Observed Spatial Characteristics of Beijing Urban Climate Impacts on Summer Thunderstorms

This study investigates interactive effects from the Beijing urban area on temperature, humidity, wind speed and direction, and precipitation by use of hourly automatic weather station data from June to August 2008-12. Results show the Beijing summer urban heat island (UHI) as a multicenter distribution (corresponding to underlying land-use features), with stronger nighttime than daytime values (averages of 1.7 degrees vs 0.8 degrees C, respectively). Specific humidity was lower in urban Beijing than in surrounding nonurban areas, and this urban dry island is stronger during day than night (maximum of -2.4 vs -1.9 g kg(-1)). Wind direction is affected by both a mountain-valley-breeze circulation and by urbanization. Morning low-level flows converged into the strong UHI, but afternoon and evening southerly winds were bifurcated by an urban building-barrier-induced divergence. Summer thunderstorms also thus bifurcated and bypassed the urban center because of the building-barrier effect during both daytime and nighttime weak-UHI (<1.25 degrees C) periods. This produced a regional-normalized rainfall (NR) minimum in the urban center and directly downwind of the urban area (of up to -35%), with maximum values along its downwind lateral edges (of >15%). Strong UHIs (>1.25 degrees C), however, induced or enhanced thunderstorm formation (again day and night), which produced an NR maximum in the most urbanized area of up to 75%.