Urban-induced aerosol effects on the stratiform clouds and precipitation in the downstream region

Many observational and numerical studies have indicated that urban-induced aerosol effects modify cloud property, precipitation, and further weather pattern over and downwind of urban region. It is important to understand its effect over and downwind of the urban region, because these effects have occurred in the urban region together. Eun et al. (2011) showed increasing trend of precipitation amount and frequency downwind of Seoul Metropolitan Area (SMA) still in process of developing from 1972 to 2007, for particularly light precipitation (less than 1 mm per day) and westerly condition only. It implies the possible influences of urban aerosol on the precipitation in the downwind region of SMA. Based on observed results, we selected golden cases (9 and 10 February 2009) to investigate the impact of urban-induced aerosol on light precipitation using the Weather Research and Forecasting (WRF) model. An initial condition of cloud condensation nuclei (CCN) number concentration is given as 1,000 #/cm3 for the sensitivity run at SMA domain with a background condition of 100 #/cm3. The boundary layer winds for the given days are westerly with mostly 5~6 m/s and cloud thickness is about 500 m within 2 km above the ground. In general, sensitivity studies shows that the enhanced CCN number concentrations advected match well with smaller effective radius (re) and more cloud droplet number concentration (Nc) over and downwind of SMA in comparison to a control run, approximately 3 ~ 5 hours after the initial CCNs given at SMA, when re and Nc vary more widely downwind (up to 100 km) of SMA, while a change in precipitation amount is trivia with a change in precipitation location. The coverage, intensity, and duration time of aerosol effect under different environmental condition such as updraft velocity, and horizontal wind strength will be discussed in detail.