Giant and Ultragiant Sea-Salt Aerosols and Caribbean Trade Wind Cumuli

Understanding the initiation of precipitation in trade wind cumuli was one of the major goals of the recent Rain in Cumulus over the Ocean (RICO) experiment. One hypothesis to explain spectral broadening in these clouds is that giant (1-10 ìm) and ultragiant (> 10 ìm) sea salt nuclei act as embryos for raindrops in the warm rain process. Although many studies of these aerosols have been conducted, no clear relationship has been established between their concentration and the onset of precipitation in marine cumulus. During RICO, sub-cloud aerosol and in-cloud particle spectra were measured under a range of conditions characteristic of the trade wind layer. Aerosol spectra below cloud base in RICO were measured using CCN counters, a PCASP, an FSSP-100 and a slide impactor/giant aerosol microscope system. Droplet spectra in cloud were measured with the FSSP-100 and optical array probes. In this paper, a comparison between wind speed and giant aerosol concentrations will be presented. A strong correlation was obtained between aircraft measurements of 100 m altitude wind speed and deliquesced giant aerosol concentrations from the FSSP-100 particle spectrometer. The concentration of dry particles of 1, 3, and 5 ìm radius were calculated assuming that aerosol particles measured by the FSSP-100 were deliquesced NaCl in equilibrium with ambient relative humidity. A near-linear correlation was observed for these concentrations as a function of wind speed over the range of 4-14 m/s. The FSSP spectra will be compared to spectra determined directly from the impact slides. Six research flights were selected to study near cloud base droplet spectra in conditions of low, medium and high wind speeds to determine possible relationships between giant aerosol concentrations and near-cloud base droplet spectra. Cloud spectral statistics are currently being compared for all clouds with similar depth and penetration altitudes, but different low level winds (and sub-cloud aerosol spectra). Initial results are just becoming available at the time this abstract was due. The latest findings will be presented at the conference.