Effects of Turbulence-Induced Collision Enhancement in Warm Clouds under Various Aerosol Concentrations

During the recent decades, it has been known that cloud turbulence has significant impacts on the development of clouds and precipitation, especially on the collision between cloud particles. This study investigates the effects of turbulence-induced collision enhancement (TICE) on clouds and precipitation in warm clouds. For this, a two-dimensional spectral (bin) microphysics cloud model is used with a grid size of 50 m and numerous idealized numerical experiments are performed under various number concentrations of cloud condensation nuclei (CCN).

The values of TICE are tabulated as pre-calculated lookup tables, which is a function of Taylor microscale Reynolds number and turbulent energy dissipation rate. The model computes turbulent kinetic energy prognostically. The Taylor microscale Reynolds number and turbulent energy dissipation rate are calculated by the computed turbulent kinetic energy using the 1.5th order turbulent closure. The thermodynamic sounding used in this study, which represents a typical sounding for trade-wind cumuli over the ocean, is characterized by a warm and humid environment near the surface and a capping inversion layer.

TICE reduces liquid water path slightly under all CCN concentrations because of the intensified surface precipitation and suppressed condensation. Also, TICE always accelerates the onset of surface precipitation irrespective of the CCN concentration. However, changes by TICE in rainwater path and surface precipitation are different with CCN concentrations. Under high CCN concentrations, because the coalescence between small droplets is inefficient and the effect of TICE that accelerates coalescence between droplets appears comparatively large, the mean cloud drop number concentrations (CDNC) decreases and the mean effective radius of drop increases as TICE is included, which makes the amount of surface precipitation increase. Under low CCN concentrations, however, the small droplets can grow by diffusion of water vapor to coalesce efficiently and the changes in the mean effective radius of drop and the mean CDNC are relatively small or even opposite to those under high CCN concentrations. A decrease in condensation due to the accelerated coalescence between droplets explains the small decrease in surface precipitation. A preliminary argument about the relationship between the aerosol concentration and precipitation in warm clouds is given.