Articles | Volume 12, issue 6
Atmos. Chem. Phys., 12, 3045–3064, 2012

Special issue: VAMOS Ocean-Cloud-Atmosphere-Land Study (VOCALS) (ACP/OS inter-journal...

Atmos. Chem. Phys., 12, 3045–3064, 2012

Research article 29 Mar 2012

Research article | 29 Mar 2012

Evaluating WRF-Chem aerosol indirect effects in Southeast Pacific marine stratocumulus during VOCALS-REx

P. E. Saide1, S. N. Spak1, G. R. Carmichael1, M. A. Mena-Carrasco2, Q. Yang3, S. Howell4, D. C. Leon5, J. R. Snider5, A. R. Bandy6, J. L. Collett7, K. B. Benedict7, S. P. de Szoeke8, L. N. Hawkins9, G. Allen10, I. Crawford10, J. Crosier10, and S. R. Springston11 P. E. Saide et al.
  • 1Center for Global and Regional Environmental Research (CGRER), University of Iowa, Iowa City, Iowa, USA
  • 2Center for Sustainability Research, Universidad Andrés Bello, Santiago, Chile
  • 3Pacific Northwest National Laboratory, Richland, WA, USA
  • 4Department of Oceanography, University of Hawaii at Manoa, Honolulu, USA
  • 5Department of Atmospheric Science, University of Wyoming, Laramie, Wyoming, USA
  • 6Department of Chemistry, Drexel University, Philadelphia, PA, USA
  • 7Colorado State University, Department of Atmospheric Science, Fort Collins, CO, USA
  • 8Oregon State University, College of Earth, Ocean, and Atmospheric Sciences, Corvallis, OR, USA
  • 9Harvey Mudd College, Department of Chemistry, Claremont, CA, USA
  • 10Centre for Atmospheric Science, University of Manchester, Manchester, M13 9PL, UK
  • 11Brookhaven National Laboratory, USA

Abstract. We evaluate a regional-scale simulation with the WRF-Chem model for the VAMOS (Variability of the American Monsoon Systems) Ocean-Cloud-Atmosphere-Land Study Regional Experiment (VOCALS-REx), which sampled the Southeast Pacific's persistent stratocumulus deck. Evaluation of VOCALS-REx ship-based and three aircraft observations focuses on analyzing how aerosol loading affects marine boundary layer (MBL) dynamics and cloud microphysics. We compare local time series and campaign-averaged longitudinal gradients, and highlight differences in model simulations with (W) and without (NW) wet deposition processes. The higher aerosol loadings in the NW case produce considerable changes in MBL dynamics and cloud microphysics, in accordance with the established conceptual model of aerosol indirect effects. These include increase in cloud albedo, increase in MBL and cloud heights, drizzle suppression, increase in liquid water content, and increase in cloud lifetime. Moreover, better statistical representation of aerosol mass and number concentration improves model fidelity in reproducing observed spatial and temporal variability in cloud properties, including top and base height, droplet concentration, water content, rain rate, optical depth (COD) and liquid water path (LWP). Together, these help to quantify confidence in WRF-Chem's modeled aerosol-cloud interactions, especially in the activation parameterization, while identifying structural and parametric uncertainties including: irreversibility in rain wet removal; overestimation of marine DMS and sea salt emissions, and accelerated aqueous sulfate conversion. Our findings suggest that WRF-Chem simulates marine cloud-aerosol interactions at a level sufficient for applications in forecasting weather and air quality and studying aerosol climate forcing, and may do so with the reliability required for policy analysis.

Final-revised paper