Effects of Cloud Condensation Nuclei Due to Fires and Surface Sources During South Florida Droughts

Abstract

Cloud condensation nuclei (CCN) at cloud base strongly affect the droplet concentration at cloud base, which in turn influences the life history of a cloud. There are usually more nuclei over land than over water because of surface sources of nuclei, and vegetation fires produce large numbers of nuclei which may keep much of a cloud's water in small droplets that fail to reach raindrop size. Smoke particles from drought fires are thought to result from the burning of four major vegetation types in South Florida. Samples of these were burned in the laboratory and produced (at 0.75% supersaturation) between 109 and 1010 nuclei per gram burned. A simple calculation, assuming reasonable burn rates for these materials, resulted in 4600 CCN cm?2 when mixed uniformly to cloud base over a large area. The drought between 1 April and 15 May of 1967 over Florida was found to be related predominantly to synoptic-scale dryness and subsiding northerly winds aloft. There was no significant large-scale lag in rainfall caused by lingering CCN from fires and the dry surface, since dynamic changes explain the onset of normal rainfall. Individual cloud rainfall may have been affected by high CCN counts as indicated by cumulus model calculations. Liquid water fallout from small clouds is affected to a greater degree than from tall clouds.