Impact of natural and anthropogenic aerosols on stratocumulus and precipitation in the Southeast Pacific: a regional modelling study using WRF-Chem

摘要

Cloud-system resolving simulations with the chemistry version of the Weather Research and Forecasting (WRF-Chem) model are used to quantify the relative impacts of regional anthropogenic and oceanic emissions on changes in aerosol properties, cloud macro- and microphysics, and cloud radiative forcing over the Southeast Pacific (SEP) during the VAMOS Ocean-Cloud-Atmosphere-Land Study Regional Experiment (VOCALS-REx) (15 October–16 November 2008). Two distinct regions are identified. The near-coast polluted region is characterized by the strong suppression of non-sea-salt particle activation due to sea-salt particles, a dominant role of first over second indirect effects, low surface precipitation rates, and limited impact of aerosols associated with anthropogenic emissions on clouds. The effects of natural marine aerosols on cloud properties (e.g., cloud optical depth and cloud-top and cloud-base heights), precipitation, and the top of atmosphere and surface shortwave fluxes counteract those of anthropogenic aerosols over this region. The relatively clean remote region is characterized by large contributions of aerosols from non-local sources (lateral boundaries), much stronger drizzle at the surface, and high aerosol-cloud-precipitation interactions under a scenario of five-fold increase in anthropogenic emissions. Clouds in this clean region are quite sensitive (e.g., a 13% increase in cloud-top height and a 9% increase in surface albedo) to a moderate increase (25% of the reference case) in cloud condensation nuclei (CCN) concentration produced by a five-fold increase in regional anthropogenic emissions. The reduction of precipitation due to this increase in anthropogenic aerosols more than doubles the aerosol lifetime in the clean marine boundary layer. Therefore, the aerosol impacts on precipitation are amplified by the positive feedback of precipitation on aerosol, which ultimately alters the cloud micro- and macro-physical properties, leading to strong aerosol-cloud-precipitation interactions. The high sensitivity is also related to an increase in cloud-top entrainment rate (by 16% at night) due to the increased anthropogenic aerosols. The simulated aerosol-cloud-precipitation interactions due to the increased anthropogenic aerosols have a stronger diurnal cycle over the clean region compared to the near-coast region with stronger interactions at night. During the day, solar heating results in more frequent decoupling of the cloud and sub-cloud layers, thinner clouds, reduced precipitation, and reduced sensitivity to the increase in anthropogenic emissions. The results of this study imply that the energy balance perturbations from increased anthropogenic emissions are larger in the more susceptible clean environment than in already polluted environment and is larger than possible from first indirect effect alone.