Surface Boundary Layer Characterization in Urban Areas

Harvard University, Cambridge MA

Investigators

Abstract

The Principal Investigator will exploit existing data sets from urban field experiments that were collected from a wide array of geographic areas and countries throughout the world. There is an opportunity, to analyze these comprehensive urban data bases from several fundamental scientific perspectives in order to develop universal scientific relations. These general relations will be tested in the context of a hierarchy of urban boundary layer models that should improve understanding and should allow more accurate modeling in the future in urban areas where such extensive observations are not available. In urban areas with buildings 10 to 100 m or more tall, much of the boundary layer of interest is below the roughness elements, which requires development of general relations that allow a transition between the below canopy boundary layer to the above canopy. Given descriptors of urban areas such as building morphology, methods will be studied to estimate the surface fluxes and the flow field, turbulence, stabilities and mixing heights. Several of the features of the urban boundary layer are governed by horizontal inhomogeneities associated with the flow of rural air into the urban area with different surface characteristics. Since each building obstacle and block and neighborhood is unique, the urban boundary layer is never fully in equilibrium, requiring development of approximations for the development of new boundary layers as the air passes from one surface type to another. For example, during the night, the urban boundary layer in the built-up downtown area is often convective (unstable) because the warm urban surface heats the inflowing colder stable rural air (i.e., the urban heat island effect). Mesoscale meteorological models need more detailed land-use and boundary layer parameterizations for urban areas. Since urban areas are growing throughout the globe, the surface fluxes, the drag, and other boundary layer processes in urban areas have increasing influence on most meteorological phenomena. The products of the research will be 1) general formulas that describe the mean and turbulence structure of the urban boundary layer, 2) relationships between urban and rural flow variables, 3) relationships describing the variation of stability as the air passes from upwind rural to downtown and back to rural, and 4) improved parameterizations of urban effects. These products will help to improve comprehensive basic models for urban areas. There will be broad impacts of the research, such as collaborations with related urban studies in North America, Europe, and Asia, development of simple but basic scientific relations that are verified over a number of urban areas, and enhancement of K-12 and undergraduate education. For example the simplified physical relations will be useful for explaining urban boundary layers in basic texts and in general meteorology courses.