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.
Because an ever-increasing fraction of the world's population is living in large cities, there is increased emphasis on weather and air pollution at street level in the built-up downtown urban areas where there are multiple skyscrapers. Many large cities (e.g., New York City, Sao Paulo, Mexico City, Beijing) have downtown areas of diameters of 10 km or more. The main focus of this research has been to use urban meteorological observations in several cities to improve our understanding of winds, heat fluxes, and turbulence (short-term variability if winds) in downtown areas so as to improve weather forecasting, climate modeling, and estimates of pollution transport in the built-up city centers. The study has made use of recent detailed field experiments in Salt Lake City (2000), Oklahoma City (2003), and Manhattan (2005), funded by the DOE, DHS, and DOD. Many highly sensitive sonic anemometers were used, which can measure rapid turbulent wind fluctuations. Tracer gases were released near street level and concentrations were measured at about 100 locations in the downtown area. These data are unique because the emphasis was on the built-up downtown areas. Our analyses of these data have been incorporated into the public electronic data archives maintained by the DOD. The analysis of surface energy fluxes as observed by the sonic anemometers reveals several key findings of use to weather forecasting, climate modeling and to air pollution dispersion modeling. For example, in the built-up area, the presence of the large buildings results in there nearly always being an upward heat flux from the surface, even at night. Thus stable conditions are inhibited. The presence of paving and building surfaces results in a large absorption of solar (sun's) energy by the surfaces and much larger ground heat fluxes than in the suburbs or surrounding rural area. The latent heat flux (due to evaporation or condensation at the surface) is minimized in areas where there are no lawn or other irrigated surfaces within about 100 m. The turbulent wind observations from the sonic anemometers reveal a relatively large turbulence intensity at all times and at all locations. This is due to mechanical mixing by the large buildings and to the variability imposed by meandering of the incoming air flow at the edge of the urban area. Similar turbulence relations are found at the three cities studied. Study of transport and dispersion patterns also shows similarities across the three cities. Due to the large turbulence intensities in all three directions, the average plume spread over 30 to 60 minutes is quite large. Tracer material is even found in the upwind direction due to the presence of along-wind turbulence speeds that exceed the mean wind speed. Typically, concentrations on tall rooftops (100 m to 200 m elevation) near the source are 1 to 5 % of those at street level. Vertical mixing is enhanced by the downdrafts on the windward side of skyscrapers and updrafts on the leeward side. The results of the analysis of transport and dispersion have been used to develop and verify a simple dispersion model for downtown areas. These basic principles have been incorporated in operational emergency response models used by the DOE and the DOD, and have been incorporated in emergency response guidelines from the DHS. For example, as a result of our findings that the plume can spread in any direction near the source location, the new guidelines emphasize that no direction is really safe within a block or two of the release. Even if the wind direction is from the west at rooftops, the wind may be from the east for short periods of time at street level. The findings regarding transport and dispersion have also been used to evaluate and improve models for dispersion of pollutants (such as fine particles) released from traffic in busy streets. These releases lead to pollution "hot spots" and consequently health problems in persons living in adjacent residential structures. Thus this six-year research program has led to increased knowledge of the fundamentals of the boudary layer and dispersion in urban downtown areas. These results are being used to improve weather forecast models, climate models, and operational urban dispersion models.
