Production of rain and drizzle by coalescence in stratiform clouds

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Abstract
Calculations have been made of the rate at which droplets originating on salt nuclei of given mass grow by both condensation and coalescence with smaller droplets in layer clouds of given uniform updraught, mean temperature, supersaturation and liquid‐water content; also of the rate at which the drops falling from the cloud will shrink by evaporation before reaching the ground. It is shown that clouds sustained by uniform updraughts of up to 10 cm sec−1 with no turbulent motion would be incapable of producing drizzle at the ground unless their liquid‐water contents were considerably higher than measured values indicate. It is suggested that turbulence plays an important part in releasing precipitation from non‐freezing layer clouds by increasing the effective fall path of some larger droplets among their small neighbours; this would lead to increased growth by coalescence. In order to start the coalescence process, a few droplets considerably larger than average must be present. It does not appear possible to account for this initial heterogeneity in the droplet‐size spectrum otherwise than by assuming that these larger droplets grow by condensation for a considerably longer period than the rest. It is suggested that while the bulk of the cloud droplets have their growth periods limited by the fact that they are continually being transferred by eddies between the interior and the boundaries of the cloud, a few remain in the interior for a long time, and it is these which initiate coalescence. These would have to exist in the cloud for 3 to 4 hr before falling out as drizzle drops.

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