I have not seen a tornado (and don't think that I want to, although a waterspout, viewed from the coast, might be cool). The ones I've seen on, say, weather channel, start at the top.....the cloud starts rotating long before you get dust spinning on the ground. This is tornados....dust devils, even big ones, may be different....there is some logic that they start at the ground....
Hmm...as I write this...for tornadoes you need to start with an updraft...to get the thunderstorms also....so it all starts with heating of the ground...and yes, from the bottom - up.
Chris