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

Abstract

Abstract. 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 low surface precipitation rates, the strong suppression of non-sea-salt particle activation due to sea-salt particles, a predominant albedo effect in aerosol indirect effects, and limited impact of aerosols associated with anthropogenic emissions on clouds. Opposite sensitivities to natural marine and anthropogenic aerosol perturbations are seen in cloud properties (e.g., cloud optical depth and cloud-top and cloud-base heights), precipitation, and the top-of-atmosphere and surface shortwave fluxes over this region. The relatively clean remote region is characterized by large contributions of aerosols from non-regional sources (lateral boundaries) and much stronger drizzle at the surface. Under a scenario of five-fold increase in regional anthropogenic emissions, this relatively clean region shows large cloud responses, for example, a 13 % increase in cloudtop height and a 9 % increase in albedo in response to a moderate increase (25 % of the reference case) in cloud condensation nuclei (CCN) concentration. 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 aerosolcloud-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. This study shows the importance of natural aerosols in accurately quantifying anthropogenic forcing within a regional modeling framework. The results of this study also imply that the energy balance perturbations from increased anthropogenic emissions are larger in the more susceptible clean environment than in already polluted environment and are larger than possible from the first indirect effect alone.

Impacts of aerosol particles on the microphysical and radiative properties of stratocumulus clouds over the southeast Pacific Ocean

“…These alkaline droplets would initially contain a large fraction of available S(IV) (aqueous SO 2 , bisulfite and sulfite) in the cloud, which may be converted irreversibly to sulfate through aqueous-phase reactions (Hegg and Hobbs, 1979). Through this process, seasalt acts as a sink for SO 2 and may limit the formation and growth of smaller sulfate aerosols (Yang et al, 2012). VO-CALS electron microscopy ( Fig.…”

Coupling spectral‐bin cloud microphysics with the MOSAIC aerosol model in WRF‐Chem: Methodology and results for marine stratocumulus clouds

“…There were significant variations in aerosol and cloud microphysical properties when midlatitude troughs reached the VOCALS campaign region during October . First, the modified ACI processes in the newly coupled model are evaluated using a simplified aerosol setup for a case occurring on 15 October 2008 at the VOCALS ''remote ocean'' region, where aerosol concentrations are low and aerosol has primary marine source with minimum influence by anthropogenic emissions [Yang et al, 2011[Yang et al, , 2012. The cloud base and top heights are about 600 m and 1400 m, respectively.…”

“…For all simulations, the initial and boundary meteorological conditions are produced from the National Center for Environmental Prediction (NCEP) Final Analysis (FNL) data with 1-degree grid spacing, and the meteorological boundary conditions are updated in every 6 h. We use 1 km horizontal grid spacing and 64 vertical layers as in Yang et al [2011Yang et al [ , 2012 and Martini et al [2014]. The layer thickness increases from 30 m at the lowest level to 50 m at around 1 km altitude, and about 90 m at 2 km altitude.…”

“…Based on both observational [Allen et al, 2011] and modeling studies [Yang et al, 2011], sulfate is the dominant aerosol composition for submicron aerosols over the Southeast Pacific during VOCALS, so for simplicity, the aerosols are assumed to consist of only sulfate. The aerosol size distribution used for initial and boundary conditions is obtained from a previous WRF-Chem simulation at 1 km resolution that was nested in a 3 km resolution domain and used boundary conditions from the 9 km resolution AERO simulation described in Yang et al [2011Yang et al [ , 2012.…”

Implementation of aerosol–cloud interactions in the regional atmosphere–aerosol model COSMO-MUSCAT(5.0) and evaluation using satellite data

“…Compared to an LES that only simulates individual cloud systems, regional climate models are able to simulate the feedbacks between clouds and aspects of the large-scale circulation and its variability reasonably well. Even though regional models do not describe part of the large-scale feedbacks, they may be considered a good optimal compromise (Bangert et al, 2011;Van den Heever and Cotton, 2007;Chapman et al, 2009;Forkel et al, 2015;Yang et al, 2012). A still often applied cloud microphysics parameterization in numerical weather prediction is a bulk one-moment scheme (Kessler, 1969;Lin et al, 1983) which uses the specific masses for different hydrometeor species as prognostic variables.…”