High fidelity numerical simulations are performed to study the atomization process of impinging jets over a broad range of operating conditions. An improved volume-of-fluid (VOF) method augmented with adaptive mesh refinement (AMR) is used to simulate the formation and breakup of the liquid sheet formed by two impinging jets. In addition, a thickness-based refinement method is developed and implemented to automatically and efficiently resolve the drastic changing of requested grid resolution. The behaviors in various Reynolds and Weber number regimes are studied systemically. The predicted liquid sheet topology, atomization, and droplet size distribution agree well with experimental measurements. Several different patterns of sheet and rim configurations are obtained, including liquid chain, closed rim, fish-bone, disintegrating sheet, disintegrating rim and impact wave. The instability mechanisms of sheets and rims are studied based on the concepts of absolute and convective instabilities. New knowledge is acquired about the onset of sheet and rim instabilities. This locking-on feature of Strouhal number of impact wave is found based on the previous detailed study. Finally, schematic diagrams of all kinds of instabilities happens in impinging jet atomization are obtained.