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

Droplet spectral broadening and initiation of precipitation in trade wind cumuli are major unresolved scientific issues in cloud physics. One hypothesis to explain spectral broadening is that giant and ultragiant nuclei act as embryos for raindrops in the warm rain process. Although many studies of giant (1μm) and ultragiant (> 10μm) aerosols have been made, no clear relationship has been established between the concentration of these aerosols and the onset of precipitation in marine cumulus. To address this issue, a comparison between wind speed and giant aerosol concentration was made during the recent RICO (Rain in Cumulus over the Ocean) experiment. The measurements we used came from a unique combination of instrumnets on the NCAR C-130 aircraft, which provide new observations on the initiation of precipitation. 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. Assuming the aerosol particles measured by the FSSP-100 were deliquesced NaCl in equilibrium with the ambient relative humidity, the concentrations of dry particles with radius of 1, 3, and 5 μm were calculated. The concentrations showed a near-linear correlation as a function of wind speed over the range of 4-14 m/s. Three research flights were selected to study rain formation in conditions of low, medium and high wind speeds to determine possible relationships between giant aerosol concentrations and droplet spectral broadening. Data from the FSSP-100, 2DC, 2DP, and 260-X probes were evaluated to obtain cloud droplet spectra just above cloud base in clouds of similar maximum height, as measured from a forward looking video in combination with inertial navigational system information. Data from CN counters, PCASP, FSSP-100 and a giant aerosol microscope system were used to obtain complete size spectra of the aerosol particles below cloud base. The aerosol spectra from below cloud base will be compared with the drop spectra measured above cloud base.