Urban Impacts On Summer Heavy Rainfall And Flooding

As one of the important anthropogenic influences on water cycle, urbanizationexhibits a direct impact on surface hydrology by modifying the characteristics of landsurface conditions; on the other hand, urbanization also alters the partition of surfaceenergy balance and urban environment (e.g., aerosols, anthropogenic heat), whichchanges the characteristics of regional rainfall regime and ultimately feed back to thesurface hydrology. This study focuses on summer heavy rainfall and flooding eventsover urban environment. Urban modification of heavy rainfall and flooding wasexplored based on multi-source observations and numerical atmospheric/hydrologicmodels. The results are of remarkable scientific as well as practical meanings to furtherthe understanding on interactions between land surface and atmosphere, and improvethe design of flood-control systems in urban areas.The connection between the spatio-temporal patterns of summer rainfall and EastAsian summer monsoon (EASM) was examined over China. Under the control ofEASM, North China is experiencing a significantly decreasing trend of summer rainfallduring1961~2012, with Beijing metropolitan region included. However, summerrainfall over the urban core region of Beijing witnessed an increasing trend after2000,in accordance with the abrupt changes of built-up areas. Two ‘hot spots’ of higherfrequency of heavy rainfall events and summer total rainfall were observed over theBeijing metropolitan region within the period of2008~2012. One is located over theurban core region, and the other resides in the climatological downwind region. WeatherResearch and Forecasting (WRF) model coupled with single layer urban canopy model(SLUCM) was used to examine the urban signature in the spatial pattern of2008~2012summer rainfall over Beijing. A new storm cell identification algorithm SCI-2D wasdesigned to analyze the dynamics of storm cells over urban region. The urban coreregion of Beijing tends to split the storm cells into smaller ones and trigger new smallcells as well. Urban thermodynamic properties play a key role in enhancing theconvective potentials by increasing the depth of planetary boundary later (PBL), liftedcondensation level (LCL) and level of free convection (LFC), which ultimately elevatedrainfall intensity and frequency over the urban region. The impact of thermodynamic properties on summer rainfall is affected by regionalcirculation, e.g., Lake Breeze for lake-shore cities. This study further examined a typicalsummer heavy rainfall event over Milwaukee-Lake Michigan region, US. The couplingmechanisms of urban circulation and Lake Breeze and their effects on the rainfallprocesses were identified. The dynamics of storm cells approaching the city, asdisclosed with the storm tracking algorithm TITAN, were in accordance with theconclusions of the study in Beijing metropolitan region.An integrated assessment of urban impact on hydrological responses was carriedout by setuping up a virtual urbanized basin and customized sensitive numericalscenarios. The numerical scenarios were the combinatins of different imperviouscoverages (as represented by urban ratios) and rainfall spatial patterns, assuming thaturbanization processes could noticeably alter the rainfall spatial patterms for urbanizedbasins. Numerical experiments show that the amplification of flood magnitude inducedby urban sprawl could be underestimated as large as50%with the ignorance of rainfallspatial pattern. In addition, rainfall spatial pattern also plays an indispensable role in theinterannual variability of annual runoff. This study synthesized the direct impact ofurbanization and the indirect impact through the feedback of land surface/atmosphereon hydrological responses, which improved the analytical framework of hydrologicalresponses over urbanized basins.