Objective  The weather environment is unpredictable, and rainy weather is even more inevitable. Studying the surface thermal characteristics of ground targets can better serve thermal infrared detection and guidance, improving the accuracy of target positioning. Most of the existing research on the thermal and infrared characteristics of ground targets is based on sunny conditions, while few studies have been conducted on the surface temperature characteristics of ground targets under cloudy and rainy conditions. The distribution of rainfall on the surface of ground targets under overcast and rainy weather conditions is affected by rainfall and wind force; The ground background is affected by the latent heat caused by moisture absorption and evaporation of permeable materials, resulting in uncertainty in the thermal characteristics of the ground target surface. Therefore, this paper proposes a calculation model for surface temperature of ground targets that integrates wind-driven rain and moisture heat coupling, and studies the thermal characteristics of surface targets under overcast and rainy weather conditions, as well as the effects of rainfall intensity and wind direction on surface temperature characteristics of targets.

  Methods  In Open FOAM, an open source platform, combining air flow model, radiation heat transfer model, wind-driven rain model, and moisture heat coupling model, a ground target surface temperature calculation model integrating wind-driven rain and moisture heat coupling is established (Fig.2), and the call and coupling process is given (Fig.3). The reliability and accuracy of the integrated model were verified using CUBI model experiments. The calculated temperature curve was similar to the temperature variation trend of the characteristic points on the surface of the CUBI model in the experiment (Fig.9), and the average absolute error was 0.95 K. The calculation model is used to study the thermal characteristics of the surface of a ground target under overcast and rainy weather conditions, as well as the effects of rainfall intensity and wind direction on the temperature characteristics of the target surface (Fig.11, Fig.13).

  Results and Discussions   The simulated value of the target surface temperature calculated by the integrated calculation model has a highly consistent trend with the measured value of the thermocouple, and the average absolute error of each surface during the 24-hour numerical calculation is 0.95 K; At the time of the rainfall, the temperature difference on the target surface is small, and due to the impact of rainfall and wind direction, the temperature on the windward side of the target is usually lower than the temperature on the leeward side due to the high rainfall capture rate; Changes in rainfall intensity, wind speed, and direction will affect the distribution of rainfall capture rate on the target surface (Fig.10, Fig.12), thereby affecting the distribution of surface temperature characteristics.

  Conclusions  In this study, a calculation model of surface temperature of ground targets considering the coupling of wind-driven rain and moisture heat is presented for overcast and rainy weather conditions. This method uses a fully integrated three-dimensional numerical model to study the temperature distribution characteristics of ground targets and backgrounds, fully considering water and heat transfer. The calculation model can provide detailed spatial distributions of velocity, temperature, and humidity, which can be used to study the temperature distribution characteristics of ground targets affected by solar radiation, air humidity, wind speed, and wind direction under complex weather conditions. The model has reliability and good accuracy, and can provide methodological support for the thermal characteristics analysis of complex ground targets under cloudy and rainy weather conditions.