Numerical study on influence of turbulent droplet clustering on radar reflectivity factor under cumulus cloud conditions

Abstract:

Spatial correlations of cloud droplets cause particulate Bragg scattering, which increases the reflected microwave intensity in radar observations. Most studies assume that particulate Bragg scattering is insignificant in clouds. However, cloud turbulence generates microscale clusters of cloud droplets due to centrifugal effects. This indicates that the influence of turbulent clustering can be a cause of observational errors. Thus, this study aims to investigate the influence of turbulent clustering of cloud droplets on the radar reflectivity factor. Droplet clustering data are obtained by performing a three-dimensional direct numerical simulation (DNS), in which an isotropic turbulence is generated by solving the Navier-Stokes equation without any turbulence model and a large number of droplet motions are tracked by the Lagrangian method. The clustering data are used to calculate the power spectrum of number density fluctuation. The results show that the turbulent Reynolds number dependency of the power spectrum is sufficiently small for enough high turbulent Reynolds number. On the other hand, the spectrum is strongly dependent on the Stokes number, which is defined as the ratio of droplet relaxation time to the Kolmogorov time. Thus, the influence of turbulent clustering on the radar reflectivity factor is estimated by using the power spectrum considering the Stokes number dependency. We will show the estimate results under ideal cumulus cloud conditions, where the droplet size distributions and the number densities are set based on the dataset of Hess et al. (1998), and discuss the influence on radar cloud observations.