Influence of Gravitational Settling on Turbulent Droplet Clustering and Radar Reflectivity Factor

This study investigates the influence of gravitational settling of droplets on turbulent clustering and the radar reflectivity factor. A three-dimensional direct numerical simulation (DNS) of particle-laden isotropic turbulence is performed to obtain turbulent droplet clustering data. The turbulent clustering data are then used to calculate the power spectrum of droplet number density fluctuations. The results show that the gravitational settling modulates the power spectrum more significantly as the settling becomes larger. The gravitational settling weakens the intensity of clustering at large wavenumbers for St≤1, whereas it significantly enlarges the intensity for St>1. The dependence on the Taylor-microscale-based Reynolds number is also investigated to discuss the contribution of large-scale eddies to the settling influence. The results show that large-scale eddies modulate the small scale clustering structure of large St droplets. The increment of radar reflectivity factor due to turbulent clustering is estimated from the power spectrum for the case of St=1.0. The result shows that the influence of gravitational settling on the radar reflectivity factor can be significant for the case of large settling velocity droplets.