Interaction between cloud droplets and turbulence*

We investigate the mechanism for the modification of turbulence spectra due to turbulence-cloud droplet interactions reported in the previous DNS study (Saito and Gotoh, 2018, New J. Phys, 20, 023001), where it was found that spectra of scalar variances (temperature and vapor mixing ratio) tend to be k^-1/3 unlike the classical Obukhov-Corrsin spectrum k^-5/3. In order to explore the physical mechanism, we explore the mechanism of the spectral modification by using the cloud microphysics simulator under the conditions simpler than for the previous study. It is found that the particle inertia plays an essential role in the spectra modification. The classical -5/3 spectrum for the scalar variance at low wavenumers changes to the shallower spectrum at moderate wavenumbers, which resembles the characteristics reported in previous observational and DNS studies. We examined the dependence of the transition in the spectra on various parameters, such as gravity, Stokes numbers, Reynolds numbers, and so on.