The Fine-Scale Structure of the Trade Wind Cumuli over Barbados—The CARRIBA project (Invited Presentation)

Shallow cumulus convection is one of the prevalent cloud forming mechanism in the tropics. Since shallow cumulus clouds are very widespread in the trade wind regions, they play an important role in the moisture transport to the free atmosphere and also for the Earth's radiation budget. Furthermore, shallow cumulus convection significantly influences the dynamics of the entire planetary boundary layer (PBL) by intensifying the vertical transport of moisture, momentum, and heat from the surface to higher levels. Although the trades were in the focus of several extensive field experiments several aspects are still not completely understood and it has to be stated that despite significant research efforts in the past, we are still lacking means to predict the behavior of shallow cumulus clouds in terms of their microphysical and radiative properties, the initiation of precipitation, and their influences on large-scale boundary layer dynamics.

Therefore, the CARRIBA-project (Cloud, Aerosol, Radiation and tuRbulence in the trade wInd regime over BArbados) was initiated. The project is embedded in a longterm initiative of the Max-Planck-Institute for Meteorology in Hamburg, which has operated an observatory at the East coast of Barbados for more than three years. This presentation will provide a general introduction of the CARRIBA-project including specific goals, the measurement strategy, and several research highlights from two measurement campaigns in November 2010 and April 2011.

During the two campaigns, Barbados was influenced by different air masses advected from the Caribbean area, the Atlantic Ocean, and the African continent which led to distinct aerosol conditions. Pristine conditions with low aerosol particle number concentrations of 100 cm^-3 were alternating with periods influenced by Saharan dust or aerosol from biomass burning resulting in comparably high number concentrations of 500 cm^-3. The biomass burning aerosol was originating from both, the Caribbean area and Africa. The shallow cumulus clouds responded to the different aerosol conditions with a wide range of mean droplet sizes and number concentrations. Here, we will present two days with different aerosol and cloud microphysical properties but almost identical meteorological conditions. The differences in the droplet number concentration and droplet sizes appear not to show any significant change for turbulent cloud mixing, but the relative roles of droplet inertia and sedimentation in initiating coalescence, as well as the cloud reflectivity, do change substantially.