No abstract

To estimate the impact of light-colored surfaces (roofs and pavements) and urban vegetation (trees, grass, shrubs) on meteorology and air quality of a city, it is essential to accurately characterize various urban surfaces. Of particular importance is the characterization of the area fraction of various surface-types, as well as the vegetative fraction. In this report, a method is discussed for developing data on surface-type distribution and city-fabric makeup (percentage of various surface-types) using aerial color photography. We devised a semi-automatic Monte-Carlo method to sample the data and visually identifi the surface-type for each pixel. The color aerial photographs for Sacramento covered a total of about 65 square km (25 square mile). At 0.30-m resolution, there were approximately 7X108 pixels of data. Five major land-use types were examined: 1) downtctwn and city center, 2) industrial, 3) offices, 4) commercial, and 5) residential. In downtown Sacramento, the top view (above the canopy) shows that vegetation covers 30% of the area, whereas roofs cover 23% and paved surface (roads, parking areas, and sidewalks) 41%. Under-the-canopy fabric consists of 52% paved surfaces, 26% roofs, and 12% grass. In the industrial areas, vegetation covers 8-14% of the area, whereas roofs cover 19-23%, and paved surfaces cover 29-44%. The surface-type percentages in the office area were 21% trees, 16% roofs, and 49% paved surfaces. In commercial areas, vegetation covers 5-20%, roofs 19-20%, paved surfaces 4-4-68% (about 25-54% are parking areas). Residential areas exhibit a wide range of percentages of surface-types. On average, vegetation covers about 36% of the area (ranging 32-49%), roofs cover about 20% (ranging 12-25%), and paved surfaces about 28% (ranging 21-34%). Trees mostly shade streets, parking lots, grass, and sidewalks. Under the canopy the percentage of paved surfaces is significantly higher. In most non-residential areas, paved surfaces cover 50-70% of the area. In residential areas, on average, paved surfaces cover about 35% of the area. Land-use/land-cover (LULC) data from the United States Geological Survey was used to extrapolate these results from neighborhood scales to metropolitan Sacramento. In an area of roughly 800km2, defining most of metropolitan Sacramento, about half is residential. The total roof area is about 150km2 and the total paved surfaces (roads, parking areas, side walks) is about 310km2. The total vegetated area is about 230km2.

This study explores how the Atlanta, Georgia (United States), urban region influences warm-season (May through September) cloud-toground lightning flashes and precipitation. Eight years (1995)(1996)(1997)(1998)(1999)(2000)(2001)(2002)(2003) of flashes from the National Lightning Detection Network and mean accumulated precipitation from the North American Regional Reanalysis model were mapped under seven different wind speed and direction combinations derived from cluster analysis. Overlays of these data affirmed a consistent coupling of lightning and precipitation enhancement around Atlanta. Maxima in precipitation and lightning shifted in response to changes in wind direction. Differences in the patterns of flash metrics (flash counts versus thunderstorm counts), the absence of any strong urban signal in the flashes of individual thunderstorms, and the scales over which flashes and precipitation enhancement developed are discussed in light of their support for land-cover-and aerosol-based mechanisms of urban weather modification. This study verifies Atlanta's propensity to conjointly enhance cloud-to-ground lightning and precipitation production in the absence of strong synoptic forcing. However, because of variability in aerosol characteristics and the dynamics of land use change, it may be a simplification to assume that this observed enhancement will be persistent across all scales of analysis.

This review of urban lightning research begins with a description of cloud‐to‐ground flash data and some of the conditioning practices related to their use. Urban lightning studies from the United States and internationally are then examined to distill findings as well as to compare and contrast the modes of inquiry among the meteorologists, climatologists, engineers, and geographers who study lightning. In summary, these investigations convey how urban heating, building‐induced surface friction, aerosols, and specificities such as local physiography and synoptic setting are intertwined causal mechanisms underlying urban flash modification. A tension among methodological approaches in these studies circumscribes how questions are posed, and how findings are valued and ultimately synthesized. Flash research tends to follow either a descriptive phenomenological approach or a foundational approach in which prediction and mechanism are paramount. Selecting the spatial extent of an urban lightning study and the methods to assess the statistical embeddedness of flash counts within thunderstorms events are two other methodological points of contention. City‐to‐city comparisons and integrated analysis of aerosols, precipitation, and flashes over multiple scales are important directions for future research.

In this report, the materials and various surface types that comprise a city are referred to as the "urban fabric". Urban fabric data are needed in order to estimate the impact of light-colored surfaces (roofs and pavements) and urban vegetation (trees, grass, shrubs) on the meteorology and air quality of a city, and to design effective urban-environmental implementation programs. We discuss the results of a semi-automatic Monte-Carlo statistical approach used to develop data on surface-type distribution and city-fabric makeup (percentage of various surface-types) using aerial color orthophotography. The digital aerial photographs for Houston covered a total of about 52 km 2 (20 mi 2). At 0.30-m resolution, there were approximately 5.8 x 10 8 pixels of data. Four major land-use types were examined: (1) commercial, (2) industrial, (3) educational, and (4) residential. On average, for the regions studied, vegetation covers about 39% of the area, roofs cover about 21%, and paved surfaces cover about 29%. For the most part, trees shade streets, parking lots, grass, and sidewalks. At ground level, i.e., view from below the vegetation canopies, paved surfaces cover about 32% of the study area. GLOBEIS model data from University of Texas and land-use/land-cover (LULC) information from the United States Geological Survey (USGS) were used to extrapolate these results from neighborhood scales to Greater Houston. It was found that in an area of roughly 3,430 km 2 , defining most of Greater Houston, over 56% is residential. The total roof area is about 740 km 2 , and the total paved surface area (roads, parking areas, sidewalks) covers about 1000 km 2. Vegetation covers about 1,320 km 2. * This work was supported by the U. S. Environmental Protection Agency through the U. S. Department of Energy under contract DE-AC03-76SF00098. v * When sunlight hits a surface, some of the incident solar radiation is reflected (this fraction is called albedo = â) and the rest is either absorbed or transmitted. Low-â surfaces of course become much hotter than high-â surfaces.