Rear flank downdraft
Encyclopedia
The rear flank downdraft or RFD is a region of dry air wrapping around the back of a mesocyclone
in a supercell
thunderstorm. These areas of descending air are thought to be essential in the production of many supercellular tornado
es. Large hail
within the rear flank downdraft often shows up brightly as a hook on weather radar
images, producing the characteristic hook echo
, which often indicates the presence of a tornado.
, which can be generated by cold anomalies produced at the rear of the supercell thunderstorm by evaporative
cooling of precipitation
or hail
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), respectively.
Vertical pressure
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 that originates in dry air and warms adiabatically can produce warmer observations out of the RFD at the surface.
(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.
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 circulation contributes to the circulation to form a hook echo. It was concluded that the presence of a hook echo can reflect downdraft intensification.
(tornadogenesis). In 1975, Ted Fujita
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.
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 that enter 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 that the air entering 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, 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 :
Mesocyclone
A mesocyclone is a vortex of air, approximately 2 to 10 miles in diameter , within a convective storm....
in a supercell
Supercell
A supercell is a thunderstorm that is characterized by the presence of a mesocyclone: a deep, continuously-rotating updraft. For this reason, these storms are sometimes referred to as rotating thunderstorms...
thunderstorm. These areas of descending air are thought to be essential in the production of many supercellular tornado
Tornado
A tornado is a violent, dangerous, 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. They are often referred to as a twister or a cyclone, although the word cyclone is used in meteorology in a wider...
es. Large hail
Hail
Hail is a form of solid precipitation. It consists of balls or irregular lumps of ice, each of which is referred to as a hail stone. Hail stones on Earth consist mostly of water ice and measure between and in diameter, with the larger stones coming from severe thunderstorms...
within the rear flank downdraft often shows up brightly as a hook on weather radar
Weather radar
Weather radar, also called weather surveillance radar and Doppler weather radar, is a type of radar used to locate precipitation, calculate its motion, estimate its type . Modern weather radars are mostly pulse-Doppler radars, capable of detecting the motion of rain droplets in addition to the...
images, producing the characteristic hook echo
Hook echo
The hook echo is one of the classical hallmarks of tornado-producing supercell thunderstorms as seen on a weather radar. The echo is produced by rain, hail, or even debris being wrapped around the supercell...
, which often indicates the presence of a tornado.
Formation
The rear flank downdraft can arise owing to negative buoyancyBuoyancy
In physics, buoyancy is a force exerted by a fluid that opposes an object's weight. In a column of fluid, pressure increases with depth as a result of the weight of the overlying fluid. Thus a column of fluid, or an object submerged in the fluid, experiences greater pressure at the bottom of the...
, which can be generated by cold anomalies produced at the rear of the supercell thunderstorm by evaporative
Evaporation
Evaporation is a type of vaporization of a liquid that occurs only on the surface of a liquid. The other type of vaporization is boiling, which, instead, occurs on the entire mass of the liquid....
cooling of precipitation
Precipitation (meteorology)
In meteorology, precipitation In meteorology, precipitation In meteorology, precipitation (also known as one of the classes of hydrometeors, which are atmospheric water phenomena is any product of the condensation of atmospheric water vapor that falls under gravity. The main forms of precipitation...
or hail
Hail
Hail is a form of solid precipitation. It consists of balls or irregular lumps of ice, each of which is referred to as a hail stone. Hail stones on Earth consist mostly of water ice and measure between and in diameter, with the larger stones coming from severe thunderstorms...
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), respectively.
Vertical pressure
Pressure
Pressure is the force per unit area applied in a direction perpendicular to the surface of an object. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.- Definition :...
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 that originates in dry air and warms 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, but the clear slot is not always evident in all 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 temperatureEquivalent potential temperature
Equivalent potential temperature, commonly referred to as theta-e \left, is a quantity related to the stability of a column of air in the atmosphere....
(θe) is cold with respect to the inflow. Moreover, the lowest wet-bulb potential temperature
Wet-bulb potential temperature
Wet-bulb potential temperature, sometimes referred to as pseudo wet-bulb potential temperature, is the temperature attained by mass of air brought adiabatically to saturation and then carried along moist-adiabat to 1000 mb ....
(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 (FFD)
Compared to the forward flank downdraft (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, but the FFD is forced down from upper levels, which start from a much cooler temperature.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 circulation contributes to the circulation to form a hook echo. It was concluded that 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 formationTornadogenesis
Tornadogenesis is the process by which a tornado forms. There are many types of tornadoes, and each type of tornado can have several different methods of formation. Scientific study is ongoing, as some aspects of tornado formation remain a mystery....
(tornadogenesis). In 1975, Ted Fujita
Ted Fujita
was a prominent severe storms researcher. His research at the University of Chicago on severe thunderstorms, tornadoes, hurricanes and typhoons revolutionized knowledge of each.- Biography :Fujita was born in Kitakyūshū, Fukuoka Prefecture, Japan...
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.
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 that enter 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 that the air entering 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, 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