Thermodynamic analysis of air-ground and water-ground energy exchange process in urban space at micro scale

With an acceleration of urbanization in China, a large number of natural underlying surface have been replaced by impervious surface, which seriously affect the urban thermal and water environment. In this study, we focus on four typical urban underlying surfaces, asphalt, cement, pervious brick, and lawn. Based on the theory of heat transfer and fluid mechanics, we establish a solar radiation model and a rainfall convection model to analyze the heat transfer process of thermal radiation, thermal conduction, and thermal convection of urban underlying surface under different meteorological conditions. The fitting effects of both models are good: For solar radiation model, 0.89 ≤ R² ≤ 0.99, 1.93 °C ≤ RMSE≤2.45 °C, 1.87 °C ≤ MAE ≤ 2.17 °C. For rainfall convection model, 0.95 ≤ R² ≤ 0.96, 0.17 °C ≤ RMSE≤0.21 °C, 0.15 °C ≤ MAE ≤ 0.2 °C. Results show that: 1) In the absence of rainfall, the land-surface temperature of asphalt, cement, and pervious brick underlying surface is higher than air temperature, which has a positive effect on urban near-surface air temperature. In addition, the lawn underlying surface with the lowest temperature and the lowest temperature difference has a negative impact on the urban surface temperature. 2) In the rainfall, the underlying surface transfers heat to the runoff in the form of convection, forcing the runoff temperature to rise. Asphalt has the most obvious heating effect on runoff and lawn has the least effect on runoff temperature. The study proposes that the land-surface temperature can be lowered by paving lawn in hot places, and the pervious underlying surface should be adopted in the areas adjacent to the water bodies to reduce the influence of the underlying surface on runoff temperature.