4:15 PM - 4:30 PM
[ACG47-16] Multi-step temporal changes of the intensity and structure of tornado-like vortices translating from smooth to rough surfaces
Keywords:Tornado, Surface roughness, Computational Fluid Dynamics, Vortices, Microscale meteorology, Wind disaster
Some studies suggest that weak waterspouts tend to dissipate within a short distance after making landfall (e.g., Bech et al. (2005), Miyagi and Sassa (2019)). One possible reason is that the vortex weakens over land due to increased surface roughness. To clarify the effect of surface roughness on tornado-like vortices translating from a smooth surface to various rough surfaces, idealised computational fluid simulations were performed.
The attached figure shows our computational domain, which was designed with inspiration from the computational domain in Wang (2022). The domain consisted of an upper cylinder and a lower slider. The upper cylinder was designed based on the Ward-type tornado chamber. The ground-relative motion of the tornado-like vortices was represented by the leftward movement of the lower slider. The left side of the slider had a flat bottom, while the right side had regularly-distributed hexahedral roughness obstacles on the bottom.
The results showed three characteristic changes during the vortex translation.
The first change was an increase in axial wind speed as the slider part started translation. This change could be seen in all cases where the translation speed was high enough, regardless of the existence of obstacles.
The second change occurred suddenly when the vortex reached the rough surface, regardless of the translation speed. Because the change in vortex intensity was accompanied by a change in the local corner flow swirl ratio, this change can be interpreted as a transition of the vortex flow scheme in response to the local changes in surface roughness.
When the low-swirl vortex translated at high speed onto rough surfaces, the third change appeared after translating over the rough surface for a while: the vortex axis bent and deformed, dramatically reducing its intensity.
Our results suggest a possibility that frictional effect on the land may filter out weak waterspouts after landfall.
The attached figure shows our computational domain, which was designed with inspiration from the computational domain in Wang (2022). The domain consisted of an upper cylinder and a lower slider. The upper cylinder was designed based on the Ward-type tornado chamber. The ground-relative motion of the tornado-like vortices was represented by the leftward movement of the lower slider. The left side of the slider had a flat bottom, while the right side had regularly-distributed hexahedral roughness obstacles on the bottom.
The results showed three characteristic changes during the vortex translation.
The first change was an increase in axial wind speed as the slider part started translation. This change could be seen in all cases where the translation speed was high enough, regardless of the existence of obstacles.
The second change occurred suddenly when the vortex reached the rough surface, regardless of the translation speed. Because the change in vortex intensity was accompanied by a change in the local corner flow swirl ratio, this change can be interpreted as a transition of the vortex flow scheme in response to the local changes in surface roughness.
When the low-swirl vortex translated at high speed onto rough surfaces, the third change appeared after translating over the rough surface for a while: the vortex axis bent and deformed, dramatically reducing its intensity.
Our results suggest a possibility that frictional effect on the land may filter out weak waterspouts after landfall.