Observations of Urban Boundary Layers – Progress and Challenges

As urbanization continues to accelerate worldwide, adverse environmental conditions in cities, such as high concentrations of pollutants and elevated heat stress, along with the related health impacts are of great concern.   The increased roughness and distinct thermal properties of urban surfaces influence atmospheric patterns at various spatial scales. Traditional boundary-layer parameterizations that assume horizontally homogenous flow are inadequate under such conditions.  Yet, accurate predictions of wind and thermal comfort in cities, and also the transport and dispersion of pollutants that are emitted at high concentrations in cities are critical for human health. Such predictions require knowledge about the structure and properties of the atmospheric boundary layer in and near metropolitan areas. An increase in the boundary layer depth near cities along with urban-scale circulations and differences in urban versus rural aerosol concentrations can also alter precipitation patterns and the dynamics of convective storms.  This talk will present an overview of important findings from major urban field campaigns conducted over the last 20 years, discuss challenges in studying the evolution and three dimensional structure of urban boundary layers, and conclude with an outlook about how newer instrumentation, such as Doppler wind lidars and passive remote sensing instruments can further advance the knowledge of boundary-layer processes in cities.