Kerry Emanuel's hypercane idea can be found online using a Google search, here's one from the hard drive:
Were Dinosaurs Blown Away?
Unlike an asteroid, a hypercane would send into the atmosphere not dust, but large quantities of water. The model shows that the water would create sheets of new clouds, which would block out the sun, disrupt radiation patterns, and perhaps trigger ozone depletion.
When large impacts happen on water, the blast wave will be worse than any kind of storm brewed up by it, it just won't last very long. And the impact itself WOULD put water into the atmosphere -- lots of it.