## 4 Validation of Water Body Temperature Field Calculation Model and Analysis of Test Results
### Analysis of the Diurnal Variation Patterns of the Cold Skin Layer in Water Bodies  
In Section 4.3, an analysis of the factors influencing cold skin properties provided preliminary insights into how variations in individual environmental parameters affect these attributes. However, in real-world scenarios, these factors interact in a coupled manner. Therefore, this section utilizes environmental parameters measured during Day 1 and Day 2, along with the air-sea flux model and the cold skin calculation model, to compute the diurnal variations in heat fluxes and cold skin properties. These results are then compared with measured data to validate the accuracy of the cold skin calculation model. Figure 18 illustrates the diurnal variations in heat fluxes for Day 1. It is evident that the sensible and latent heat fluxes remain relatively low throughout the day. During periods of strong solar radiation in the daytime, the water surface absorbs a significant amount of shortwave radiation, which substantially reduces the net heat flux at the interface. At night, when solar radiation ceases, the shortwave radiation absorbed by the water surface drops sharply to zero. At this time, the water temperature exceeds the ambient air temperature, and net longwave radiation dominates, causing the net heat flux at the interface to increase rapidly. This results in an enhanced cooling effect at the water surface during nighttime. Figure 19 presents the calculated and measured results for cold skin thickness. The cold skin thickness predominantly remains within the range of 2-5 mm throughout the day, with minimal diurnal variation. The measured cold skin thickness is generally higher than the calculated values, but the magnitudes are comparable, indicating that the calculation model can predict the cold skin thickness parameter with reasonable accuracy. Figure 20 provides the calculated and measured results for the cold skin temperature difference. Analysis reveals that during the period from 08:00 to 09:40, the sensible and latent heat fluxes remain at relatively high levels, while the shortwave radiation absorbed at the interface is low. This results in a larger net heat flux at the interface, with the cold skin temperature difference ranging between 0.08 and 0.12 K. From 10:00 to 15:30, solar radiation increases significantly,  
Figure 16: Effect of temperature difference on each heat flux. Figure 17: Effect of temperature difference on cold skin properties. leading to a rise in the radiation absorbed by the water surface and a rapid decrease in the net heat flux at the interface. During this period, the cold skin temperature difference remains at a low level, mostly below 0.02 K. After sunset, the shortwave radiation absorbed at the interface drops sharply, while the net longwave radiation increases steeply, causing the cold skin temperature difference to rise rapidly, exceeding 0.2 K. Figure 20 compares the calculated and measured diurnal variations in the cold skin temperature difference, showing that the trends of the measured and calculated results are generally consistent, with an average error within 23%. Figure 21 illustrates the diurnal variations in heat fluxes for Day 2. Compared to Day 1, the solar radiation intensity on Day 2 is significantly higher, with shortwave solar radiation exceeding 500 W/m\({}^{2}\) between 1000 and 1500 and peaking at 800 W/m\({}^{2}\). During this  
Figure 19: Cold skin thickness calculation and measured results of Day 1. Figure 18: Diurnal heat flux variation curve of Day 1. Figure 20: Cold skin temperature difference calculation and measured results of Day 1.  
period, the shortwave radiation energy absorbed by the water surface dominates. Under intense sunlight, the rate of air temperature increase exceeds that of the water temperature, leading to negative net longwave radiation. Although high solar irradiance can enhance water evaporation and heat conduction at the air-liquid interface, the magnitude of this effect is far outweighed by the shortwave radiation energy absorbed at the surface.