Thermal Emission Hot-Spot Effect of Crop Canopies—Part I: Simulation

This paper is the first part of a three-part article series. Simulations of directional brightness temperature over both simple canopies with triangular leaves and the row-planted wheat and corn were used to analyze the thermal emission hot-spot effect on crop canopies. Two models, Cupid and TRGM, were successively used to simulate the thermal hot-spot signatures under conditions which cannot be easily captured in reality. The investigation includes the planting row structure, the leaf area index (LAI), the leaf angle distribution (LAD), the component temperature distribution as well as variations in the microclimate. The results show that there are typically three types of directional emission shapes in the solar principle plane: the bowl, dome and bell shape. Regardless of the shape, the hot spot is significant and can be accurately fitted (R² = 0.98 and RMSE = 0.04°C) with a function of the phase angle (ξ), the hot-spot amplitude (ΔT_HS) and the half width of the hot spot (ξ₀)> which can be quantified with the half width in the RED band. The planting row structure can reduce the ΔT_HS by a maximum amount (about 1.2°C) when compared with an unstructured horizontal canopy. The ΔT_HS is linearly related to the component temperature differences between sunlit and shadowed parts. The linear equation can be used to predict the component temperature differences from ΔT_HS. The accuracy is very good for the horizontal canopies with triangular leaves (RMSE <; 0.4°C and R² > 0.99), and acceptable for the virtual wheat and corn canopies (RMSE <; 1.8°C and R² > 0.81).