Observations on Urban Fluxes and Boundary Layers in Beijing

Half the global population will be in cities by 2025, many having more than 10 million people.  Urbanization modifies atmospheric energy and moisture balances, forming local climates, e.g. urban heat islands (UHIs) and enhanced precipitation.  These produce significant challenges to science and society, e.g. flooding, heat waves strengthened by UHIs, and air pollutant hazes.  The Beijing megacity experiences such severe events, e.g., 2012 flooding killed 79 and caused losses of $2B.

Despite significant research into urban effects on weather and air quality, important science issues remain, e.g., urban-thermodynamic and aerosol impacts on summer convective precipitation and interactions between urban and regional climate changes.  Observations are fundamental for understanding these interactions, improving forecasts, and providing useful information to end-users.  Previous large urban field campaigns have not been coordinated by a group such as the Beijing Institute of Urban Meteorology (IUM), with its responsibilities for both boundary layer research and real time urban weather forecasting.  The overall science objective of the 2014-8 SURF Project is thus a better understanding of urban, terrain, convection, and aerosol interactions for improved forecast accuracy.  Beijing is a test case, but the improved understandings are transferable to many large cities globally.

Using the turbulence measurement from the IAP tower, we confirmed the HOST hypothesis proposed by Sun et al. (2016) over urban canopies, i.e., turbulence at a given height is dominated by large coherent eddies, and turbulence intensity is constrained by the turbulence energy conservation within the layer where dominant turbulence eddies are generated.

The atmospheric boundary layer (ABL) depth is a key variable to describe the boundary layer structure, and in many applications such as weather and air-quality forecast. We studied the ABL evolution by using Doppler lidar wind measurements in central Beijing during the summer 2015 SURF field campaign. We developed a composite method for estimating the diurnal variations of the ABL depth using the Doppler lidar.

The surface energy balance characteristics observed by urban and rural towers will be analyzed. Recent significant developments and open challenges will be discussed.