Abstract
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.
Highlights
Clouds play a major role in Earth’s radiative balance (Ramanathan et al, 1989; Cess et al, 1989)
There is an imperative need for reducing uncertainty and improving the atmospheric models used in studies of aerosolcloud interactions at scales needed for numerical weather prediction (NWP), air quality predictions, and policy assessments
2011), we perform model simulations designed to address the questions: what are the effects on cloud dynamics and microphysics from changing the subcloud aerosol loads? And do these effects bring model results closer to observations when aerosol loads are in better agreement to measurements? To address these questions results from two model simulations, with and without wet deposition (NW) were analyzed
Summary
Clouds play a major role in Earth’s radiative balance (Ramanathan et al, 1989; Cess et al, 1989). Uncertainties in the processes that affect cloud optical properties and modify this balance are still high (Solomon et al, 2007). These processes are driven by the indirect climatic effects of aerosols (Lohmann and Feichter, 2005), which can modify cloud albedo (Twomey, 1991) and lifetime (Albrecht, 1989), evaporate clouds (Graßl, 1979), change thermodynamics in deep convective clouds (Andronache et al, 1999), increase precipitation in ice clouds (Lohmann, 2002), and change the surface energy budget (e.g., Liepert, 2002).
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