A Numerical Investigation of the Aerosol Effects on a Mesoscale Convective System




Roy, Priyanka

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Mesoscale Convective Systems (MCSs) are frequent occurrences during summer months in mid-west USA and bring almost 30% rainfall to the region. This work investigates the effects of anthropogenic aerosols, like sulfate and black carbon, and natural aerosols like dust on a MCS. The coupled meteorology and chemistryWeather Research and Forecasting{ Chemistry (WRF-Chem) version 3.1.1 model was employed for the numerical study of the aerosol effects on MCS. The selected MCS occurred on June 20, 2007 covering large parts of Kansas, Oklahoma and northern Texas. In the WRF-Chem model, the aerosol effects are analyzed by inputting the aerosol optical properties into the shortwave radiation scheme and physical properties into the microphysics scheme. The interaction of aerosols with the incoming shortwave radiation is higher due to the wavelength being similar to particulate sizes found in the atmosphere. The spatial resolution which resolves the features of the MCS reliably well was found by conducting sensitivity studies at coarse and fine resolution. At the coarse resolution (18 km) the MCS was not very well resolved, with delays in cloud and precipitation formation. However, the direct and indirect effects of anthropogenic aerosols were prominent, by showing large scale scattering of the shortwave radiation and by suppressing the precipitation, respectively. The nested domain simulations have higher inner domain resolutions (6 and 1.5 km) and as a result resolved the MCS better than the single coarse resolution simulation. The combined aerosol effects are investigated by increasing the amount of the sulfate, black carbon and dust aerosols, and considering their dominant characteristics. Sulfates are the major constituents of the anthropogenic emissions, and they are scattering and reflecting in nature. On the other hand, black carbon and dust absorb radiation, evaporating clouds and also warming the atmosphere. The dust particulates form giant cloud condensation nuclei (CCN), which can enhance precipitation in the presence of moisture in the atmosphere. The combination of the radiative effects due to each of these aerosols has shown, that scattering due to aerosols is a dominant factor for all the types of aerosols. The presence of aerosols interacting with the microphysics and radiation schemes produces a more organized MCS structure, as well as more liquid and ice clouds. The black carbon particulates do not solely warm the atmosphere, but also prevent a large amount of the solar radiation from reaching the surface. The giant CCN due to dust particles instead of suppressing the precipitation enhances it. Thus, two absorbing aerosols when increased in amounts show very different effects on cloud cover and precipitation during MCS. This is one of the few studies to use coupled chemistry and meteorology model to study the effects of aerosols on MCS. It brings to light the fact that inspite of their concentration, some dominant characteristics of each aerosol type may be lost while others may be emphasized, on the surface energy balance and in the non-linear process of precipitation even during MCS. This work shows that the aerosol concentration and composition are prominent on the surface energy and while the aerosol size and concentration are vital for the precipitation processes.



WRF-Chem, Convection, Aerosol, Clouds, Mesoscale Convective System, Precipitation