft is air from aloft transported down to the surface by colliding and mixing with the storm.[ Secondly, hook echoes form through advection of precipitation from the rear of the main echo around the region of strong updraft.][ Thus, precipitation loading and evaporation cooling induced by the hook echo can enhance the downdraft. Some observations showed the presence of an enhanced downdraft in the vicinity of the strongest low-level rotation, behind the main storm updraft.
Dry environmental air is also entrained into the downdraft and evaporative cooling helps create more negatively buoyant air. As precipitation falls and cool entrained air circulated downward and eventually reaching the surface. This contributes to the circulation to form a hook echo. It was concluded the presence of a hook echo can reflect downdraft intensification.
]
Association with tornadoes
It has been realized by many researchers that rear flank downdrafts, especially those associated with hook echoes, are fundamentally critical to tornado formation
Tornadogenesis is the process by which a tornado forms. There are many types of tornadoes and these vary in methods of formation. Despite ongoing scientific study and high-profile research projects such as VORTEX projects, VORTEX, tornadogenesis ...
(tornadogenesis). In 1975, Ted Fujita
was a Japanese-American meteorologist whose research primarily focused on severe weather. His research at the University of Chicago on severe thunderstorms, tornadoes, hurricanes, and typhoons revolutionized the knowledge of each. Although he is ...
originated the ''recycling hypothesis'' of tornadogenesis:[ First, downdraft air is recirculated into the (developing) tornado, which results in an appreciable convergence on the back side of the (still developing) tornado. Then the downward transport of the angular momentum by precipitation, and the recycling of air into the tornado, will create a tangential acceleration required for the intensification of the tornado as a ]positive feedback loop
Positive feedback (exacerbating feedback, self-reinforcing feedback) is a process that occurs in a feedback loop which exacerbates the effects of a small disturbance. That is, the effects of a perturbation on a system include an increase in the ...
.
Observations of low-level vorticity couplets within RFDs indicate that tilting of vorticity by the RFD is important in the formation of tornadoes within supercell storms. During the tornadogenesis phase in supercells, the parcels of air infiltrating the tornado or incipient tornado regularly seem to pass through the hook echo and RFD, which can serve as the basis for Fujita's ''recycling hypothesis''. Furthermore, observations of the clear slot during and just prior to the tornadic stage, imply the air infiltrating the tornado may come from the RFD.
Regularly, generation of large vertical vorticity close to the surface in an environment which is required for tornadogenesis
Tornadogenesis is the process by which a tornado forms. There are many types of tornadoes and these vary in methods of formation. Despite ongoing scientific study and high-profile research projects such as VORTEX, tornadogenesis is a volatile pro ...
, is attributed to downdraft. Tornadoes may arise, however, in the absence of a downdraft in environments containing preexisting vertical vorticity at the surface, such as in some cases of nonsupercell tornadogenesis.
Downdraft may have the following roles in near-ground mesocyclogenesis:
# tilts horizontal vorticity to produce vertical vorticity
# transports air containing vertical vorticity from mid-level to the surface
# enhances the near-ground vorticity convergence beneath the updraft tremendously by entering the updraft and stretching vertically
See also
*Hook echo
A hook echo is a pendant or hook-shaped weather radar signature as part of some supercell thunderstorms. It is found in the lower portions of a storm as air and precipitation flow into a mesocyclone, resulting in a curved feature of reflectivity. ...
*Supercell
A supercell is a thunderstorm characterized by the presence of a mesocyclone: a deep, persistently rotating updraft. Due to this, these storms are sometimes referred to as rotating thunderstorms. Of the four classifications of thunderstorms (s ...
*Thunderstorm
A thunderstorm, also known as an electrical storm or a lightning storm, is a storm characterized by the presence of lightning and its acoustic effect on the Earth's atmosphere, known as thunder. Relatively weak thunderstorms are someti ...
*Tornado
A tornado is a violently rotating column of air that is in contact with both the surface of the Earth and a cumulonimbus cloud or, in rare cases, the base of a cumulus cloud. It is often referred to as a twister, whirlwind or cyclone, altho ...
*Vertical draft
In meteorology, an updraft is a small-scale current of rising air, often within a cloud.
Overview
Localized regions of warm or cool air will exhibit vertical movement. A mass of warm air will typically be less dense than the surrounding region, ...
References
Bibliography
*
*{{cite book
, last = Bluestein
, year = 1993
, title = Synoptic-Dynamic Meteorology in Midlatitudes II
, pages = 491, 493–495, 501
Wind
Tornadogenesis