Effects of Turbulence-Induced Collision Enhancement on Mixed-Phase Deep Convective Clouds under Different Basic-State Winds and Aerosol Concentrations

Turbulence in clouds enhances collisions between cloud particles. This study investigates the effects of turbulence-induced collision enhancement (TICE) on mixed-phase deep convective clouds. For this purpose, a 2-D dynamic model with bin microphysics that considers TICE for the drop-drop collisions and the drop-ice collisions is used. Two basic-state wind profiles (uniform and shear flows) and two aerosol concentrations (low and high aerosol concentration) are considered to examine the sensitivity of the effects of TICE to these factors. In all cases, TICE reduces cloud water path at the early stage of cloud development. Also, it accelerates the onset of raindrop formation and the surface precipitation because of enhanced collision between small droplets. In spite of accelerated surface precipitation, TICE acts to reduce the amount of accumulated surface precipitation in all cases. These are in agreement with the results of previous studies. At first, changes in the size distribution of graupel particles can decrease surface precipitation. Graupel particles with moderate sizes occupy some portion of graupel mass in the cases with TICE, whereas graupel particles with large sizes occupy a significant portion of graupel mass in the cases without TICE. The change in the size distribution of graupel particles by TICE results in a decrease in the mass-averaged mean graupel terminal velocity. Therefore, the downward flux of graupel mass, and hence the melting of graupel particles, is reduced by TICE, leading to the decrease in the amount of accumulated surface precipitation. The decrease in the relative humidity can also decrease surface precipitation. TICE reduces the relative humidity in the lower atmosphere because TICE accelerates the coalescences between small droplets and makes them fall to the ground more quickly. The decrease in the relative humidity results in the decrease in surface precipitation by increasing the sublimation of ice particles. However, it is minor compared to the decrease in downward flux of graupel mass in reducing the surface precipitation. The decrease in the amount of accumulated surface precipitation is more pronounced under the high aerosol concentration than under the low aerosol concentration and is also larger in the uniform flow than in the shear flow.