Wednesday, 9 July 2014
Mesoscale Convective Systems (MCS) are important contributors to rainfall in the United States as well as producers of severe weather such as large hail, severe winds and tornadoes. The goal of this research is to understand the susceptibility of a Derecho-producing MCS to the effects of varying aerosol characteristics such as number concentration and chemical composition. The indirect effects of anthropogenic aerosols on the different modes of convection that constitute an MCS and their subsequent impacts on precipitation and severe wind formation in the storm are investigated. In this study the May 8th 2009 MCS was successfully simulated using the Regional Atmospheric Modeling System (RAMS). Three model simulations were performed using a triple nested-grid, where the finest grid contains the entire domain of the MCS storm, from genesis to decay. Convection and cloud-microphysics are explicitly represented in the simulated cloud development of the MCS. The simulation configuration of the three simulations was identical apart from the aerosol distribution and chemical composition, represented by Kappa, at initialization. In the first simulation, which serves as the control run, the initial aerosol field was defined by spatially homogeneous clean aerosol concentrations. In the control run, only one species of aerosols was considered, the fine mode of ammonium sulfate. The second and third simulations were initialized with an aerosol distribution from output of the Goddard Earth Observing System-Chemistry (GEOS-CHEM) model: a three dimensional atmospheric model that includes emissions, formation and transport of different aerosol species. Both organic and inorganic aerosol species from GEOS-CHEM were incorporated into the RAMS simulations, where they serve as potential cloud condensation nuclei (CCN). The main purpose of introducing GEOS-CHEM aerosols into RAMS is to provide realistic estimates of horizontally heterogeneous aerosol concentrations with varying hygroscopicity. The second RAMS simulation was initialized with GEOS-CHEM aerosols with anthropogenic aerosols, and the third RAMS simulation was initialized with GEOS-CHEM aerosols without anthropogenic components. These two GEOS-CHEM data sets differed both in number concentrations and in chemical composition. The comparison between the two simulations containing GEOS-CHEM aerosols allows us to investigate the indirect impacts of anthropogenic emissions on the precipitation and the surface severe straight line winds (Derecho) produced by the simulated MCS. By comparing the output of these simulations advancements in our understanding of the impacts of anthropogenic aerosols serving as cloud condensate on MCS dynamics will be made. Preliminary analyses suggest that adding anthropogenic aerosols increases the overall surface accumulated precipitation from the simulated storm, while decreasing the surface wind magnitudes and their spatial distribution. Such findings, if prove robust, have implications for the severity of derechoes.
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