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Keywords:

  • aerosol indirect effects;
  • shallow cumulus;
  • large eddy simulation

[1] To determine conditions over the Indian Ocean, under which cloud fields are most susceptible to modification from aerosols, and to study how turbulent activities and shallow cumuli vary for different meteorological scenarios, a three-dimensional large-eddy simulation model was initialized using data collected during the Indian Ocean Experiment (INDOEX). Radiosonde data were used to construct six soundings encompassing the range of temperature and humidity observed. A total of 18 meteorological scenarios were then obtained by adding either an average transition layer (TL), a strong inversion layer (IL), or no stable layer to each sounding. Separate simulations were conducted for each scenario assuming pristine or polluted conditions as observed during INDOEX. For aerosol profiles measured during INDOEX, aerosol semidirect effects always dominated indirect effects, with the positive daytime net indirect forcing (semidirect plus indirect forcings) varying between 0.2 and 4.5 W m−2. Anthropogenic aerosols had a larger net indirect forcing when the environmental relative humidity (RH) was higher and in the absence of the IL and TL. Changes in meteorological factors had larger impacts on the cloud properties than did anthropogenic aerosols, indicating large uncertainties can be introduced when solely using observations to quantify aerosol effects without examining their meteorological context. Because mean lateral detrainment and entrainment rates depended on RH, aerosols, and the presence of stable layers, mass-flux parameterizations in climate models should not use single values for such rates that may not represent the range of conditions observed where trade cumuli form.