Assessing regional scale predictions of aerosols, marine stratocumulus, and their interactions during VOCALS-REx using WRF-Chem

Abstract

Abstract. This study assesses the ability of the recent chemistry version (v3.3) of the Weather Research and Forecasting (WRF-Chem) model to simulate boundary layer structure, aerosols, stratocumulus clouds, and energy fluxes over the Southeast Pacific Ocean. Measurements from the VA-MOS Ocean-Cloud-Atmosphere-Land Study Regional Experiment (VOCALS-REx) and satellite retrievals (i.e., products from the MODerate resolution Imaging Spectroradiometer (MODIS), Clouds and Earth's Radiant Energy System (CERES), and GOES-10) are used for this assessment. The Morrison double-moment microphysics scheme is newly coupled with interactive aerosols in the model. The 31-day (15 October-16 November 2008) WRF-Chem simulation with aerosol-cloud interactions (AERO hereafter) is also compared to a simulation (MET hereafter) with fixed cloud droplet number concentrations in the microphysics scheme and simplified cloud and aerosol treatments in the radiation scheme. The well-simulated aerosol quantities (aerosol number, mass composition and optical properties), and the inclusion of full aerosol-cloud couplings lead to significant improvements in many features of the simulated stratocumulus clouds: cloud optical properties and microphysical properties such as cloud top effective radius, cloud water path, and cloud optical thickness. In addition to accounting for the aerosol direct and semi-direct effects, these improvements feed back to the simulation of boundary-layer characteristics and energy budgets. Particularly, inclusion of interacCorrespondence to: Q. Yang (qing.yang@pnnl.gov) tive aerosols in AERO strengthens the temperature and humidity gradients within the capping inversion layer and lowers the marine boundary layer (MBL) depth by 130 m from that of the MET simulation. These differences are associated with weaker entrainment and stronger mean subsidence at the top of the MBL in AERO. Mean top-of-atmosphere outgoing shortwave fluxes, surface latent heat, and surface downwelling longwave fluxes are in better agreement with observations in AERO, compared to the MET simulation. Nevertheless, biases in some of the simulated meteorological quantities (e.g., MBL temperature and humidity) and aerosol quantities (e.g., underestimations of accumulation mode aerosol number) might affect simulated stratocumulus and energy fluxes over the Southeastern Pacific, and require further investigation. The well-simulated timing and outflow patterns of polluted and clean episodes demonstrate the model's ability to capture daily/synoptic scale variations of aerosol and cloud properties, and suggest that the model is suitable for studying atmospheric processes associated with pollution outflow over the ocean. The overall performance of the regional model in simulating mesoscale clouds and boundary layer properties is encouraging and suggests that reproducing gradients of aerosol and cloud droplet concentrations and coupling cloud-aerosol-radiation processes are important when simulating marine stratocumulus over the Southeast Pacific.

Limited-are a modelling of stratocumulus over South-Eastern Pacific

“…Although there was a significant variability of the observed profiles (the largest for the horizontal velocity components within the boundary layer), the variability does not explain the systematic differences in the boundary layer height between the observations and simulations. Overall, significant underprediction of the height is consistent with previous limited-area simulations of cloud-topped marine boundary layer (e.g., Wyant et al, 2010;Abel et al, 2010;Yang et al, 2011;Wang et al, 2011). In our case, the underprediction results from a combination of two factors.…”

“…This is consistent with several previous investigations, such as Wyant et al (2010), Abel et al (2010), Yang et al (2011. The observed top of the approximately well-mixed boundary layer is between 1 and 1.5 km, but the model predicts the depth of between 0.5 and 1 km.…”

Simulating aerosol–radiation–cloud feedbacks on meteorology and air quality over eastern China under severe haze conditionsin winter

“…Similar results are found when we replace the Lin microphysics scheme by the two-moment Morrison scheme (Morrison et al, 2009). Previous model assessments also showed that the inclusion of aerosol indirect effects reduced cloud water content over the South Pacific Ocean and improved model comparison with aircraft observations (Yang et al, 2011). The relatively small precipitation, as well as small changes of precipitation due to aerosol feedbacks over the north of the domain, suggests that precipitation has a minor effect on near-surface aerosols in January 2013.…”

Aerosol indirect effect on the grid-scale clouds in the two-way coupled WRF–CMAQ: model description, development, evaluation and regional analysis

“…Aerosol particles can influence the earth's climate both directly by scattering and absorption of incoming solar radiation and terrestrial outgoing radiation, and indirectly by affecting cloud radiative properties through their role as cloud condensation nuclei (CCN) and ice nuclei (IN) (Twomey, 1974(Twomey, , 1991Charlson et al, 1992;Yu, 2000;Yu et al, 2000Yu et al, , 2001aYu et al, , b, 2003Yu and Zhang, 2011;Lohmann and Feichter, 2005;Menon et al, 2002Menon et al, , 2008IPCC, 2007;DeFelice et al, 1997;Chapman et al, 2009;Gustafson et al, 2007;Zhang et al, 2010aZhang et al, , b, 2012Tao et al, 2012;Hansen et al, 1997;Haywood and Boucher, 2000;Ramanathan et al, 2001;Rosenfeld et al, 2008;Saxena and Yu, 1998;Saxena et al, 1997;F. Yu et al, 2012a, b;Saide et al, 2012;Yang et al, 2011;Liu et al, 2011;McKeen et al, 2007;Yu et al, 2004Yu et al, , 2007bYu et al, , 2008. The aerosol indirect effect (AIE) can be split into the first, second, and glaciation aerosol indirect effects.…”

“…The resulting cloud drop and ice number concentrations are added to the Morrison cloud microphysics scheme (Morrison et al, 2009(Morrison et al, , 2005, and this allows us to estimate aerosol effects on cloud and ice optical depth and microphysical process rates for indirect aerosol radiative forcing (including first, second and glaciation indirect aerosol forcing) by tying a two-moment treatment of cloud water (mass and number) and cloud ice (mass and number) to precipitation (the Morrison et al two-moment cloud microphysics scheme, Morrison et al, 2009Morrison et al, , 2005 and two radiation schemes (the Rapid Radiative Transfer Model for General Circulation Models (GCMs) (RRTMG), Iacono et al, 2008, andCAM, Collins et al, 2004) in the WRF model. The RRTMG and CAM radiation schemes are selected because these two schemes are used in many studies (Liu et al, 2007;Collins et al, 2004;Iacono et al, 2008;Yang et al, 2011;Saide et al, 2012). The comparison results of the WRF-CMAQ/CAM and WRF-CMAQ/RRTMG simulations can indicate the effects of radiation schemes on the model performance on air quality and cloud properties.…”