Observation and analysis of the urban heat island effect on soil in Nanjing, China

The heat island effect in urban meteorology has received significant attention in the recent years. In order to investigate the heat island effect on urban soil, two observation stations were built, respectively, in an urban area and a rural area of Nanjing city, China. The temperatures of underground soil (0-300 cm depth) were recorded continuously for 1 year from June 2009 to June 2010. The data show that the urban soil temperature is generally higher than the rural soil temperature, and reveal an obvious heat island effect in urban soil with average intensity of 2.02A degrees C over the 1-year period. The intensity varies between days, months and seasons: the daily urban heat island intensity (UHII) of soil ranges from 0.37A degrees C to 3.98A degrees C; the monthly UHII of soil ranges from 1.34A degrees C (November) to 3.05A degrees C (July); the order of seasonal UHII is summer (2.45A degrees C) > winter (2.03A degrees C) > spring (1.63A degrees C) > autumn (1.53A degrees C). The temperature data indicate that the maximum influence depth of daily synoptic events on the subsurface temperature is approximately 60 cm; the UHII generally increases with increasing depth. In addition to soil temperature, the temporal-spatial variation of soil moisture in a 100 cm profile depth was also investigated in this study. It is found that the moisture content of urban soil is generally lower than that of rural soil, which reveals an obvious dry island effect with average intensity of -7.2% over the 1-year period; the maximum single-day urban dry island intensity (UDII) in soil is -28.0%; the maximum average monthly UDII is -19.1%, observed in July; the seasonal UDII shows a tendency of summer (-13.8%) > spring (-6.3%) > autumn (-5.2%) > winter (-3.7%). In profile, soil moisture content generally increases with increasing depth, and the maximum UDII is -25.8% at 40 cm depth. In addition, the large-scale measurement results of 600 general points also confirm that the heat island and dry island effects are exist in urban soil.