By "deep", we mean a significant fraction of the depth of the cumulonimbus cloud in which the circulation is embedded (several kilometers). By "persistent", we mean in comparison to a convective time scale, defined by the time it takes for air parcels to rise from within the inflow layer of the updraft to the anvil outflow (a few tens of minutes).
 Note that because a radar (even at its lowest elevations) scans a storm above the surface, a region with little or no surface precipitation may still be within radar-detectable precipitation aloft.
 Recall the discussion in the Introduction, distinguishing between the limited number of abstract mechanisms for creating intense vortices and the processes operating at storm scales to allow the vortex dynamics to operate.
 See the discussion between Fankhauser et al. 1983a,b, Doswell 1983, and Moller 1983 for some sense of the terminology issues; although that debate concerns the names for cloud features, its flavor is characteristic of terminology debates in general.
 Of course, some waterspouts do arise from supercells. They have been called tornadic waterspouts by Golden (1971) and appear to be virtually identical to tornadoes associated with supercells over land. The distinction between a tornado and a waterspout is basically of little or no scientific value.
 At the risk of being repetitious, it is the presence of a deep, persistent mesocyclone which defines a supercell, not the depth of convection. When the mesocyclonic circulation exists through a substantial fraction of the depth of the storm, it doesn't matter if the storm is relatively shallow; it is a supercell. Storms poleward of, say, 45 deg latitude often have low tops because the environment is relatively cold, with a correspondingly low tropopause.
 Interestingly, some citizens observing the deadly Cheyenne, Wyoming tornado of 16 July 1979 thought they were seeing a dust devil; this confusion may have arisen because of the relative rarity of tornadoes in Wyoming, along with the absence of a visible condensation funnel for the early part of the tornado's life.