Effects of in-cloud nucleation and turbulence on droplet spectrum formation in cumulus clouds

Drop spectrum evolution is investigated using a moving mass grid microphysical cloud parcel model containing 2000 mass bins and allowing turbulent effects on droplet collisions. Utilization of precise methods of diffusion and collision drop growth eliminates any artificial droplet spectrum broadening. Simulation of continental, intermediate and maritime clouds is conducted using different concentrations of cloud condensation nuclei and different vertical velocities at the cloud base. An increase of the collision kernel in turbulent surroundings is found to be an important factor in the acceleration of large droplet and raindrop formation. Droplet spectrum formation was found to be affected by three stages of in-cloud droplets' nucleation: (a) nucleation near the cloud base, formaning the primary mode of the droplet spectrum; (b) nucleation within a parcel, where supersaturation exceeds its maximum at the cloud base, this type of nucleation forming the secondary spectral mode; and (c) nucleation within the zone of intensive collisions, when a rapid decrease in drop concentration leads to an increase in supersaturation. It is shown that the secondary mode in the droplet spectrum contributes significantly to raindrop formation, therefore the absence of the secondary mode (the single-mode spectrum) can reduce or even inhibit formation of raindrops. The contributions of diffusion and collision growth to drop spectrum formation are compared. Effective collisions are found to start when the effective radius attains about 15 μm. The level where the effective radius attains 15 μm can be considered as the level of the first radar echo. This height is shown to crucially depend on cloud dynamics (in particular, on the vertical velocity at the cloud base) and on the concentration of aerosol particles.