  We examine observational characteristics of multi-phase turbulent flows in
the diffuse interstellar medium (ISM) using a synthetic radiation field of
atomic and molecular lines. We consider the multi-phase ISM which is formed by
thermal instability under the irradiation of UV photons with moderate visual
extinction $A_V\sim 1$. Radiation field maps of C$^{+}$, C$^0$, and CO line
emissions were generated by calculating the non-local thermodynamic equilibrium
(nonLTE) level populations from the results of high resolution hydrodynamic
simulations of diffuse ISM models. By analyzing synthetic radiation field of
carbon lines of [\ion{C}{2}] 158 $\mu$m, [\ion{C}{1}] $^3P_2-^3P_1$ (809 GHz),
$^3P_1-^3P_0$ (492 GHz), and CO rotational transitions, we found a high ratio
between the lines of high- and low-excitation energies in the diffuse
multi-phase interstellar medium. This shows that simultaneous observations of
the lines of warm- and cold-gas tracers will be useful in examining the thermal
structure, and hence the origin of diffuse interstellar clouds.
