cells [@KlahrHenning1997] and between small-scale eddies [@Cuzzi+etal2008] occurring near the dissipative scale of the turbulence. Larger m-sized particles pile up on large scales (i.e. larger than the gas scale height) in long-lived geostrophic pressure bumps surrounded by axisymmetric zonal flows [@Johansen+etal2009a]. In the model presented in [@Johansen+etal2007] \[hereafter referred to as J07\], approximately meter-sized particles settle to form a thin mid-plane layer in balance between sedimentation and stirring by the gas which has developed turbulence through the magnetorotational instability [@BalbusHawley1991]. Particles then concentrate in nearly axisymmetric gas high-pressure regions which appear spontaneously in the turbulent flow [@FromangNelson2005; @Johansen+etal2006; @Lyra+etal2008a], reaching local column densities up to ten times the average. The passive concentration is augmented as particles locally accelerate the gas towards the Keplerian speed, which leads to accumulation of particles drifting rapidly in from exterior orbits [a manifestation of the streaming instability of @YoudinGoodman2005]. The gravitational attraction between the particles in the overdense regions becomes high enough to initiate first a slow radial contraction, and as the local mass density becomes comparable to the Roche density, a full non-axisymmetric collapse to form gravitationally bound clumps with masses comparable to the 950-km-diameter dwarf planet Ceres ($M_{\rm
Ceres}\approx9.4\times10^{20}\,{\rm kg}$). Such large planetesimal 