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GEOPHYSICAL RESEARCH LETTERS, VOL. 33, L20814, doi:10.1029/2006GL027487, 2006

Influence of low-level wind speed on droplet spectra near cloud base in trade wind cumulus

Marilé Colón-Robles

Department of Atmospheric Sciences, University of Illinois at Urbana-Champaign, Urbana, Illinois, USA


Robert M. Rauber

Department of Atmospheric Sciences, University of Illinois at Urbana-Champaign, Urbana, Illinois, USA


Jørgen B. Jensen

Earth Observation Laboratory, National Center for Atmospheric Research, Boulder, Colorado, USA


Abstract

Data collected during the Rain In Cumulus over the Ocean (RICO) campaign demonstrate a relationship between the low-level wind speed, droplet concentrations and the presence of large cloud droplets near cloud base in trade wind clouds in a clean marine background atmosphere. Weak winds were associated with fewer activated cloud droplets, larger mean droplet sizes, and more large cloud droplets, even though the concentration of giant/ultragiant sea-salt aerosol increased with increasing near-surface wind speed. The data suggest that in unpolluted trade wind cumuli: (1) the production of large cloud droplets near cloud base is controlled primarily by the intensity of the cloud base updraft rather than the concentration of giant/ultragiant sea-salt particles; (2) higher droplet concentrations are more likely under conditions of stronger low-level wind speeds, primarily because stronger low-level wind speeds are associated with more intense cloud base updrafts.

Received 6 July 2006; accepted 20 September 2006; published 26 October 2006.

Index Terms: 0305 Atmospheric Composition and Structure: Aerosols and particles (0345, 4801, 4906); 0320 Atmospheric Composition and Structure: Cloud physics and chemistry; 3311 Atmospheric Processes: Clouds and aerosols.

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Citation: Colón-Robles, M., R. M. Rauber, and J. B. Jensen (2006), Influence of low-level wind speed on droplet spectra near cloud base in trade wind cumulus, Geophys. Res. Lett., 33, L20814, doi:10.1029/2006GL027487.