Growth of cloud droplets by turbulent collision-coalescence

In summary, we studied the impact of air turbulence on the growth of cloud droplets using new collision kernel parametrization and an accurate bin integral (Table presented) method for KCE. Based on the recent studies by Wang et al. (1998), Zhou et al. (2001), Riemer and Wexler (2005), and Ayala (2005), we considered four different turbulent collision kernels and compare several time scales for warm rain initiation relative to the hydrodynamical-gravitational kernel of Hall (1980). We only consider the effects of air turbulence on the geometric collision kernel through local flow shear, local fluid acceleration, and preferential concentration. The general observation is that the time evolution of the growth process is quite similar for the Ayala kernel, the mZWWb kernel, and the Hall kernel, except that the three kernels result in different times for the switch from the autoconversion phase to the accretion phase to take place. If we take the Ayala kernel as the most appropriate kernel for the description of collision-coalescence rate in clouds, then the air turbulence can shorten the time for the formation of drizzle drops by 39 based on t ₁ or based on t₂, when compared with the base case (the Hall kernel). This does not include the effect of air turbulence on the collision efficiency. Wang et al. (2006b) speculated that the combined effect of air turbulence on the geometric collision rate and collision efficiency can lead to at least a factor of two speedup in the warm rain initiation as compared to the gravitational mechanism alone. In general, we expect the gravity is still the dominate mechanism for collision-coalescence for droplets large than 60 μm. Wthout gravity, air turbulence alone (as in mZWWa) is not capable of producing rain in a reasonable time interval. We also developed a novel method to unambiguously identify the time intervals for the three phases of collection growth as defined qualitatively by Berry and Reinhardt (1974). We used the maximum and minium of the net mass-density transfer rate to locate the time intervals of the three phases. We found that the air turbulence have the strongest impact on the autoconversion phase, which is typically the longest phase for warm rain initiation. The overall implication is that a moderate increase of collection kernel of small droplets by air turbulence can have a significant impact on the warm rain initiation. At this stage, much remains to be done to accurately quantify the effects of air turbulence on collision rate and collision efficiency.