Thank you. let me back it up and say that in this picture, the part of the "shelf cloud" (arcus clouds for weather weenies) that extend to the surface look heavily photoshopped and blurred to make it look like a tornado touchdown. Nor is it the base of the "mesocyclone". That term gets thrown around sometimes, but it simply refers to the general circulation caused by the upward momentum of rising air within a severe thunderstorm being tilted by changing direction of winds aloft (wind shear), causing a somewhat vertical rotation in the storm (defining the storm as a supercell). The mesocyclone itself is hardly visible from the ground until it produces a funnel cloud or an on-the-ground tornado.

In this image, we see a low shelf cloud, which is a broad circular cloud base which extends down from the leading edge of a thunderstorm. The shelf cloud usually heralds the arrival of the "outflow", or the horizontal rain-cooled winds caused by a downdraft of air out from the thunderstorm's base which hits the ground and spreads out in all directions. The shelf cloud itself is a byproduct of this sinking cold air meeting the warm air out ahead of the thunderstorm. Here's a good zoomed out view of a shelf cloud and the turbulent clouds and rain column behind it

Hope I've explained this thoroughly enough.