Urbanization effects and contributions on surface thermal trends: Diurnal and climatic differences across urban gradients in Asian cities

Urban expansion and internal densification reshape urban surface structures and thermal conditions, yet their effects and contributions across development intensity gradients remain unclear. This study examined land surface temperature (LST) conditions and trends across urban gradients in Asian cities from 2000 to 2023. Impervious surfaces were classified into low, medium, and high density urban zones, with rural reference areas included. Daytime LST, nighttime LST, and day–night temperature difference was derived. Long-term trends were estimated, and urbanization effects and contributions were quantified from urban density trend differences. The total impervious surface area increased by 156.9%, with high density areas expanding by 328.2% and accounting for 50.9% of the net increase. Mean daytime and nighttime LST rose consistently with impervious surface density, increasing by 2.10 and 1.58 °C, respectively, from rural to high density zones. In contrast, long-term LST trends showed no consistent monotonic gradient across density classes, while the day–night difference exhibited negative trends in all spatial classes. Daytime urbanization effects increased from 0.0167 to 0.0217 °C/year, with relative contributions rising from 0.1772 to 0.2614. Nighttime effects and contributions shifted from negative values at low and medium densities toward zero at high density, whereas effects on day–night difference shifted from positive to near zero. The Warm climate zone showed the clearest density gradients during daytime. These findings reveal distinct density-related patterns in urban thermal change. High thermal conditions did not necessarily correspond to rapid warming, highlighting the need for density and climate specific heat mitigation strategies.