Potential impacts of aerosol pollution on a Derecho-producing MCS

Mesoscale Convective Systems (MCS) are important contributors to rainfall as well as producers of severe weather such as hail, tornados and straight-line wind events known as Derechos. Past studies have shown that through the aerosol indirect effect on hydrometeor distribution and characteristics, changes in aerosol concentrations may lead to either an increase or decrease in evaporation and melting within convective downdrafts. This in turn affects the strength of the downdraft, the propagation of the cold-pool and the velocity of the gust front. In this study, the characteristics of a severe wind event produced by an MCS to increased anthropogenic aerosol concentrations is examined by simulating a case study using the Regional Atmospheric Modeling System (RAMS). The case study of the 8 May 2009 “Super-Derecho” MCS was chosen since it produced widespread wind damage over a broad region from the High Plains of western Kansas to the foothills of the Appalachians. Aerosol concentrations of 12 species from the 3D chemical transport model, GEOS-Chem were incorporated into RAMS where they served as cloud concentration nuclei. Two RAMS simulations were performed, one with GEOS-Chem aerosol concentrations from only natural sources (Clean) and the second with GEOS-Chem aerosols sourced from both natural and anthropogenic sources (Polluted). The two simulations differed both in aerosol number concentrations and chemical composition. Analysis between the two simulations included back-trajectory analysis of parcels within the gust flow to examine the changes within the air source of the Derecho due to enhanced aerosol concentrations. Results of this study show that increased aerosol concentrations produced a weaker derecho due to a decrease in precipitation melting and evaporation. An analysis of the response of the MCS microphysical and the dynamical feedback on the formation of the derecho among the two simulations will be presented.