This study provides the first climatological synthesis of how urbanization augments warm-season convection among a range of cities in the southeastern U.S. By comparing the location of convection in these cities and adjacent control regions via high-resolution, radar reflectivity and lightning data, we illustrate that demographic and land-use changes feed back to local atmospheric processes that promote thunderstorm formation and persistence. Composite radar data for a 10-year, JuneAugust period are stratified according to specific "medium" and "high" reflectivity thresholds. As surrogates for potentially strong (medium reflectivity) and severe (high reflectivity) thunderstorms, these radar climatologies can be used to determine if cities are inducing more intense events. Results demonstrate positive urban amplification of thunderstorm frequency and intensity for major cities. Mid-sized cities investigated had more subtle urban effects, suggesting that the urban influences on thunderstorm development and strength are muted by land cover and climatological controls. By examining cities of various sizes, as well as rural counterparts, the investigation determined that the degree of urban thunderstorm augmentation corresponds to the geometry of the urban footprint. The research provides a methodological template for continued monitoring of anthropogenically forced and/or modified thunderstorms.

In 1888, Iowa weather researcher Gustavus Hinrichs gave widespread convectively induced windstorms the name "derecho". Refinements to this definition have evolved after numerous investigations of these systems; however, to date, a derecho climatology has not been conducted. This investigation examines spatial and temporal aspects of derechos and their associated mesoscale convective systems that occurred from 1986 to 1995. The spatial distribution of derechos revealed four activity corridors during the summer, five during the spring, and two during the cool season. Evidence suggests that the primary warm season derecho corridor is located in the southern Great Plains. During the cool season, derecho activity was found to occur in the southeast states and along the Atlantic seaboard. Temporally, derechos are primarily late evening or overnight events during the warm season and are more evenly distributed throughout the day during the cool season.

A 15 yr (1986)(1987)(1988)(1989)(1990)(1991)(1992)(1993)(1994)(1995)(1996)(1997)(1998)(1999)(2000) climatology of derecho-producing mesoscale convective systems (DMCSs) is presented in order to better delineate their spatial and temporal patterns. Several significant results emerged from the analysis, including the development of the NW flow corridor as the dominant derecho activity region in the climatology. Results suggest that, as the sample size of DMCSs increases (230 events), the prominent derecho activity corridors across the eastern US become located in the northern Plains through the Ohio Valley, with a secondary maximum in the southern Plains. Evidence further suggests that climatological factors strongly control the distribution of derechos. For example, an anomalously strong 500 hPa height gradient existed coincident with the northern US derecho activity corridor. Another aspect of derecho development is related to the temporal distribution. Evidence suggests that derecho systems tend to occur in groups or 'families', several events occurring within several days. The synoptic environment also appears to be responsible for activating these corridors and providing an environment conducive to DMCS grouping. KEY WORDS: Derecho · Climatology · Mesoscale convective systemsResale or republication not permitted without written consent of the publisher Clim Res 24: 129-139, 2003 warm season anticyclones appears to favour and subsequently activate some derecho corridors, particularly in the Great Plains and northern tier of the US. Once synoptic-scale conditions are favourable for DMCS development, the DMCSs, after being initiated by localized forcing, tend to group or 'train' over a similar geographic region. In other words, a specific region may experience a series of DMCSs over successive days. DATA/METHODOLOGYThe primary data source for this investigation is the Storm Prediction Center's Severe Plot 2.0 software (Hart 2000) containing a database of severe convective wind events (≥ 26 m s ) and documented wind-damage reports, taken from the NOAA publication Storm Data. In order to assure consistency, the identification criteria used in BM98 is also used in this investigation (Table 1). The derecho events were then mapped to facilitate visual inspection and ensure temporal and spatial continuity. Individual wind damage reports from each derecho event were then gridded on an 1°× 1°grid-cell domain using the Interactive Data-common Language (IDL) so that contours could be generated in order to discern the geographical and frequency distribution of DMCSs.Composite reflectivity values obtained from radar archives were also examined to ensure convective characteristics of many cool-season (September-February) DMCSs. These images were obtained from the National Climate Data Center's (NCDC) on-line radar archive (available at http://lwf.ncdc.noaa.gov/oa/ radar/radardata.html).Data from the NCEP/NCAR reanalysis were used to reconstruct the 500 hPa environment during months of DMCS activity. The reanalys...

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.

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