Urban heat island observations over Nashville in 1999

Cities have different radiative properties than their surroundings, and under appropriate circumstances we expect to find differences in the intensity and depth of turbulent mixing due to the "urban heat island." However, the magnitude and timing of these differences is not well documented. At the 1999 Southern Oxidant Study Nashville Intensive, radar wind profilers were deployed around the city as well as in one location very near downtown. Daytime boundary layer heights measured by the profilers are consistently higher in the city than outside, and on one day (4 July) the difference at midday is 700 m. This "urban dome" affects the transport and fate of pollutants emitted in the city. On 4 July, the urban plume emitted into the dome is found several hours later downwind above the rural boundary layer. To accurately model the regional atmospheric chemistry, models must account for the strong urban dome. Similar results were seen under somewhat different conditions in 1995.