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The response of precipitation to aerosol through riming and melting in deep convective clouds

by Z. Cui, S. Davies, K. S. Carslaw, A. M. Blyth
Atmospheric Chemistry and Physics ()

Abstract

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.

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