Urban Heat Island Effect and Its Contribution to Urban Warming in Lanzhou in Recent 50 Years

Urban heat island effect of Lanzhou city is analyzed using the average temperature, the maximum temperature and the minimum temperature during 1956-2005. Then the heat island intensity trends and contribution rates are calculated using the linear trends of three temperatures of Lanzhou city and the suburb. Results show that the differences of three temperatures between Lanzhou city and the suburb are all taking on increasing trends. The linear trends of average temperature, the maximum temperature and the minimum temperature are 0.371degC /10a, 0.169degC /10a and 0.654degC /10a, respectively. The minimum temperature increases most obviously. In recent 50 years. temperatures increase mainly in the latter 25 years (1981-2005), and the linear trends of average temperature, maximum temperature and the minimum temperature of Lanzhou city are 0.789degC/10a, 0.997degC/10a and 0.625degC/10a, respectively while those of the suburb are 0.395degC/10a, 0.188degC/10a and 0.674degC/10a. The heat island contribution rates of three temperatures are 87.0%, 49.6% and 100%, respectively. The linear trends of average temperature and the minimum temperature in winter are the most in both city and the suburb, but the heat island contribution rate of the winter temperatures are not the largest, this maybe mainly related with the severe air pollution in winter in Lanzhou city, because it can hold back urban heat island in some extent. In the latter 25 year, urban heat island effect of Lanzhou city was becoming stronger, but the contribution rates of average temperature and the maximum temperature are declining. This study is important to city planning and studying urbanization and the effect of environmental change on urban temperature.