Effects of aerosols on trade wind cumuli over the Indian Ocean: Model simulations

An Eulerian non-hydrostatic anelastic fluid model using a warm cloud bulk parametrization is used to study responses of trade wind cumuli to increased cloud drop concentrations produced by enhanced aerosol numbers and to absorption from aerosols containing soot. Simulations are initialized using temperature and moisture profiles measured during the Indian Ocean Experiment (INDOEX) and sea surface temperatures retrieved from the Tropical Rainfall Measuring Mission microwave imager. Cloud covers between 4 and 10% and cloud-averaged liquid water contents of between 0.01 and 0.6 g m^-3 more closely match values retrieved and measured in situ during INDOEX than those modelled in past studies; previous studies found larger cloud coverage associated with cloud spreading below a strong inversion not present in the INDOEX soundings. As in previous studies, a diurnal cycle with reductions of cloud cover and liquid water path (LWP) after sunrise is seen, with absorbing aerosols within cloud augmenting the decrease in cloud cover. Variations in aerosol vertical profiles can change semi-direct radiative forcing at the surface (top of the atmosphere) from + 1.6 (+1.3) W m^-2 when aerosols are confined to cloud compared to -0.4 (-0.6) W m^-2 when aerosols occur below cloud. Low-level water-vapour mixing ratios decrease as vertical motions increase, the vertical motions being enhanced by below-cloud solar heating and cloud-top radiative cooling and suppressed by in-cloud heating. Cloud cover and LWP are larger for stronger vertical motions, explaining the dependence of radiative forcing on aerosol profile. In the late afternoon and evening, increases in radiative cooling and sensible and latent heat flux, occurring due to reductions in mixed-layer temperature and vapour, allow cloud cover and LWP to recover to values like those found in simulations without absorbing aerosols.