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Weixin Xu http://www.met.utah.edu/people/graduate

Graduate student, Department of Meteorology, University of Utah. ---

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Rear flank downdraft has been updated!! The existing article about Rear flank downdraft on Wikipedia is a very simple introduction. There is a need to add in-depth and detailed description of it, including its characteristics, formation and its role in tornadogenesis.
 * Extend and give scienfific depth to the article of Rear flank downdraft!


 * Extension content:

RFD may presents clear slot wrapping itself at least two-thirds of the way around the tornado, but clear slot is not necessary for RFD. Many documents indicated that surface pressure excesses up to a few millibars exist within RFDs. Some finding showed that within the RFDs and equivalent potential temperature (θe) are cold with respect to the inflow. Moreover, the lowest wet-bulb potential temperature (θw) values were observed at the surface were within the RFD. But there are also observations of warm, high-θe air within RFDs.
 * a. Thermodynamic Characteristics:

Rear-flank downdraft has 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 echo formed 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 enhanced downdraft in the vicinity of the strongest low-level rotation, behind the main storm updraft. Further, it was concluded that the presence of hook echo can reflect downdraft intensification.
 * b. Association with hook echo:

Compared to the forward flank downdraft (FFD) the rear flank downdraft (RFD) consists of warm and dry air. This is due to the fact that the RFD is forced down from the mid-levels of the atmosphere, but the FFD from upper levels which starting from a much cooler temperature.
 * c. Difference from Forward flank Downdraft (FFD):

The Rear Flank Downdraft can arise owing to negative buoyancy, which can be generated by cold anomalies produced by evaporative cooling or hail melting, or by precipitation loading, 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. (more details will be updated)
 * d. Formation:

It has been realized by many investigators that the Rear Flank Downdrafts, especialy those associated with hook echoes is fundamentally critical to tornado formation. Fujita (1975) initiated the Recycling Hypothesis: Firstly, downdraft air is recirculated into the (developing) tornado, which results in an appreciable convergence on the back side of the (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.
 * e. Role in tornadogenesis:

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. But tornadoes may arise 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: 1). tilts horizontal vorticity to produce vertical vorticity, 2) transports air containing vertical vorticity from mid-level to the surface, 3) enhances the near-ground vorticity convergence beneath the updraft tremendosly by entering the updraft and being stretched vertically.

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 * Related links:
 * References on Mesoscale Meteorology
 * Hook echos and Rear-Flank Downdrafts:A Review
 * Fujita T. T., 1975b: New evidence from the April 3–4, 1974 tornadoes.
 * Davies-Jones R. P., 1982a: A new look at the vorticity equation with application to tornadogenesis.
 * Lemon L. R., and C. A. Doswell, 1979: Severe thunderstorm evolution and mesocyclone structure as related to tornadogenesis.