Radial distributions of the gas surface density (gray dashed curves, left y-axis) and the continuum opacity at 230 GHz (black solid curves, right y-axis) for (a) Model 1, (b) Model 2, and (c) Model 3. The vertical dotted lines indicate the radial locations where the total grain surface density has a local maximum. Note the radial variations of the continuum opacity across the disks. We adopt the optical constants from Ricci et al. (2010) to calculate the grain opacity.