I do like me my silly megaprojects.
n the US, most devastating tornadoes occur in Tornado Alley, which is a strip of land between the Appalachian Mountains and the Rocky Mountains, including most American Midwest states. In 2013, there were 811 confirmed tornadoes in USA, 57 in Europe and 3 in China. Among 811 tornadoes in USA, most of them, especially the most devastating ones, occurred in Tornado Alley. What causes such huge differences?
From the atmospheric circulation point of view, Tornado Alley is inside the "zone of mixing", where the warm and moist air flows northbound and cold air flows southbound. At a certain season, the warm air flow front clashes with the cold air flow front at some place in Tornado Alley. Major tornadoes in Tornado Alley all start with such clashes (Fig.1). Especially as there is no east-west mountain in Tornado Alley to weaken or block the air flows, some clashes are violent, creating vortex turbulence. Such violent vortexes, supercells, are initially in horizontal spinning motion at the lower atmosphere, then tilt as the air turns to rise in the storm's updraft, creating a component of spin around a vertical axis. If the vortex stretching during the vortex tilting intensifies the vertical vorticity enough to create a tornado, the vortex size is getting much smaller as the rotation speed gets much faster. About 30% of supercells lead to tornadoes (Fig.2).
Calculations show that the chance to produce tornadoes depends on the wind speeds during the clashes. For example, if both cold wind and warm wind have speed 30 miles/h (13.3m/s), the chance to develop tornados from the clash is very high. On the other hand, if the both winds have speed below 15 miles/h, there is almost no chance for the clash to develop into tornadoes. Hence reducing the wind speed and eliminating the violent air mass clashes are the key to prevent tornado formation in Tornado Alley. We can learn from the Nature how to do so.
United States and China have similar geographic locations. In particular, the Northern China Plain and the Eastern China Plain is also in the zone of mixing, similar to Tornado Alley. However, very few violent tornadoes occur in this region of China because there are three east-west mountain ranges to protect these plains from tornado threat. The first one is 300km long Yan Mountain which lies at the northern boundary of these plains. The second one is 600km long Nanling (Nan Mountains) at the south boundary of these plains. The third one is 800kom long Jiang-Huai Hills through the middle of the plains. Especially, Jiang-Huai Hills are only about 300 meters above sea level, but effectively eliminate the major tornado threat for the areas. This is evidenced by the following fact.
Jiang-Huai Hills do not extend to Pacific ocean, leaving a small plain area, north part of Jiangsu province, unprotected. This small area, similar to US Tornado Alley, has annually recurring tornado outbreaks. For example, the city Gaoyou in this area has a nickname "Tornado hometown", which has tornado outbreaks once in two years on average. It is thus clear that Jiang-Huai Hills are extremely effectively in eliminating tornadoes formation. Without Jiang-Huai Hills, a quite big area in China would become "Tornado Hometown"
While there are no mountains in Tornado Alley to play the same role as Jiang-Huai Hills etc in China, there are two small mountains, Ozarks Mountains and Shawnee Hills, which significantly reduce tornado risk for some local areas.
The solution: 3 huge walls to act as artificial mountain ranges, slow the winds and stop tornados ever forming in the USA.
How big?
300 meter high and 50 meter wide.
To eliminate the tornado threat for the entire Tornado Alley, we may need to build three great walls. The first one should be close to the northern boundary of the Tornado Alley, maybe in North Dakota. The second one should be in the middle, maybe in the middle of Oklahoma and going to east. The third one can be in the south of Texas and Louisiana.
