Using WRF-Chem to understand interactions between synoptic and microphysical variability over the southeast Pacific

Aerosol indirect effects contribute strongly to the uncertainty in estimates of the anthropogenic contribution to climate change. Uncertainties in how aerosols affect microphysical cloud properties and how clouds respond to changes in their microphysics can be complicated by the coincident meteorological conditions. Changes in cloud properties such as droplet concentration and liquid water path can arise due to meteorological/thermodynamic drivers and/or aerosol effects, and separating the two remains a challenge. For example, synoptic meteorological variability in the southeast Pacific (SEP) is associated with variations in droplet concentration of up to 50% of the mean value. Important follow-up questions involve determining the extent to which this association is due to (i) aerosol effects on clouds and aerosol transport via the meteorology; (ii) direct dynamical mechanisms independent of aerosol changes (e.g. variations in updraft speed); (iii) aerosol removal due to variations in precipitation. Parsing dominant mechanisms determining the variability of cloud properties necessitates the use of a model.

We investigate aerosol and meteorological mechanisms dominating low cloud variability in the SEP using the WRF-CHEM model. We study the SEP because a large and persistent stratocumulus deck often covers the region, the coasts of South America bordering the SEP contains many aerosol sources (e.g. copper smelters near the coasts of Chile and Peru provide a major source of oxidized sulfur emissions), and the VOCALS Regional Experiment(REx) is providing much detailed information about the character and behavior of the low clouds there. We run WRF-CHEM simulations forced with NCEP FNL meteorological data. MOZART4 (Emmons et al. 2009) data provides time-varying chemical boundary and initial conditions. We compare model output to VOCALS Rex flight observations and MODIS data from the NASA Terra satellite to validate the model. Initial results indicate that the model can reasonably capture some mean-state parameters. It reproduces the LWP field (though the magnitude is smaller) well and the large droplet concentration magnitude near the coast, for example.

With these model runs we explore how important aerosol variability is to the macrophysical and microphysical cloud property variability, with the controlling large-scale meteorology in mind. We investigate the transport and extent of the aerosols a as well as the thermodynamic conditions that affect the model cloud. We attempt to reproduce cloud features noted during VOCALS Rex and understand those that are successful using available data from the campaign. Unreproducible features are investigated for potential reasons within the model.