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Airflow over a City in Terrain of Moderate Relief

Francis Pooler Jr.U. S. Weather Bureau Research Station, Robert A. Taft Sanitary Engineering Center, Cincinnali, Ohio

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Abstract

Meteorological data from a network set up to study local air pollution problems were used to study local air flow patterns at Louisville, Ky., an urban area located in a region of relatively minor topographic features. Periods of stability and instability were defined primarily in terms of temperature differences obtained from hygrothermographs installed at different ground elevations. For each stability group, hourly wind data from each of five surface stations were subdivided according to wind directions and speeds obtained from an installation at the 524-ft level of a television tower located in downtown Louisville.

The flow during unstable hours was reasonably homogeneous; the data for unstable periods were used principally to infer differences in instrumental responses. The flow during stable hours appeared to be approximately antitriptic, i.e., determined by pressure and frictional forces only. The pressure forces could be considered as consisting of a large-scale and a local component. The television tower winds were used to indicate the large-scale component, and the local component could be inferred from a consideration of the effects of topography and urban heating. The area flow patterns for a number of television tower wind direction groups, representing a number of large-scale pressure gradient directions, show the effects of varying topographic blocking. The flow pattern with null large-scale gradient showed an inflow toward the urban center; this effect could be detected whenever the over-all flow was weak.

Abstract

Meteorological data from a network set up to study local air pollution problems were used to study local air flow patterns at Louisville, Ky., an urban area located in a region of relatively minor topographic features. Periods of stability and instability were defined primarily in terms of temperature differences obtained from hygrothermographs installed at different ground elevations. For each stability group, hourly wind data from each of five surface stations were subdivided according to wind directions and speeds obtained from an installation at the 524-ft level of a television tower located in downtown Louisville.

The flow during unstable hours was reasonably homogeneous; the data for unstable periods were used principally to infer differences in instrumental responses. The flow during stable hours appeared to be approximately antitriptic, i.e., determined by pressure and frictional forces only. The pressure forces could be considered as consisting of a large-scale and a local component. The television tower winds were used to indicate the large-scale component, and the local component could be inferred from a consideration of the effects of topography and urban heating. The area flow patterns for a number of television tower wind direction groups, representing a number of large-scale pressure gradient directions, show the effects of varying topographic blocking. The flow pattern with null large-scale gradient showed an inflow toward the urban center; this effect could be detected whenever the over-all flow was weak.

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