天津市热环境时空演变及影响因素分析

随着城市化进程加快，城市热环境发生显著变化，对人类活动造成很大影响。为探究天津市城市热环境时空变化及其影响因素，基于2005—2020年四期Landsat遥感数据，利用辐射传输方程法反演地表温度，计算城市热岛比例指数，用标准差椭圆法分析城市热环境发展和布局，利用地表温度并结合土地利用分类研究城市地物覆盖类型与地表温度的响应关系，并采用地理探测器探究高程、绿地、建筑用地和水体对地表温度变化的影响差异。结果表明：天津市热岛足迹沿主干道由市内六区向环城四区发展，天津城市热环境呈现先增高后降低的趋势；由标准差椭圆分析可知，城市热岛呈现发展主轴维持在东北—西南方向，整体空间格局呈现放射状分布；天津市中心城区建筑用地面积占比逐年增加，水体面积占比逐年降低，各类型地物平均温度排序为：建筑用地＞绿地＞水体；地理探测器分析表明：NDBI和NDVI是影响地表温度变化的主要因子，NDVI和MNDWI对LST交互作用最强，是城市热环境驱动作用最强的因子组合。

Spatio-temporal evolution of thermal environment in Tianjin City and its influencing factors

Background, aim, and scope Since the Beijing-Tianjin-Hebei coordinated development plan was promoted as a national strategy in 2014, the economic system of Tianjin City has been continuously improved. Currently, Tianjin City has been promoted to a super-city, the urbanization process has entered a new development stage, and the urban thermal environment has also undergone significant changes. To understand the influence of the urban heat island effect on the city, it is vital to investigate the spatio-temporal changes of the urban heat island effect in Tianjin City, analyze the influencing factors, and give reasonable suggestions. Materials and methods To investigate the current potential effect of data and deeply study the change law and influencing factors of heat island in the central urban area of Tianjin City in recent years, this paper collected Landsat remote sensing image data from 2005 to 2020, conducted land surface temperature inversion, and calculated the urban heat island proportion index of 10 main urban areas of Tianjin City by employing the radiative transfer equation method. The annual variation of the urban thermal field was demonstrated using the standard deviation ellipse method, and the local key areas of temperature anomaly were determined. Then, the response relationship between land cover type and land surface temperature was investigated through land surface temperature and land use classification. Finally, the geographical detector was employed to investigate the influence and interpretation degree of elevation, green space, built-up land, and water body on heat island change. Results From 2005 to 2020, the heat island footprint of downtown Tianjin developed from six districts to four suburban districts along the main road. The urban heat island ratio index increased from 0.23 in 2005 to 0.30 in 2014 and then decreased to 0.27 in 2020. Meanwhile, from 2005 to 2020, the main axis of urban heat island development was in the direction of northeast to southwest, and the deflection angle ranged from 118.97° to 129.80°. Also, both the proportion of built-up land in the central urban area and the proportion of water bodies in Tianjin City are increasing year by year. The average temperature of various types of land objects is ranked as built-up land＞green space＞water body. The results of geographic detector analysis suggest that NDBI and NDVI are the main factors affecting land surface temperature change, and NDVI and MNDWI are the strongest combinations of factors driving the urban thermal environment. Discussion The urban thermal environment in the central urban area of Tianjin exhibits a trend of first increasing and then decreasing, and the distribution direction of the heat island is obvious and gradually develops from a single-center structure to a multi-center structure. The surface temperature is jointly affected by many factors, and the comprehensive influence of two factors is greater than that of a single factor. The research indicates that the remote sensing indices NDBI and NDVI, which are closely related to urban construction and human activities, significantly affect the spatial distribution of surface temperature. This is because the built-up land has a large heat capacity, while the vegetation reduces the surface heat capacity due to its transpiration and the shielding of the surface, making the spatial differentiation of surface temperature crucial. As important components of the urban ecosystem, urban green space and water bodies play an important role in regulating the urban heat island effect. Then, the cooling efficiency of the two components in the heat island effect was quantitatively analyzed. Conclusions The heat island effect in Tianjin City exhibited a trend of increasing first and then decreasing. The urban heat island proportion index reached its highest value in 2014 and decreased again in 2020. The regional spatial distribution of the heat island radiates outward, and the four districts around the city are undergoing rapid urbanization, which is a key factor for the change in urban heat island spatial patterns. The results obtained by the geographic detector suggest that vegetation and water have the most significant mitigation effect on Tianjin’s urban heat island. Recommendations and perspectives In future urban construction, the layout and planning of urban green space and water bodies should be strengthened, and the landscape pattern of blue-green space should be optimized to mitigate the influence of the urban heat island effect.