The changes in precipitation in north-eastern North America caused bychemistry – and particularly anthropogenic aerosols – are investigatedusing the Weather Research Forecasting with Chemistry (WRF/Chem v3.2) model. The simulations were carried out for a five-monthperiod from April to August 2009. The model results show that non-negligiblechanges in both convective and cloud-resolved (non-convective) precipitationare caused by chemistry and/or aerosols over most parts of the domain. Thechanges can be attributed to both radiative and microphysical interactionswith the meteorology. A chemistry-induced change of approximately −15%is found in the five-month mean daily convective precipitation over areaswith high convective rain; most of this can be traced to radiative effects.Total convective rain is greater than total non-convective rain in thedomain, but a chemistry-induced increase of about 30% is evident inthe five-month mean daily non-convective precipitation over the heavilyurbanized parts of the Atlantic coast. The effects of aerosols on cloudmicrophysics and precipitation were examined for two particle size ranges, 0.039–0.1 μm and 1–2.5 μm, representing the nucleation andaccumulation modes respectively. Strongly positive spatial correlationbetween cloud droplet number and non-convective rain are found for activated(cloud-borne) aerosols in both size ranges. Non-activated (interstitial)aerosols have a positive correlation with cloud droplet number andnon-convective rain when they are small and an inverse correlation forlarger sizes.
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