## 3 Results and Analysis
### Quantitative Impact of Urbanization on Compound Floods
#### 3.2.2 Impervious Surface  
In addition to climate change, urbanization is another major factor leading to an increase in compound flood risk and challenging future urban flood management strategies [75; 76]. The impact of urbanization on compound floods should consider not only the thermodynamic and dynamic changes of the urban environment and atmosphere related to climate change on a large scale, but also the complex effects of changes in local-scale near-surface material and energy movement on meteorological and hydrological characteristics.  
Firstly, due to the urban heat island effect, the atmospheric water-holding capacity and stability in urban areas increase, enhancing convection within the atmospheric boundary layer and creating water vapor convergence zones, which raises the likelihood of heavy rainfall events [77]. Secondly, changes in cloud condensation nuclei in urban environments caused by anthropogenic aerosol emissions are widely believed to be a cause of urban-induced rainfall changes [78]. Using the Chemistry version of the Weather Research and Forecasting model (WRF-Chem), it was found that the anthropogenic aerosol effects in Houston increased the peak precipitation rate by 30% and the frequency of large rain rates by about five times [79].  
Finally, the rapid expansion of urban areas has introduced complex changes to the underlying surface, primarily through the encroachment on green spaces, rivers, and lakes for urban construction, which reduces flood control capacity. High rates of urban hardening lead to an increase in impermeable areas, a decrease in surface infiltration, and an increase in runoff. The SWAT hydraulic model was applied to study the impact of urban land use change on flood risk in China. The results showed that from 1977 to 2018, important surface areas such as roads, squares, and airports in cities increased by 47,205 km\({}^{2}\) (140.37%), directly leading to a decrease in urban flood regulation service capacity (UFRS) from 31.34% to 17.95%, and almost all cities experienced a decline in UFRS [80].  
Statistical analysis of observed data is a fundamental method for studying the impact of urbanization on compound floods. Statistical model studies indicate that urbanization increased the probability of Houston experiencing extreme flood events related to Hurricane Harvey by approximately 21 times and exacerbated the total storm rainfall [81]. Through statistical analysis of hourly precipitation data from seven meteorological stations in Shenzhen, China, it was found that the annual maximum precipitation in various parts of the city generally increased, and the changes in extreme precipitation were more significant in areas with faster urbanization rates [82]. Climate models are also valuable for simulating the future evolution of compound floods under urbanization. For instance, [PERSON] (2023) [83] used a convection-permitting (4 km) weather research and forecasting model to examine the effects of future urbanization and climate warming on extreme precipitation in the Guangdong-Hong Kong-Macao Greater Bay Area of China. Urbanization led to an increase of 27.8% to 37.6% in the quantity, 22.9% to 32.55% in the duration, and 3.7% to 7.2% in the intensity of extreme precipitation, especially in summer. Other commonly used numerical simulation methods include SWAT, LISFLOOD-FP, as shown in Table 4.