Rear Flank Downdraft
Introduction
The Rear Flank Downdraft (RFD) is a meteorological phenomenon associated with severe thunderstorms, particularly those that produce tornadoes. It is a region of dry, cool air descending on the back side of a thunderstorm. The RFD plays a crucial role in the formation and maintenance of tornadoes, making it a key factor in severe weather forecasting.
Formation and Characteristics
The RFD forms due to the evaporation of precipitation and the downward transport of higher momentum air from aloft. This process is facilitated by the presence of a mesocyclone, a rotating updraft within a severe thunderstorm. The RFD begins as a downdraft on the rear side of the mesocyclone, which then wraps around the mesocyclone and descends toward the ground.
The RFD is characterized by a sharp drop in temperature, increase in wind speed, and a shift in wind direction. It is often accompanied by a "clear slot" or "slot" in the cloud cover, which is a region of clear air resulting from the descending dry air. The clear slot can often be seen visually on the back side of a thunderstorm.
Role in Tornado Formation
The RFD plays a critical role in the formation of tornadoes. As the RFD descends and wraps around the mesocyclone, it can help to tighten the rotation and increase the vorticity, or spin, of the air. This can lead to the formation of a tornado vortex signature, a radar-indicated rotation that often precedes the development of a tornado.
In addition to its role in tornado formation, the RFD can also influence the path and intensity of existing tornadoes. The interaction between the RFD and the tornado can lead to changes in the tornado's direction of movement, as well as fluctuations in its intensity.
Observation and Detection
The RFD can be observed visually as a clear slot or region of descending air on the back side of a thunderstorm. However, it can also be detected using weather radar. The presence of a hook echo, a radar signature associated with tornado-producing storms, can indicate the presence of an RFD.
In addition to radar, the RFD can also be detected using surface observations. A sudden shift in wind direction, increase in wind speed, and drop in temperature can all indicate the passage of an RFD.
Impact and Significance
The RFD can have significant impacts on the ground, including strong winds, rapid temperature changes, and severe weather conditions. These impacts can pose hazards to both people and property.
In the field of meteorology, the study of the RFD is crucial for understanding and predicting severe weather events. The characteristics and behavior of the RFD can provide valuable information about the potential for tornado formation and the likely path and intensity of tornadoes.