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We present synthetic Hi and CO observations of a simulation of decayingturbulence in the thermally bistable neutral medium. We first present thesimulation, with clouds initially consisting of clustered clumps. Self-gravitycauses these clump clusters to form more homogeneous dense clouds. We apply asimple radiative transfer algorithm, and defining every cell with <Av> > 1 asmolecular. We then produce maps of Hi, CO-free molecular gas, and CO, andinvestigate the following aspects: i) The spatial distribution of the warm,cold, and molecular gas, finding the well-known layered structure, withmolecular gas surrounded by cold Hi, surrounded by warm Hi. ii) The velocity ofthe various components, with atomic gas generally flowing towards the moleculargas, and that this motion is reflected in the frequently observed bimodal shapeof the Hi profiles. This conclusion is tentative, because we do not includefeedback. iii) The production of Hi self-absorption (HISA) profiles, and thecorrelation of HISA with molecular gas. We test the suggestion of using thesecond derivative of the brightness temperature Hi profile to trace HISA andmolecular gas, finding limitations. On a scale of ~parsecs, some agreement isobtained between this technique and actual HISA, as well as a correlationbetween HISA and N(mol). It quickly deteriorates towards sub-parsec scales. iv)The N-PDFs of the actual Hi gas and those recovered from the Hi line profiles,with the latter having a cutoff at column densities where the gas becomesoptically thick, thus missing the contribution from the HISA-producing gas. Wefind that the power-law tail typical of gravitational contraction is onlyobserved in the molecular gas, and that, before the power-law tail develops inthe total gas density PDF, no CO is yet present, reinforcing the notion thatgravitational contraction is needed to produce this component. (abridged)

Molecular cloud formation as seen in synthetic Hi and molecular gas observations