First evaluation of near-real-time photosynthetically active radiation from Himawari remote sensing observations under different aerosol, precipitable water, and cloud conditions in the tropical environment of Thailand

This study presents the first comprehensive assessment of near-real-time photosynthetically active radiation (PAR) derived from Himawari satellite products (PAR_Himawari) in the complex tropical environment of Thailand, which is characterized by variable aerosol concentration, precipitable water vapor, and cloud movement. Level 2.0 quality-assured PAR_Himawari data were validated against ground-based EKO quantum sensor measurements (PAR_EKO) at three regions (Chiang Mai, Nakhon Pathom, and Songkhla) from 2018 to 2022. Results show strong linear correlations between PAR_Himawari and PAR_EKO (R² > 0.7). However, the magnitude of estimates remains uncertain. The Nash-Sutcliffe equation (NSE) was negative at Chiang Mai (-1.10), indicating poor estimation performance, while moderate NSE values of 0.29 and 0.50 were found at Nakhon Pathom and Songkhla, respectively. The relative root mean square error (rRMSE) ranged from 33.15 % to 64.32 %, and the relative mean bias error (rMBE) ranged from 23.82 % to 55.09 %, suggesting systematic overestimation by the satellite product. Further analysis confirmed that PAR_Himawari residuals increase with aerosol optical depth (AOD) and precipitable water vapor. For cloud optical thickness (COT), residuals median values were positive under clear-sky conditions, approached zero under intermediate sky conditions, and shifted negative under overcast conditions. Overall, PAR_Himawari shows higher errors when providing high PAR values, underscoring the need for improved aerosol and moisture corrections to enhance satellite PAR retrievals in tropical environments for ecological and agricultural applications.