Growth of Cloud Drops by Collision and Coalescence

Abstract

As we have seen in Chapter 1, it has long been established that the presence of ice is not necessary for precipitation formation in tropical cumuli. Also, radar observations of clouds outside the tropics have shown that the formation of echoes, indicating the presence of precipitation, can occur at temperatures warmer than 0°C. In such cases the Wegener-Bergeron-Findeisen mechanism of precipitation formation (Section 13.3.1) is absent, and the flow of water up the spectrum from small droplets to rain must occur by the process of collision and coalescence. This is also often referred to as the collection process, or as the ‘warm rain’ process. The latter designation is somewhat inappropriate, since collection growth occurs also in clouds colder than 0°C. For example. Braham (1964) found evidence of collection growth of supercooled drops in summer cumuli over the central U.S.