Collision rate enhancement in turbulent clouds of different types

Collision efficiencies and collision kernels between cloud droplets are calculated under turbulent conditions typical of actual clouds at 1000 mb and 500 mb pressure levels. These calculations are performed using a new method, according to which collisions are calculated within a small air volumes with linear scales of order of the Kolmogorov microscale. Scale analysis suggests that in such volumes turbulent shears and accelerations can be considered uniform in space and invariable with time during the process of hydrodynamic droplet interaction (HDI). A statistical model is used to generate long series of turbulent shears and accelerations reproducing probability distribution functions (PDF) at high Reynolds numbers, as they were obtained in recent laboratory and theoretical studies. The droplet fluxes of droplets of one size on droplets of another size are calculated for each sample of an acceleration-shear pair. The collision efficiency is calculated as a ratio of droplet fluxes in the presence of HDI and droplet flux in the absence of HDI. The modified superposition method is applied in calculations of collision kernel and collision efficiency. Collision efficiencies and kernels were calculated for each of many thousand turbulent flow samples (elementary volumes). PDF of collision efficiencies and collision kernels, as well as mean values of these quantities are calculated. As a result, detailed tables (with the resolution of 1 micron) of mean collision efficiencies and kernels between cloud droplets of radius ranged from 1 to 20 microns have been calculated for dissipation rates and Reynolds numbers typical of different clouds from stratiform clouds to Cb. The tables are available upon request. The comparison of the effects of different turbulent mechanisms on collision rate (increase in relative velocity between droplets, clustering effect and effects on the droplet hydrodynamic interaction) is presented. It is shown that the main turbulence effect is related to the hydrodynamic droplet interaction. The tables of collision kernels were implemented into cloud models with detailed microphysics. Effects of turbulence on droplet concentration and droplet collisions are parameterized using the results of statistical analysis of long series of drop arrival times measured in situ. Numerical results indicate significant effects of turbulence on raindrop formation.