Atmos. Chem. Phys. Discuss., 11, 29723-29775, 2011
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This discussion paper has been under review for the journal Atmospheric Chemistry and Physics (ACP). Please refer to the corresponding final paper in ACP.
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, S. Howell3, D. C. Leon4, J. R. Snider4, A. R. Bandy5, J. L. Collett6, K. B. Benedict6, S. P. de Szoeke7, L. N. Hawkins8, G. Allen9, I. Crawford9, J. Crosier9, and S. R. Springston10
1Center for Global and Regional Environmental Research (CGRER), University of Iowa, Iowa City, Iowa, USA
2Center for Sustainability Research, Universidad Andrés Bello, Santiago, Chile
3Department of Oceanography, University of Hawaii at Manoa, Honolulu, USA
4Department of Atmospheric Science, University of Wyoming, Laramie, Wyoming, USA
5Department of Chemistry, Drexel University, Philadelphia, PA, USA
6Colorado State University, Department of Atmospheric Science, Fort Collins, CO, USA
7Oregon State University, College of Oceanic and Atmospheric Sciences, Corvallis, OR, USA
8Harvey Mudd College, Department of Chemistry, Claremont, CA, USA
9Centre for Atmospheric Science, University of Manchester, Manchester, M13 9PL, UK
10Brookhaven National Laboratory, Upton, MA, 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 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 wet (NW) 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, 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, including the reliability required for policy analysis and geo-engineering applications.

Citation: Saide, P. E., Spak, S. N., Carmichael, G. R., Mena-Carrasco, M. A., Howell, S., Leon, D. C., Snider, J. R., Bandy, A. R., Collett, J. L., Benedict, K. B., de Szoeke, S. P., Hawkins, L. N., Allen, G., Crawford, I., Crosier, J., and Springston, S. R.: Evaluating WRF-Chem aerosol indirect effects in Southeast Pacific marine stratocumulus during VOCALS-REx, Atmos. Chem. Phys. Discuss., 11, 29723-29775, doi:10.5194/acpd-11-29723-2011, 2011.
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