Spatial Distribution of Convective Activity in the Las Vegas Valley and its Relation to the Urban Growth Boundary

Urban areas have a large influence on the environment due to the effects urbanization has in regards to land use and land cover change. Since the land and atmosphere are coupled within the hydrologic cycle the conversion of land from its natural state, to an urban surface, can alter the micro-climatology of a city and its adjoining regions. Past research has shown that cities can manipulate regional convection and precipitation. This is a product of several factors such as: increased temperatures and lift caused by the urban heat island effect (UHI), alteration of the land use and land cover (this consists of surface roughness changes, surface albedo, and the change in natural vegetation), pollution, and the design of the urban surface (which causes augmented friction and convergence). This study analyzed if the rapid growth and transformation of the natural landscape in the Las Vegas Valley (LVV) has had any effect upon the climatology of the region. This research created a convective climatology and then analyzed the spatial distributions of the convective patterns occurring in the LVV to determine if there have been any changes in the patterns that coincide with the rapid urbanization of this arid regime. Radar composite reflectivity data was used to create a convective climatology for the LVV covering the period from 1995 to 2008. The LVV study region was divided into six directional sections: northwest (NW), west (W), southwest (SW), northeast (NE), east (E), and southeast (SE). Through the creation of a convective climatology for the LVV the storm patterns and trends were analyzed. The activity during the study period revealed three key areas of high convective activity. The western LVV along the Spring Mountains, the Black Mountain area in the SE section, in the McCullough Range, and around the Gass Peak area in the north-central valley in the NE section. The mean 700mb wind direction during the study period was 197°, a south-southwesterly wind direction. This mean wind direction makes part of the SE, the E, and the NE sections the downwind areas of the study region. The eastern sections demonstrated statistically significant increases in the daily percentage of convective activity during storm events days. Also the urban area showed a statistically significant enhancement in convective activity. The overall trend in the LVV has been a decrease in the average strength of the 30+ dBZ values in the valley as a whole. When those trends are analyzed on a sectional basis four of the six sections exhibit the same trend; the NW, W, SW, and NE. The SE trend displays no change in average dBZ level. The E section displays a slight increase of the average dBZ value over the span of this study, this further supports what has been observed in other urban-modification climate studies; an enhancement of convective activity over and downwind of the urban area