Flow and Turbulence in Urban Canopies

Rapid urbanization and associated issues such as urban security and air quality have necessitated a better understanding of airflow, turbulence and heat and mass transfer in urban canopies. Such knowledge is key to the development of detailed predictive models as well as parameterization of urban effects in meso- scale models. To this end, field and laboratory experiments were conducted to improve our understanding of flow, turbulence and dispersion in urban areas. The field component was carried out in conjunction with two large field campaigns: Mock Urban Setting Test (MUST) conducted at the U.S. Army Dugway Proving Ground during September 2001 and Joint Urban 2003 (JU 2003) in Oklahoma City during the summer of 2003. The MUST set up consisted of a 180m x 176m rectangular array of 120 containers that simulated an urban environment. Balloons carrying tethersondes provided background profiles of important meteorological quantities while the ultrasonic anemometers located inside and outside of the grid provided mean and turbulent characteristics of the flow. The JU 2003 was conducted in a real urban area and data were collected in different parts of the city including downtown Central Business District (CBD), commercial/industrial areas as well as suburban areas. This wide distribution of measurements sites enabled a better understanding of how local site characteristics influence flow and its stability. In JU 2003, the ASU group deployed two meteorological towers, one inside an urban canyon within CBD (Park Avenue) and the other 1.3 km northwest outside of the CBD on a grassy surface surrounded by low-level buildings and some vegetation (Surface Energy Budget Station). The towers were equipped with infrared thermometers that provided surface temperatures, instruments for measurements of incoming and outgoing radiation, thermistors, cup anemometers, krypton hydrometer and ultrasonic anemometers. A Lidar was also deployed for obtaining the 3-D wind field above the city. These together with data of different groups participated in the experiment were analyzed to provide detailed profiles of salient quantities inside the urban canyon as well as above the urban canopy level. Some existing formulations were verified using the two datasets and new concepts for flow through urban canyons were advanced.