Urban effect on trends of extreme temperature indices at Beijing Meteorological Station

This research analyzes the urban effect on annual and seasonal trends and the extreme temperature indices at Beijing Meteorological Station,using daily maximum and minimum temperature data of 6 meteorological stations from 1960 to 2008.The results show that the warm nights,warm days,mean maximum temperature and mean minimum temperature increase,the frost days,cold days,cold nights and mean diurnal temperature range decrease at Beijing Observatory during the time period,and these changes are significant statistically.The extreme temperature indices based on daily minimum temperature including frost days,cold nights,warm nights,mean minimum temperature and mean diurnal temperature range undergo more significant changes than those indices based on daily maximum temperature such as cold days,warm days and the mean maximum temperature.The effects of urbanization causes a significant decrease of frosts days(-5.78 d/10a),cold nights(-17.83 d/10a)and the average daily temperature range or DTR(-0.73 ℃/10a),and a significant increase of warm nights(14.76 d/10a)and the average minimum temperature(0.70 ℃/10a)in Beijing Station.Urban effects on the trends of the indices series of Beijing Station relative to daily minimum temperature are significant at the 0.01 confidence level,and the contributions of the urban effect to the overall changes of the annual mean extreme indices series all reach 100%,indicating that the changes observed are overwhelmingly caused by the urban effect.Due to the urban effect,cold nights and DTR significantly decrease,and warm nights and mean minimum temperature significantly increase for all seasons(significant at the 0.01 confidence level).The urban effects on trends of the indices series of mean minimum temperature and mean DTR are the largest in wintertime,and the urban effects on trends of the indices series of warm nights are mainly in summertime.It is essential to pay more attention to the urban effects on extreme temperature indices trends of China meteorological stations near big cities like Beijing.