Effect of anthropogenic aerosol emissions on precipitation in warm conveyor belts in the western North Pacific in winter – a model study with ECHAM6-HAM

摘要

While there is a clear impact of aerosol particles on the radiationbalance, whether and how aerosol particles influence precipitation iscontroversial. Here we use the ECHAM6-HAM global climate model coupled to anaerosol module to analyse whetheran impact of anthropogenic aerosol particles on the timing and amount ofprecipitation can be detected in North Pacific warm conveyor belts. Warmconveyor belts are the strongest precipitation-producing airstreams inextratropical cyclones and are identified here with a Lagrangian technique,i.e. by objectively identifying the most strongly ascending trajectories inNorth Pacific cyclones. These conveyor belts have been identified separatelyin 10-year ECHAM6-HAM simulations with present-day and pre-industrial aerosolconditions. Then, the evolution of aerosols and cloud properties has beenanalysed in detail along the identified warm conveyor belt trajectories. Theresults show that, under present-day conditions, some warm conveyor belttrajectories are strongly polluted (i.e. high concentrations of black carbonand sulfur dioxide) due to horizontal transport from eastern Asia to theoceanic region where warm conveyor belts start their ascent. In thesepolluted trajectories a weak delay and reduction of precipitation formationoccurs compared to clean warm conveyor belt trajectories. However, all warmconveyor belts consist of both polluted and clean trajectories at the timethey start their ascent, and the typically more abundant clean trajectoriesstrongly reduce the aerosol impact from the polluted trajectories. The mainconclusion then is that the overall amount of precipitation is comparable inpre-industrial conditions, when all warm conveyor belt trajectories areclean, and in present-day conditions, when warm conveyor belts consist of amixture of clean and polluted trajectories.