关键词: 城市热环境, 感热通量, 潜热通量, 人为热

Abstract:

Urban heat island is a phenomenon with higher temperature in urban area than that in rural area. China has experienced unprecedented rapid urbanization since 1990s, urban heat island became complicated recently and attracted more attention. In order to understand formation mechanism of urban heat environment better, this study analyzed urban heat environment spatial pattern, from the view point of surface energy balance. Shijiazhuang City, with a history of only 100 years and population of 2.86 million, is the largest city of Hebei Province. We used a Land sat TM image recorded on Aug. 12, 2009 for analysis, including extraction of the information on land surface temperature, water index and impervious surface index. The land cover of study area was classified into high density impervious surface, low density impervious surface, urban vegetation, water body, agricultural vegetation and bare land by using decision tree method. Then net radiation, sensible heat flux, latent heat flux, storage heat flux and anthropogenic heat were simulated for investigating the impact of the energy balance to urban heat environment. At last, quantitative analysis on energy per unit area comparison, energy conversion among different underlying surfaces and correlation between energy and area percentage of different underlying surfaces were analyzed. Results show that high density impervious surface(17.9%), low density impervious surface (29.2%) and agricultural vegetation (26.0%) are major land cover types, and all underlying surfaces including urban vegetation (18.6%), water body (0.7%) and bare land (7.7%) play an important role in urban heat environment formation. We also found that the amount of sensible energy per unit area of high density impervious surface (314.1 W/m²) and low density impervious surface (262.4 W/m²) were higher than other underlying surfaces, and there were 49.9% and 42.9% of net radiation conversed to sensible heat. Anthropogenic heat generated from high density impervious surface is 41.3 W/m². While the highest amount of latent energy per unit area occurred at water body (479.0 W/m²). In the city, there were 61.2% and 65.9% of net radiation conversed to latent heat from urban vegetation and water body, and it indicated that they cooled the city. Percentage of impervious surface is significant positively correlated with sensible heat flux/net radiation. Q_H/Q^* will increase by 0.3%, along with 1% increasing of impervious surface. On the other hand, sensible heat flux/net radiation will increase rapidly along with urban vegetation and water body area percentage decreasing when it is less than 30%, while if it is more than 30%, Q_H/Q^* with decrease slowly. In sum, urban heat environment is mainly caused by sensible heat exchange from impervious surface, while urban vegetation and water body can cool the city through latent energy.

Key words: urban heat environment, sensible heat flux, latent heat flux, anthropogenic heat