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The effects of an initially overlying layer of solar-absorbing aerosol on thetransition of stratocumulus to trade cumulus clouds are examined usinglarge-eddy simulations. For lightly drizzling cloud the transition isgenerally hastened, resulting mainly from increased cloud droplet numberconcentration (Nc) induced by entrained aerosol. The increasedNc slows sedimentation of cloud droplets and shortens theirrelaxation time for diffusional growth, both of which accelerate entrainmentof overlying air and thereby stratocumulus breakup. However, the decrease inalbedo from cloud breakup is more than offset by redistributing cloud waterover a greater number of droplets, such that the diurnal-average shortwaveforcing at the top of theatmosphere is negative. The negative radiative forcing is enhanced by sizablelongwave contributions, which result from the greater cloud breakup and areduced boundary layer height associated with aerosol heating. A perturbationof moisture instead of aerosol aloft leads to a greater liquid water path anda more gradual transition. Adding absorbing aerosol to that atmosphereresults in substantial reductions in liquid water path (LWP) and cloud coverthat lead to positive shortwave and negative longwave forcings on averagecanceling each other. Only for heavily drizzling clouds is the breakupdelayed, as inhibition of precipitation overcomes cloud water loss fromenhanced entrainment. Considering these simulations as an imperfect proxy forbiomass burning plumes influencing Namibian stratocumulus, we expect regionalindirect plus semi-direct forcings to be substantially negative to negligibleat the top of the atmosphere, with its magnitude sensitive to background andperturbation properties

Impacts of solar-absorbing aerosol layers on the transition of stratocumulus to trade cumulus clouds