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The Mystery of the Super Tornado: How did RFD become the secret weapon of tornadoes?
In superstorms, the downward air flow from behind, also known as rearward precipitation cooling (RFD), plays a vital role. These areas of dry air surround the back of a mesocyclone, curling around like the eye of a storm in darkness. RFD is considered one of the important factors in the formation of many superstorm tornadoes. When weather radar captures large hail in an RFD, it often shows a characteristic hook echo, which often indicates the presence of a tornado.
Many studies have shown that rear-side precipitation and cooling are closely related to the formation of tornadoes.
Formation mechanism
The formation of precipitation and cooling on the rear side is mainly due to negative buoyancy. This phenomenon may be due to the cold anomaly generated behind the super thunderstorm. These cold air come from the evaporative cooling of precipitation or the melting of hail. At the same time, dry and cold air Injected into the cloud. Vertical disturbance pressure differences can also be caused by factors such as vertical gradients of vertical vorticity, stationary ambient flow in the updraft area, and pressure disturbances due to vertical buoyancy changes. As the air sinks, this dry air heats adiabatically, forming gaps in the cloud layer known as clear troughs. This clear trough may surround the tornado or appear in a horseshoe shape below or to the side of the tornado.
Thermal properties
Precipitation and cooling on the rear side may appear as a clear trough around the tornado, but this clear trough is not clearly visible in all cases. Some studies have shown that the surface pressure excess in RFD can reach several millibars. In addition, the equivalent potential temperature (θe) in the RFD is usually cooler relative to the airflow, and the lowest wet bulb potential temperature (θw) values observed at the surface are also usually within the RFD. Although warm and high θe air has also been observed inside the RFD.
Differences from front-side precipitation and cooling
Compared to the front-side precipitation cooling (FFD), the rear-side precipitation cooling (RFD) is mainly composed of dry and warm air. This is because the RFD is forced downward from the middle atmosphere, causing compression heating of the descending air parcel. The FFD is formed due to the precipitation load and evaporative cooling on the precipitation core of the super thunderstorm. Compared with the RFD, the FFD is cold and moist. Regardless, both are considered important in the formation of tornadoes.
Role in Tornado Generation
The association between cooling by rear-side precipitation and hook echoes is well established. Initial backside precipitation cooling occurs when air from aloft is collided and mixed to the ground. A hook echo is formed by the movement of precipitation along the back of the main echo. Therefore, the precipitation load and evaporative cooling induced by the hook echo can further enhance the intensity of precipitation. Observations show that enhanced precipitation near the strongest low-level vortex contributes to the formation of the hook echo, and dry ambient air is also introduced into the precipitation, further strengthening the negative buoyancy.
The presence of RFD may result in strong precipitation enhancement, thus promoting the formation of tornadoes.
Tornado Connection
Many researchers have recognized that cooling of trailing precipitation, especially associated with the hook echo, is critical to the formation of tornadoes. As early as 1975, Ted Fujita published the recycling hypothesis of tornado formation. First, the air introduced by precipitation is recycled into the developing tornado, and then the angular momentum brought by precipitation is transferred downward, eventually forming a strong circulation. It’s the positive feedback loop needed to intensify a tornado. Observations show low-level vorticity associations within the RFD, indicating that the RFD is critical for tornado formation. These observational data related to precipitation and cooling on the rear side provide support for Fujita's recycling hypothesis.
ConclusionAccording to current research, the cooling of precipitation on the rear side actually plays a guiding role in super thunderstorms, helping to promote the formation of tornadoes. As science continues to advance, the exploration of its precise mechanisms is still ongoing. However, what we know today is just the tip of the iceberg: What other unknown elements do you think may affect the formation of tornadoes?
