The response of precipitation to aerosol through riming and melting in deep convective clouds
We have used a 2-D axisymmetric, non- hydrostatic, bin-resolved cloud model to examine the im- pact of aerosol changes on the development of mixed-phase convective clouds. We have simulated convective clouds from four different sites (three continental and one tropi- cal marine) with a wide range of realistic aerosol loadings and initial thermodynamic conditions (a total of 93 differ- ent clouds). It is found that the accumulated precipitation responds very differently to changing aerosol in the marine and continental environments. For the continental clouds, the scaled total precipitation reaches a maximum for aerosol that produce drop numbers at cloud base between 180–430cm−3 when other conditions are the same. In contrast, all the trop- ical marine clouds show an increase in accumulated precip- itation and deeper convection with increasing aerosol load- ing. For continental clouds, drops are rapidly depleted by ice particles shortly after the onset of precipitation. The pre- cipitation is dominantly produced by melting ice particles. The riming rate increases with aerosol when the loading is very low, and decreases when the loading is high. Peak pre- cipitation intensities tend to increase with aerosol up to drop concentrations (at cloud base) of ∼500cm−3 then decrease with further aerosol increases. This behaviour is caused by the initial transition fromwarm to mixed-phase rain followed by reduced efficiency of mixed-phase rain at very high drop concentrations. The response of tropical marine clouds to in- creasing aerosol is different to, and larger than, that of conti- nental clouds. In the more humid tropical marine environ- ment with low cloud bases we find that accumulated pre- cipitation increases with increasing aerosol. The increase is driven by the transition from warm to mixed-phase rain. Our study suggests that the response of deep convective clouds to aerosol will be an important contribution to the spatial and temporal variability in cloud microphysics and precipitation.