## 4 Validation of Water Body Temperature Field Calculation Model and Analysis of Test Results
### Verification of the Existence of Cool Skin Layer in Natural Water Bodies  
During a particular experiment, an intriguing phenomenon was observed, providing strong evidence for the identification and analysis of the cold skin phenomenon. The experiment was conducted in a natural environment where aquatic organisms were present in the water. During a nighttime test, a small aquatic organism, such as a water coral, swam across the water surface and passed through the infrared field of view, as illustrated in Figure 9. The organism entered the field from the upper left corner, swam downward, made contact with a central wire rope, by-passed it outside the field of view, and then swam  
Figure 8: Time–space evolution error map of vertical temperature profile in water body.  
Figure 7: Simulation results of temperature profiles in water bodies over time and space.  
diagonally downward to the right of the wire rope before exiting the field. As the organism moved across the water surface, it left behind a distinct \"thermal trace\", which was clearly captured by the infrared camera. The length of this thermal trace significantly exceeded the organism's own size, ruling out the possibility that it was caused by the organism's thermal radiation. Additionally, the organism's small size and limited propulsion capability, which relied solely on its own movement, meant it lacked the strong stirring capacity of a ship's propeller. Consequently, it could not entrain lower water layers to the surface. To further investigate the cause of this phenomenon, the temperature profile of the water body at that moment was measured.  
Figure 10 displays the vertical temperature distribution of the water body, as measured by the CTD instrument under the prevailing environmental conditions. The data reveal an isothermal layer within the 0-0.35 m depth range, with a temperature of 25.15 K. Below 0.35 m, the water temperature decreased with increasing depth. Even if the organism had possessed sufficient stirring capability to bring lower water layers to the surface, the surface would have exhibited a \"cold\" characteristic, making the formation of the \"thermal trace\" even more perplexing. In the current water environment, the organism was moving within the isothermal layer, while the cold water layer lay beneath it. This raises the following question: why did a thermal trace with a temperature difference of approximately 0.3 K form? Drawing on marine environmental science, it is hypothesized that a temperature boundary layer likely exists at the water surface. The movement of the organism disrupted this boundary layer, resulting in the thermal trace. Extensive literature research in marine hydrometeorology supports this hypothesis [6; 9; 29; 30; 42], indicating that in natural environments, factors such as latent heat flux, sensible heat flux, long- and shortwave radiation, wind speed, and air pressure influence the formation of a cold skin at the gas-liquid interface when the net heat flux is positive. This cold skin is typically a few tenths of a degree cooler than the underlying water and is extremely thin, in the order of millimeters. This theoretical framework explains the thermal trace phenomenon: the small  
Figure 9: An infrared temporal map of the ‘heat trail’ left by moving aquatic organisms.  
organism's movement disrupted the cold skin's microstructure at the water-air interface, altering its temperature distribution. To further validate this inference, the temperature distribution of the water surface layer was measured using a single-point method, as shown in Figure 11. The results indicate a low-temperature gradient layer within the 0-2.3 mm range, with temperatures approximately 0.3 K lower than the underlying water layer. This temperature difference aligns closely with that of the thermal trace captured by the infrared thermal imager.