(Abridged) We present local 2D and 3D hybrid numerical simulations of particles and gas in the midplane of protoplanetary disks (PPDs) using the Athena code. The particles are coupled to gas aerodynamically, with particle-to-gas feedback included. Magnetorotational turbulence is ignored as an approximation for the dead zone of PPDs, and we ignore particle self-gravity to study the precursor of planetesimal formation. Our simulations include a wide size distribution of particles, ranging from strongly coupled particles with dimensionless stopping time tau_s=Omega t_stop=1e-4 to marginally coupled ones with tau_s=1 (where Omega is the orbital frequency, t_stop is the particle friction time), and a wide range of solid abundances. Our main results are: 1. Particles with tau_s>=0.01 actively participate in the streaming instability, generate turbulence and maintain the height of the particle layer before Kelvin-Helmholtz instability is triggered. 2. Strong particle clumping as a consequence of the streaming instability occurs when a substantial fraction of the solids are large (tau_s>=0.01) and when height-integrated solid to gas mass ratio Z is super-solar. 3. The radial drift velocity is reduced relative to the conventional Nakagawa-Sekiya-Hayashi (NSH) model, especially at high Z. We derive a generalized NSH equilibrium solution for multiple particle species which fits our results very well. 4. Collision velocity between particles with tau_s>=0.01 is dominated by differential radial drift, and is strongly reduced at larger Z. 5. There exist two positive feedback loops with respect to the enrichment of local disk solid abundance and grain growth. All these effects promote planetesimal formation.
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