MCS precipitation intensity, distribution and efficiency response to increased aerosol concentrations

Mesoscale Convective Systems (MCS) are important contributors to rainfall in the High Plains of the United States. It is therefore of interest to understand how different aerosol concentrations serving as cloud condensation nuclei (CCN) may impact the amount, intensity and spatial distribution of precipitation produced by MCSs. In this study, different aerosol concentrations and their effects on precipitation produced by an MCS are examined by simulating a case study using the Regional Atmospheric Modeling System (RAMS). Four simulations were conducted, where each simulation differed in the initial aerosol concentrations in order to examine the effects of increased aerosols concentrations on MCS precipitation. In order to represent realistic aerosol concentrations and their spatial distributions, aerosol concentrations from the 3D chemical transport model GEOS-Chem were incorporated into RAMS where they serve as potential CCN. It is known that enhanced CCN concentrations produce a narrower cloud droplet spectrum with smaller droplet diameters leading to the suppression of warm rain production and an increase in the amount of cloud water mixing ratio which can be transported to higher levels of the storm. Therefore, higher aerosol concentrations lead to larger amounts of supercooled water which affect the size and number concentrations of ice hydrometeors through the various processes of freezing. This in turn impacts storm dynamics via the invigoration of the vertical velocity due to enhanced latent heat release of freezing. Previous studies have shown that the size of ice hydrometeors can affect the precipitation efficiency of an MCS, where smaller ice hydrometeors cause a reduction in precipitation efficiency due to enhanced sublimation and evaporation at higher levels. Past studies have also showed that although enhanced aerosol concentration suppressed the formation of warm rain in a storm, the impact of aerosols on the dynamics of a storm can cause an increase in precipitation. Results of this study suggest that although increased aerosol concentrations reduce the MCS precipitation rates, the storm accumulated precipitation quantities increases. An analysis of the response of MCS microphysical and dynamical processes due to aerosol loading leading to enhanced precipitation will be presented.