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We use 3D numerical MHD simulations to follow the evolution of cold,turbulent, gaseous systems with parameters representing GMC conditions. Westudy three cloud simulations with varying mean magnetic fields, but identicalinitial velocity fields. We show that turbulent energy is reduced by a factortwo after 0.4-0.8 flow crossing times (2-4 Myr), and that the magneticallysupercritical cloud models collapse after ~6 Myr, while the subcritical clouddoes not collapse. We compare density, velocity, and magnetic field structurein three sets of snapshots with matched Mach numbers. The volume and columndensities are both log-normally distributed, with mean volume density a factor3-6 times the unperturbed value, but mean column density only a factor 1.1-1.4times the unperturbed value. We use a binning algorithm to investigate thedependence of kinetic quantities on spatial scale for regions of column densitycontrast (ROCs). The average velocity dispersion for the ROCs is only weaklycorrelated with scale, similar to the mean size-linewidth relation for clumpswithin GMCs. ROCs are often superpositions of spatially unconnected regionsthat cannot easily be separated using velocity information; the same difficultymay affect observed GMC clumps. We analyze magnetic field structure, and showthat in the high density regime, total magnetic field strengths increase withdensity with logarithmic slope 1/3 -2/3. Mean line-of-sight magnetic fieldstrengths vary widely across a projected cloud, and do not correlate withcolumn density. We compute simulated interstellar polarization maps at varyingorientations, and determine that the Chandrasekhar-Fermi formula multiplied bya factor ~0.5 yields a good estimate of the plane-of sky magnetic fieldstrength provided the dispersion in polarization angles is < 25 degrees.Comment: 56 pages, 25 figures; Ap.J., accepte

Density, Velocity, and Magnetic Field Structure in Turbulent Molecular Cloud Models