Influence of gravitational settling on turbulent droplet clustering and radar reflectivity factor

For radar cloud observations, the microwave refectivity of clouds is measured as the radar reflectivity factor Z, which is dependent on the cloud microphysical properties. Previous study has clarified that turbulent droplet clustering can be a cause of significant increase of the radar reflectivity factor. However, the influence of gravitational settling still remains to be clarified. This study, thus, investigates the influence of gravitational settling on the spatial distribution of turbulent clustering droplets and the radar reflectivity factor. A three-dimensional direct numerical simulation (DNS) of particle-laden isotropic turbulence is conducted for several values of the nondimensional terminal velocity S_v. The results show that the power spectrum of droplet number density fluctuation, which is calculated from the DNS data, depends on S_v as the droplet settling becomes stronger, the spectrum increases at small wavenumbers and decreases at large wavenumbers. The radar reflectivity factor Z is estimated from the power spectrum under ideal cloud conditions. The results indicate that the influence of turbulent clustering is suppressed under strong gravitational-settling conditions.