The effect of urbanization and urban-induced aerosol on the warm cloud in mid-Korean peninsula

Several early observational and numerical studies have indicated that urbanization and/or urban-induced aerosols by the presence of cities modify cloud macro/micro physical processes, and potentially change the location, intensity, and type of precipitation over and downwind of urban region. It is important to understand its effects over and downwind the urban region, because these effects have occurred in the urban region together, and it is not yet clear quantitatively how urbanization and aerosol have an influence on clouds and the precipitation (Tao et al., 2012). Our previous study (Eun et al., 2011) presented that the precipitation amount (PA) and frequency (PF) together in the downwind region of Seoul Metropolitan Area (SMA) significantly increased for the westerly and light precipitation case (less than 1mm per day), such that PA and PF for the downwind area vs. SMA remarkable increased. SMA in Korea has experienced a rapid urbanization for a long time, becoming one of the largest urban area in Korea, which a population of about 24 million. It implies the possible influences of urbanization and/or urban-induced aerosols on the precipitation in the downwind region of SMA. Based on observed results, we selected golden cases (9 February 2009) to investigate the impact of urban heat island (UHIR) and urban-induced aerosol (CCNR) on the light precipitation using the Weather Research and Forecasting (WRF) model. The result of sensitivity studies shows that cloud water content experiment for urbanization effect (UHIR) increases in downwind region of SMA, mostly attributable to the enhanced convergence and its associated vertical velocity. On the other hand, increasing number concentration of Cloud Condensation Nuclei (CCN) in the SMA domain leads to smaller effective radius (re) and more cloud droplet number concentration (Nc) over and downwind of SMA in comparison to a control run, which eventually makes cloud change in the downwind region after 3 hour. The combined experiment (urbanization and aerosol effect) is similar to CCNR (aerosol effect) experiment, except for the extensive area of cloud field. The cloud coverage and thickness, precipitation intensity and influence distance of urbanization and/or aerosol effect will be discussed in detail.