The fraction of dx  ×  dy  ×  d(log τ) cells with supersonic flows versus optical depth for disk center viewing with μ = 1. Each curve represents one of the 20 analyzed snapshots. The supersonic flow may be in any direction. The thick solid curve corresponds to the snapshot with the largest abundance of supersonic flows (S1, lower row in Fig. 3), while the thick dashed curve corresponds to the initial snapshot (S0, upper row in Fig. 3).