How does hillside urban expansion affect the surface urban heat island effect? empirical evidence from China

The main findings are as follows: (1) The dynamic process of HUE at different scales was dominated by low-intensity expansion; the center of gravity of the static slope ratio was consistently located in the second quadrant (South and Southwest China), with its overall movement trajectory perpendicular to the Heihe–Tengchong Line during the study period. The proportion of medium- and high-intensity slope-climbing cities increased slowly, and high-intensity slope-climbing cities were mostly distributed in the second quadrant.

(2) The distribution of SUHI intensity showed scale similarity and regional heterogeneity from 2001 to 2023. The combined proportion of cities with cold islands and strong heat islands remained stable at approximately 60%, and the spatial scope of the SUHI effect generally exhibited an expansion trend characterized by “widespread dispersion and localized clustering.” (3) Natural environmental and human activity factors jointly shaped the driving pathways of the SUHI effect.

The cumulative contribution of key influencing factors such as built-up height, the kernel normalized difference vegetation index, annual average temperature, and nighttime light intensity reached 76.4%, exhibiting significant temporal heterogeneity and spatial nonlinearity. (4) The effect of HUE on the SUHI effect followed a cascade process of “direct driving—interaction enhancement—threshold constraint—pathway transmission.” No statistically stable HUE intensity threshold was identified at the national scale, whereas in the second quadrant, two independent methods consistently indicated that the marginal warming effect of HUE tends to saturate once its intensity exceeds 0.27.

This study proposes differentiated policy recommendations, providing an important reference for mitigating the thermal effects of HUE and promoting sustainable urban development.</p>